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enough to Transportation Engineering - II

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Planning, design, construction and operation of

highways, roads and other vehicular facilities as well as

their related bicycle and pedestrian realms.

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2

Page Numbers mentioned in this Copy are from the Text Book

HIGHWAY ENGINEERING

by KHANNA and JUSTO

eighth edition

2001

notes by Shambhu Kumar Shah enough to Transportation Engineering - II

softcopy by :- nissan email: [email protected]

3

wearing course

base course

sub-base

soil sub-grade

fig: layers of pavement

1. HIGHWAY PAVEMENT

Road Pavement

A relatively stable layer constructed over the natural soil or soil sub-grade

is known as road pavement.

- The main function of pavement is to support and transfer the wheel loads of

vehicles over a wider area on the underlying sub-grade soil.

- A pavement consists of one or more layers, normally sub-grade, sub-base,

base course and surface wearing course.

(i) Sub-grade: It is an integral part of the road pavement

as it provides the support to the pavement from

beneath.

(ii) Sub-base: The aggregate/gravel located between

pavement sub-grade and base course to provide

additional support in the distributing the stress is

called sub-base.

(iii) Base course: It is the layer of graded materials located below the wearing

course to transfer the stress to the sub-grade through pavement sub-base.

(iv) Wearing course: It is the top most layer of pavement, the purpose of which is

to provide smooth, abrasion resistant, pressure resistant, water tight and a

strong layer.

Types of Road Pavement

Based on the structural behaviour, road pavements are generally

classified into two categories:

1. Flexible pavements

2. Rigid pavements

3. Semi-rigid pavements

1) Flexible Pavement :

The pavements which have negligible flexure strength but flexible in their

structural action under the loads are known as flexible pavements.

- The flexible pavement layers transmit the vertical compressive stresses to the

lower layers by grain-to-grain transfer through the points of contact.

- The vertical compressive stress is maximum on the pavement surface directly

under the wheel load and is equal to the contact pressure under the wheel.

- A typical flexible pavement consists of four components:

(i) Soil subgrade

(ii) Sub-base course

(iii) Base course

(iv) Surface course



4

Layers of flexible pavement:

A typical flexible pavement consists of following four layers:

(i) Soil subgrade :

The soil subgrade is a layer of natural soil prepared to receive the layers of

pavement materials placed over it.

- The loads on the pavement are ultimately received by the soil subgrade for

dispersion to the earth mass.

- It is essential that at no time, the soil subgrade is overstressed.

- The soil subgrade has the lowest stability among the four typical layers of

flexible pavement.

(ii) Sub-base course :

These layers are made of smaller size graded aggregate / gravel or soil-

aggregate mixes. In some places boulder stones or bricks are also used as

sub-base or soling course.

- Sub-base course have no proper interlocking and therefore have lesser

resistance to sinking into the weak subgrade soil when wet.

- Sub-base courses are used to improve the load supporting capacity by

distributing the load through a finite thickness.

(iii) Base course :

Base course is the layer of broken stone with bound aggregate.

- The main function of base course is as same as sub-base course i.e.:

a) To improve the load supporting capacity by distributing the load through

a finite thickness.

b) To provide a stress transmitting medium to spread the surface wheel

loads in such manner as to prevent shear and consolidation deformation.

(iv) Surface wearing course :

The top most layer of pavement is called wearing course, the purpose of

which is to give a smooth riding surface that is dense.

- It resists pressure exerted by tyres and takes up wear and tear due to the

traffic.

- Wearing course also offers a water tight layer against the surface water

infiltration.

- In flexible pavement, normally a bituminous surfacing used as a wearing

course.



5

2) Rigid Pavements :

The pavements which have worthy flexural strength are known as rigid

pavements.

- These are made of plain, reinforced or pre-stressed concrete.

- The rigid pavements have the slab action and are capable of transmitting the

wheel load stresses through a wider area below.

- These are normally analysed and designed based on elastic theory assuming

the pavement as an elastic plate resting over an elastic plate or a viscous

foundation.

- The plain cement concrete slabs are expected to take-up about 40 kg/cm2

flexural stresses.

3) Semi-rigid Pavements :

The pavements which have flexural strength in between rigid and flexible

pavement are known as semi-rigid pavements.

- It is made up of lean cement concrete, soil cement or pozzolanic concrete.

- The semi-rigid pavements have low resistance to impact and abrasion and

therefore are usually provided with flexible pavement surface course.

Factors Controlling the Pavement Design

The various factors to be considered for the design of pavements are:

(a) Design Wheel Load:

Design of pavement thickness primarily depends upon design wheel load.

- Higher wheel loads obviously need thicker pavement, provided other design

factors are same.

- The various wheel load factors to be considered in pavement design are:

i.) Maximum wheel load iii.) Equivalent single wheel load (ESWL)

ii.) Contact pressure iv.) Repetition of loads

Maximum wheel loads:

The wheel load configurations are important to know the way in which the

loads of a given vehicles are applied on the pavement surface.

- According to IRC maximum axle load is specified by 8170 kg with a max

equivalent single wheel load of 4085 kg.

single axle Tandem

Axle

TRACTOR TRAILER

Fig: Wheel Configuration of Tractor Trailer Unit



6

Contact pressure:

As the depth of pavement increases, the tyre pressure decrease and

finally diminishes at a specified depth. Hence the material used in different

layers are of varying quality i.e. the quality of materials in upper layers should

be better as compared to lower layers.

- The intensity of tyre pressure under the wheel is maximum as compared in

between wheels.

Equivalent Single Wheel Load (ESWL):

To maintain the maximum wheel load within the specified limit and to carry

greater load, it is necessary to provide dual wheel assembly to the rear axles

of the road vehicles.

- In doing so the effect on the pavement through a dual wheel assembly is

obviously not equal to two times the load on any one wheel.

- The load dispersion is assumed to be at an angle of 450, as shown in figure.

- In dual wheel load assembly, let;

d = clear gap between the two wheels

S = spacing between the centre of the wheels

a = radius of the circular contact area of each wheel, then;

S = d + 2a

- Up to the depth of d/2 each wheel load ‘P’ acts independently and after this

point the stresses induced to each load begins to overlap.

- At depth 2S and above, the stresses induced are due to the effect of both

wheels and the area of overlap is considerable.

- So the total stresses due to the dual wheels at any depth greater than 2S is

considered to be equivalent single wheel load (ESWL) of magnitude ‘2P’.



7

(b) Subgrade Soil:

The properties of soil subgrade are important in deciding the required

thickness of pavement.

- A subgrade with lower stability requires thicker pavement to protect it from

traffic loads.

(c) Climatic Factors:

Among the climatic factors; rain fall affects the moisture condition in the

subgrade and the pavement layers.

- The daily and seasonal variation in temperature has significance in the design

and performance of rigid pavements and bituminous pavements.

- Variation in moisture content of subgrade affect the ground water table,

drainage conditions, type of pavement and shoulders.

- Where freezing temperatures are prevalent during winter, the possibility of

frost action in the subgrade and the damaging effects should be considered at

the design stage.

(d) Pavement Component Materials:

The stress distribution characteristics of the pavement component layers

depend on characteristics of the material used.

- The fatigue behaviour of these materials and their durability under adverse

conditions of weather should also be give due consideration.

(e) Environmental Factors:

The environmental factors such as height of embankment and its

foundation details, depth of cutting, depth of subsurface water table etc. affect

the performance of the pavement.

Comparison between Rigid pavement and Flexible pavement

Rigid pavement Flexible pavement

1. Its life time is up to 40 yrs. 2. Initial cost is more. 3. Maintenance cost is less. 4. It is done at a time.

5. The characteristics of pavement surface are good.

6. Minimum 28 days requires for traffic opening.

7. Less hazardous to environment.

8. Design of pavement is done with accuracy.

1. Its life is up to 20 yrs. 2. Initial cost is less. 3. Maintenance cost is more. 4. Can be done in stage

construction. 5. The characteristic of pavement

surface is worse as compared to rigid pavement.

6. Traffic opening after rolling.

7. More hazardous to the environment.

8. Design of pavement is done with less accuracy.



8

Method of Flexible Pavement Design

The flexible pavements are built with number of layers. In the design

process, it is to be ensured that, under the application of load none of the

layers is overstressed.

- This means that at any instance no section of the pavement structure is

subjected to excessive deformation to form a localised depression or

settlement.

- The maximum intensity of stresses occurs in the top layer of the pavement

and magnitude of loads stresses reduces at lower layer.

- Hence the superior pavement materials are used in top layers of flexible

pavement.

Out of various flexible pavement design methods available, the following

are some common methods:

i. Group index method v. McLeod method

ii. CBR (California Bearing Ratio) method vi. Burmister method

iii. Stabilometer method vii. Asphalt institute method

iv. Triaxial test method

i. Group index method: D.J. Steel in 1945 provided a discussion on the paper dealing with the

Highway Research Board method of soil classification which included the

suggested thickness requirements based on Group Index (GI) values.

- The GI values of soils vary in the range of 0 to 20.

- The higher the GI values, weaker the soil subgrade and for a constant value

of traffic volume, the greater would be the thickness requirement of the

pavement.

ii. CBR (California Bearing Ratio) method: The CBR tests were carried out by the California State Highway

Department on existing pavement layers including subgrade, sub-base and

base course. This method is done in two steps:

(a) Calculation of CBR value: It consists of following apparatuses:

(i) 150 mm diameter mould with base plate (ii) Loading frame having cylindrical plunger of 50 mm. diameter (iii) Dial gauges

Procedure:

Soil subgrade specimen is

placed in mould in four layers.

- The water absorption values are noted,

after soaking and swelling.

- A surcharge load equal to the water

absorbed values are applied along with

the plunger on the top of specimen.



9

- The load is applied at 1.25 mm/minute and the load values are noted

corresponding penetration values of 0.0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 4.0,

7.5 and 12.5 mm.

- The CBR value is calculated by using the relation:

( )

- The standard load values obtained from the average of a large number

of tests on crushed aggregates are 1370 and 2055 kg (70 & 105

kg/cm2) respectively at 2.5 and 5.0 mm penetration.

(b) Calculation of thickness of pavement: The thickness of pavement is obtained by using the formula:-

√ [

]

[

]

⁄

This expression is applicable only when the CBR value of the

subgrade soil is less than 12%.

Where, t = pavement thickness, cm

P’ = wheel load, kg

CBR = California Bearing Ratio, %

P = tyre pressure, kg/cm2

A = area of contact, cm2 (

)

iv. Triaxial test method: The triaxial compression test is used in determining the values of elastic

moduli for various materials.

The pavement thickness (Tp) is calculated from the relation:

,√(

)

- (

)

⁄

Where, Tp = thickness of pavement

P = wheel load (kg.)

Es = modulus of elasticity of subgrade in kg/cm2

Ep = modulus of elasticity of pavement materials

Δ = deflection

a = radius of contact area (cm)

X = traffic coefficient

Y = rainfall coefficient

v. McLeod method: Norman W. McLeod through Canadian Department of Transport

conducted extensive plate bearing tests on airfield and highway pavements

and developed a design method.

- The repetitive bearing test procedure was employed using various sizes of

plates.



10

From the plate load tests an empirical design equation was recommended:

Where, T = required thickness of gravel base, cm

P = gross wheel load, kg

S = total subgrade support in kg, calculated for 30 cm dia.

plate at 0.5 cm

vi. Burmister’s (Layered System) method: The total mass of pavement and subgrade does not possess a constant E

value as assumed by Boussinesq’s analysis. Since, the flexible pavement

sections are composed of layers and the elastic modulus of the top layer is

the highest.

- If the layers of soil subgrade, sub-base course and base course are assigned

elastic moduli of Es, Esb, Eb then as per Boussineq’s analysis, it is considered

Es = Esb = Eb; where as in layered analysis, it is taken that Eb > Esb > Es.

vii. Asphalt institute method:

Design Procedure:

i) Calculate the equivalent single axle load (EASL) over the design life

= estimated traffic at the base period × growth factor × no. of working

days × no. of axles × lateral displacement factor

ii) Lateral displacement factor (LDF) for various lane:

Lane Width LDF Single lane

Intermediate lane

Two lane

Four lane

3.75 m

5.5 m

7 m

15 m

2

1.5

0.75

.4

iii) Growth factor = ( )

iv) Calculate the thickness from chart.

v) Equivalent thickness; form formula, *

+

Where, Mr = equivalent elastic modulus

= 10 × CBR

Stresses due to Load, Temperature differential & Subgrade friction on Rigid pavement:

(i) Stresses due to Load:

The position of wheels of the vehicles are not constant, these may be

located at corner, edge or in between both. Some of the researchers have

given single empirical formula while some of them have given according to

position.



11

i.) According to A-T Goldback:

Stress load due to corner load (Sc) is given by:

Where, Sc = stress due to corner load (kg/cm2)

P = corner load assumed as a concentrated

h = thickness of slab (cm)

ii.) According to Westergaard’s:

The critical stresses at different locations are given by:

(a) Interior loading,

[ (

⁄ ) ]

(b) Edge loading,

[ (

⁄ ) ]

(c) Corner loading,

[ (

√

)

]

Where, Si, Se, Sc = max. stress at interior, edge & corner loading (kg/cm2)

h = slab thickness (cm)

P = wheel load (kg)

a = radius of wheel load distribution (cm)

ℓ = radius of relative stiffness (cm)

b = radius of resisting section (cm)

(ii) Stresses due to Temperature differential:

Temperature stresses are developed in cement concrete pavement due to

variation in slab temperature.

- The variation in temperature across the depth of slab is caused by daily

variation whereas an overall increase or decrease in slab temperature is

caused by seasonal variation in temperature.

- During summer season, as the mean temperature of the slab increases, the

concrete pavement expands towards the expansion joints. Due to frictional

resistance at the interface, compressive stress is developed at the bottom of

slab as it tends to expand.

- Similarly during winter season, the slab contracts causing tensile stress at

the bottom due to frictional resistance.

(iii)Stresses due to Friction:

Due to uniform temperature rise and fall in the cement concrete slab, there

is an overall expansion and contraction of the slab.

- Since the slab is in contact with soil subgrade or the sub-base, the slab

movements are restrained due to the friction between the bottom layer of the

pavement and the soil layer.

- This frictional resistance therefore tends to prevent the movements thereby

inducing the frictional stress in the bottom fibre of cement concrete

pavement.



12

- Unit stress developed in cement concrete pavement is given by:

( ⁄ )

Where, W = unit wt. of concrete, (kg/cm3) {about 2400 kg/m3}

Combination of Stresses:

The stresses developed due to different reasons should occur at a time or

separately. The following critical combinations are considered;

i.) During Summer:

The critical combination at interior and edge regions during midday occurs

when the slab tends to warp downward.

- During this period maximum tensile stress is developed at bottom fibre due to

warping and this is cumulative with the tensile stress due to the loading.

- However, frictional stress is compressive during monsoon.

Critical stress = load stress + warping stress – frictional stress

ii.) During Winter:

The critical combination of stresses at the above regions occurs at the

bottom fibre during the midday when the slab tends to warp downward.

- Critical stress combination = Load stress + Warping stress + Frictional stress

- Since the differential temperature is of lower magnitude during winter than in

summer, the combination (i) is worst for most of region of the country.

iii.) At the corner region the critical combination occurs at the top fibre of slab

during the midnight, when the slab tends to warp upward.

- Critical stress combination = Load stress + Warping stress

Design of Rigid Pavements

Since, the Rigid Pavements are made of cement concrete, the load

carrying capacity is mainly due to the rigidity and high modulus of elasticity of

slab.

- Rigid pavements should be analysed by Plate theory instead of Layered

theory.

Westergaard’s Analysis

Westergaard considered the rigid pavement as thin elastic resting

on liquid foundation.

- The upward reaction at any point is assumed to be proportional to the

deflection at that point. i.e. P = k Δ

Or, k = ⁄

Where, k = modulus of subgrade reaction

P = pressure sustained (kg/cm2)

Δ = displacement level (taken as 0.125 cm)

k = ⁄ kg/cm2



13

Relative stiffness of slab to subgrade:

A certain degree of resistance to slab defection is offered by the

subgrade. This is dependent upon the stiffness or pressure-deformation

properties of the subgrade material.

- The tendency of the slab to deflect depends upon its properties of flexural

strength.

- The resultant deflection of the slab which is also the deformation of subgrade

is a direct measure of the magnitude of subgrade pressure.

- The pressure deformation characteristic of rigid pavement is thus a function of

relative stiffness of slab to that of subgrade.

Westergaard defined this term as the Radius of relative stiffness:

*

( ) +

⁄

where, ℓ = radius of relative stiffness (cm)

E = modulus of elasticity of cement concrete (kg/cm2)

μ = Poisson’s ratio for concrete [ = 0.15 ]

h = slab thickness (cm)

k = subgrade modulus or modulus of subgrade reaction (kg/cm3)

Temperature stresses in Rigid Pavement

Temperature stresses are developed in cement concrete pavement due to

variation in slab temperature.

- The variation in temp. across the depth of the slab is caused by daily variation

whereas an overall increase or decrease in slab temperature is caused by

seasonal variation in temp.

- Following are the types of temp. stresses in Rigid pavement:

1. Warping stresses 2. Frictional stresses

1. Warping Stresses:

Whenever the top and bottom surfaces of a concrete pavement

simultaneously possess different temperature, the slab tends to warp

downward or upward inducing warping stresses.

- The difference in temp between the top and bottom of the slab

depends mainly on the slab thickness and the climatic conditions of the

region.

- The warping stress at the edge region is given by:

( )

- For the corner region, warping stress is given by:

( )

( )√

Where,

E = modulus of elasticity of concrete (kg/cm2)

(whichever is higher)



14

e = thermal coeff. of concrete per degree Celsius

t = temp. difference between top & bottom of slab in 0c

Cx = coefficient based on ⁄ in desired direction

Cy = coeff. based on ⁄ in right angle to above direction

μ = Poisson’s ratio ( 0.15 )

a = radius of contact

ℓ = radius of stiffness

2. Frictional Stresses:

Due to uniform temperature rise and fall in the cement concrete

slab, there is overall expansion and contraction of the slab.

- Since the slab is in contact with soil subgrade or the sub-base, slab

movements are restrained due to the friction between the bottom layer

of pavement and soil layer.

- Hence, the frictional resistance is induced in bottom fibre of the

pavement.

- In short slab; stress induced due to this, is negligibly small whereas in

long slab; frictional stress developed is higher due to the greater

movement i.e. 0.15 cm.

- Unit stress developed in cement concrete pavement is given by:

Where, W = unit wt. of concrete, (kg/cm3) {about 2400 kg/cm3}

f = coefficient of subgrade restraint

L = slab length in m.

B = slab width in m.

IRC recommendations for design of rigid pavements:

1. Design Parameters:

- The design wheel load is taken as 5100 kg with equivalent circular area of

15 cm and tyre inflation pressure ranging from 6.3 to 7.3 kg/cm2.

Traffic volume, A = P ( 1 + r ) n + 20 ( in CV / day )

- The mean daily and annual temperature cycles are collected.

- The flexural strength of cement concrete used in the concrete should not

be less than 40 kg/cm2.

2. Calculation of Stresses:

- The wheel load stresses at the edge and corner region is calculated for the

designed slab thickness as per Westergaard’s Analysis.

- The temperature stress at edge is calculated as per Westergaard’s

Analysis.



15

3. Design steps of Slab thickness:

- The width of slab is decided based on joint spacing and lane width.

- The length of cement concrete slab is equal to the spacing of contraction

joints.

- A trial thickness of slab is assumed for calculation the stresses. The

warping stress at edge region is calculated and this value is subtracted

from allowable flexural stress in concrete to find the residual strength in

the pavement to support edge loads.

- The load stress in edge region is found. The available factor of safety in

edge load stress w.r.t. the residual strength is found.

- The total stresses at the corner due to wheel load and warping is checked

for slab thickness (h cm).

- The design thickness (h) is adjusted for the traffic intensity or classification

at the end of design life and using the adjustment value from table

recommended by IRC to obtain the final adjusted slab thickness.

4. Spacing of joints:

- The maximum spacing recommended for 25 mm wide expansion joints is

140 m when the foundation is rough, for all slab thickness.

- When the foundation surface is smooth,

Spacing may be 90m for slab thickness up to 20cm

& 120m for slab thickness up to 25cm.

- The maximum contraction joint spacing may be kept at 4.5m in

unreinforced slab of all thickness and for reinforced slab; it may be 13m for

slab thickness 15cm.

5. Design of Dowel bars:

Dowel bars of expansion joints are mild steel round bars of short

length. Half-length of this bar is bonded in one cement concrete slab and

the remaining portion is embedded in adjacent slab, but is kept free for the

movement during expansion and contraction of the slab.

- The main function of dowel bar is:-

a) Allow opening and closing of the joint.

b) Maintaining the slab edge at the same level.

c) Load transference is affected from one slab to the other.

6. Design of Tie bars:

Tie bars are made of mild steel deformed bars of limited length. It is

used across the longitudinal joint of cement concrete pavements. Tie bars

ensure two adjacent slabs to remain firmly together.



16

Numerical s

Design a flexible pavement, if 129 CV/day (commercial vehicles per

day) exists and growth rate is 7% construction period of the pavement

is 2 yrs. and the maximum life of the pavement is 10 yrs. CBR of

subgrade is 5%. The compacted sub-base poorly graded gravel layer

has a CBR value of 30%. Base material has CBR value of 80%.

Pavement will have bituminous surfacing.

Solution:

No. of CV/day, P = 129 cv/day

Annual growth, r = 0.07

Construction period, y = 2 yrs.

Design life of pavement, n = 10 yrs.

Now, number of commercial vehicles per day for design is given by,

A = P (1 + r) n + y

= 129 (1 + 0.07)10+2 = 290.53 291 cv/day

From Chart 7.11 (CBR Design Chart; page no. 351)

For A = 291; curve D is chosen

From Curve D, for CBR = 5% (subgrade)

Total thickness of pavement over the subgrade = 38 cm = 380 mm

For CBR = 30%,

Total thickness over sub-base = 130mm (base + surface course)

Actual thickness of sub-base = 380 – 130 = 250 mm

Adopting, thickness of wearing course = 50 mm

Thickness of base course = 130 – 50 = 80 mm #

Design a flexible pavement by using Asphalt Institute method from the

following data of a stretch of existing two lane road.

Correct traffic of 80kN equivalent single axle load=0.95×103

ESAL/day

Traffic growth rate = 7.5 %

Design period = 15 yrs.

Construction period = 16 months

CBR value of subgrade = 5 %

Elastic modulus of Asphalt Concrete surface course = 2500 MPa

Elastic modulus of bituminous treated base course = 1200 MPa

Elastic modulus of granular sub-base course = 125 MPa

Solution:

Estimated traffic at the end of design life = estimated traffic at base period ×

growth factor × no. of working days × no. of axles × lateral displacement factor Where,

( )

( ) ⁄



17

Estimated traffic at the end of design life = 0.95 * 103 ×30.10 ×365 ×2 ×0.75

= 1.57 * 107 ESAL/day

We have, Resistant modulus (Mr) = 10 × CBR

= 10 × 5 = 50 MPa

From graph,

Mr = 50 MPa and 1.57 * 107 ESAL/day

Full depth of asphalt concrete = 390 mm

Use of AC surface course = 75 mm

Remaining depth of asphalt concrete = 390 – 75 = 315 mm

Use 200mm base course and 115mm sub-base course.

For base course,

Equivalent thickness of bituminous treated base course,

( )

⁄

(

)

⁄

For sub-base course,

Equivalent thickness of granular sub-base,

( )

⁄

(

)

⁄

A 2-lane, 2-way road is at present carrying traffic of 1000 CV/day at

plain terrain. The rate of growth of traffic is 10% per annum, period of

construction is 5 yrs. the pavement is to be designed for 15 years after

construction. Calculate the standard axle to be used in design.

Solution:

[( ) ] [ ]

Here, VDF = 2.5

LDF = 0.75, P = 1000 CV/day

r = 0.1, n =15

y = 5

[( ) ] [ ]

Design a highway pavement for a wheel load of 4100 kg with a tyre

pressure of 5 kg/cm2 by McLeod method. The plate bearing carried

out on sub-grade soil using 30 cm dia. plate yielding a pressure of 2.5

kg/cm2 after 10 repetition of load at 0.5 cm deflection.

Solution:

Radius of contact (a):

√



18

Perimeter over area ratio,

⁄

Using Chart 7.20 (page 367),

The ratio of unit subgrade support on 32.2cm dia. plate at 0.5cm deflection is 0.95

Unit support at 0.5cm deflection = 0.95 × 2.5 = 2.44 kg/cm2

Design sub-grade support on 32.2 cm dia. plate,

Base course constant (k) for 32.2cm dia. plate is obtained as 90, from graph 7.19

Required thickness of gravel base,

Provide 5cm of bituminous surfacing out of this thickness. #

Plate bearing test conducted on subgrade soil using 30cm diameter

plate yielded 2.5 kg/cm2 after 10 repetitions at 5mm deflection.

Design a highway pavement for a wheel load of 5100 kg and tyre

pressure of 7 kg/cm2 for allowable deflection of 7.5mm by McLeod

method. (Refer charts).

Solution:

Radius of contact (a):

√

Perimeter area ratio,

⁄

Using Chart 7.20 (page 367),

The ratio of unit subgrade support on 30.4cm dia. plate at 0.5cm deflection is 0.99

Unit support at 0.5cm deflection = 0.99 × 2.5 = 2.48 kg/cm2

Design sub-grade support on 30.4 cm dia. plate,

Base course constant for 30.4cm dia. plate is obtained from chart 7.19, as 89

I.e. k = 89

Required thickness of gravel base,

Provide 5cm of bituminous surfacing out of this thickness. #



19

Soil sub grade sample was obtained from the project site and CBR test

was conducted at field density. The following were the result;

Penetration (mm) Load (kg) Penetration, mm Load, kg

0.0

0.5

1.0

1.5

2.0

2.5

0.0

5.0

16.2

28.1

40.0

48.5

3.0

4.0

5.0

7.5

10.0

12.5

56.5

67.5

75.2

89.0

99.5

106.5

It is desired to use following materials for different pavement layers,

Compacted sandy soil with 7% CBR

Poorly graded gravel with 20% CBR

Well graded gravel with 95% CBR

Minimum thickness of bituminous concrete surfacing may be

taken as 5cm.

Traffic survey revealed present ADT of commercial vehicles as

1200. Annual rate of growth of traffic is found to be 8%. The pavement

construction is to be completed in three years after last traffic count.

a) Design pavement section by CBR method as recommended by IRC.

b) Suggest design without use of poorly graded gravels.

Solution:

The plot is made between loads (kg) vs. penetration (mm).

0

10

20

30

40

50

60

70

80

90

100

110

0 1 2 3 4 5 6 7 8 9 10 11 12 13

Load

(kg

) →

Penetration (mm) →

2.5

5.0



20

From graph,

Load at 2.5 & 5 mm penetration (after correction) are 55 & 78 kg resp.

Area of plunger of dia. (d) 5cm is, A = π ×

= 19.6 cm2

Pressure at 2.5 mm penetration = ⁄ kg/cm2

Similarly, Pressure at 5mm penetration =

kg/cm2

CBR values of soil at,

2.5mm penetration =

Adopt CBR value = 4 %

Calculation for design thickness of different layers:

No. of vehicles for design, A= P (1 + r) n + 10

= 1200 [ 1 +

] 3 + 10 = 3260 vehicles/day

For A = 3260 vehicles/day, design curve F is used (from fig. 7.11, page 351)

From curve, for CBR = 4 %,

Total pavement thickness over subgrade = 55 cm

Compacted sandy soil with CBR = 7 %

Pavement thickness above compacted subgrade = 40 cm

Poorly graded gravel with 20 % CBR,

Pavement thickness required = 21 cm

Well graded gravel with 95 % CBR

Pavement thickness required = 8 cm

#

Design the pavement section by triaxial test method using the

following data:

Wheel load = 4100 kg

Radius of contact area = 15 cm

Traffic coefficient, X = 1.5

Rain fall coefficient, Y = 0.9

8 cm Bituminous surfacing ↕8cm

Soil subgrade,

CBR = 4 %

15 cm Compacted soil,

CBR = 7 %

19 cm Poorly graded gravel,

CBR = 20 %

13 cm Well graded gravel,

CBR = 95 %

21

cm

40

cm

55

cm



21

Design deflection, Δ = 0.25 cm

E-value of subgrade soil, Es = 100 kg/cm2

E-value of base course material, Eb = 400 kg/cm2

E-value of 7.5 cm thick bituminous concrete surface course = 1000 kg/cm2

Solution:

Assuming the pavement to consist of single layer of base course

material only; the pavement thickness is given by:

{√(

)

} ( )

⁄

{√(

)

} (

)

⁄

= 65.9 cm

Here, E-value for 7.5 cm bituminous concrete surface course, Ec = 1000 kg/cm2

Let, Ec be equivalent to the thickness tb of base course. i.e.:

[ ]

[ ]

(

)

⁄

Required thickness of base course = 65.9 – 10.2 = 55.7 cm. #

#

Calculate the stresses at interior, edge and corner regions of a cement

concrete pavement using Westergaard’s stress equation. Use the

following data:

Wheel load, P = 5100 kg

Modulus of elasticity of cement concrete, E= 3 × 105 kg/cm2

Pavement thickness, h = 18 cm

Poisson’s ratio of concrete, μ = 0.15

Modulus of subgrade reaction, k = 6 kg/cm3

Radius of contact area, a = 15 cm

Solution:

Radius of relative stiffness (ℓ ) is given by:

*

( ) +

⁄

*

( ) +

⁄

The equivalent of resisting section is given by: ⁄ ⁄ = 0.833 (< 1.74)

√ √

7.5 cm Bituminous surfacing ↕7.5cm

Soil subgrade,

Es = 100 kg/cm2

55.7 cm Base course,

Eb = 400 kg/cm2 63

.2 c

m



22

Stress at interior region,

* (

⁄ ) +

[ (

⁄ ) ]

Stress at the edge,

* (

⁄ ) +

[ (

⁄ ) ]

Stress at the corner,

[ (

√

)

]

[ (

√

)

]

Design a cement concrete for the following condition:

i) Design wheel load = 4100 kg

ii) Present traffic = 300 CV/day

iii) Design life = 20 yrs.

iv) Traffic growth rate = 7.5 %

v) Temperature variation, t = 13.10C

vi) Modulus of sub-grade reaction, k = 6 kg/cm3

vii) Concrete flexure strength = 40 kg/cm2

viii) Modulus of elasticity of concrete, E = 3 × 105 kg/cm2

ix) Poisson’s ratio, μ = 0.15

x) Coefficient of thermal expansion (α) = 10 –5 /0C

Solution:

Trial 1:

Select the thickness of slab (h) = 20 cm (20~25)

Contraction joint spacing (L) = 4.5 m

Lane width (w) = 3.5 m

Now, Radius of relative stiffness,

*

( )+

⁄

*

( )+

⁄

Here,

[ ⁄

⁄ ⁄

⁄ ]



23

From chart 7.25 (page 380), For

= 5.9, Cx = 0.9

For

= 4.6, Cy = 0.65

Use, Cx ; Cx > Cy

Edge temperature stress,

( )

( )

Residual strength of concrete,

= Flexure strength – Edge temp stress = 40 – 17.69 = 22.31 kg/cm2

For, h = 20 cm and k = 6 kg/cm2;

edge load stress (Se) is obtained from Chart 7.23 (page 376)

Se = 29 kg/cm2

Available factor of safety =

= 0.769 < 1, not safe.

Trial 2:

Try, h = 23 cm

*

( )+

⁄

*

( )+

⁄

⁄

For ⁄ = 5.29, Cx = 0.77 (From chart 7.25, page 380),

Edge temperature stress,

( )

Residual strength of concrete,

= Flexure strength – Edge temp stress = 40 – 15.13 = 24.87 kg/cm2

For, h = 23 cm and k = 6 kg/cm2;

edge load stress (Se) = 24 kg/cm2 (from Chart 7.23, page 376)

Available factor of safety = ⁄ = 1.04 > 1, ok.

Now, Corner load stress (Sc), for h = 23 cm and k = 6 kg/cm2

Sc = 24 kg/cm2 (from chart 7.24, page 377)

< 40 kg/cm2, ok.

Design traffic volume = P (1 + r) n

= 300 (1 + ⁄ ) 20 = 1275

From chart, ht = 0

h = hs + ht = 23 + 0 = 23 cm

So, Adopt slab thickness = 23cm. #



24



25

2. ROAD CONSTRUCTION TECHNOLOGY

Tools, Equipment and Plants used in Road construction

A. Tools:

i) Hand shovel → for excavation in soft soil

ii) Peak → for excavation in hard soil

iii) Chisel → for excavation in hard soil

iv) Hand rammer → for compaction

v) Brushes → for cleaning

vi) Wheel barrow → for carrying materials

vii) Trowel → for mixing of mortar and masonry

B. Equipment:

1.) Earth moving equipment:

i) Bull dozer, Scrapper → shallow excavation

ii) Power shovel → excavation in soft and medium soil

iii) Dragline → to excavate soft soil

iv) Clamshell → excavation in soft and medium soil

v) Excavator → for excavation work

vi) Trench digger → for deep excavation

2.) Levelling equipment:

i) Grader → to make surface of certain gradient

3.) Compaction equipment:

i) Vibrating roller → in cohesion less soil

ii) Pneumatic roller → in cohesive and cohesion less soil

iii) sheep’s foot roller → in cohesive soil

iv) Smooth wheel roller → for granular soil like gravel, crust, soil etc.

v) Impact rammer → used in small area, where difficult to

operate other equipment.

4.) Paving equipment:

i) Binder spreader → to spread the binder

ii) Aggregate spreader → to spread aggregate

iii) Heating kettle for binder

iv) Cement concrete mixer → to mix cement, sand & aggregate

v) Bituminous pavers

vi) Cement concreter pavers

5.) Lifting equipment:

i) Backhoe → to lift construction materials to certain heights

ii) Crane → to lift construction materials as per crane type



26

6.) Transporting equipment:

i) Dumping trucks (Tipper) → to shift materials. Not needing

manpower for unloading.

ii) Trucks → to shift the construction materials in large

quantity and longer distances.

iii) Mini dumpers → to shift the construction materials in small

quantity and shorter distances.

C. Plants:

i) Cement concrete plant → mixing cement concrete

ii) Asphalt concrete cement → mixing of course aggregate,

fine aggregates and binders

iii) Cold premix mixing plant → mixing of CA, FA and binders

iv) Aggregate crusher plant → breaking stones as per size of aggregate

v) Washing plant → to wash aggregate

vi) Screening plant → to screen aggregate

Earth excavation equipment and their suitability

i.) Bulldozer & Scraper:

These are used for shallow excavation work and for hauling the

earth for relatively short distances.

- Bulldozer is suitable for clearing site, opening up pilot roads and hauling

earth for short distance about 100m.

- Scrapper is suitable for digging, hauling and discharging the material in

uniformly thick layer.

ii.) Power shovel:

It is primarily used to excavate earth of all classes except rock and

to load it into wagons.

- The power shovel can effectively operate to excavate earth from lower

level when the depth of the face to be excavated is not too shallow.

iii.) Drag line:

Dragline is suitable to excavate soft earth and to deposit in nearby

or to load into wagons.

iv.) Clamshell:

This equipment is useful for excavation of soft to medium materials

and loose materials at or below existing ground surface.

v.) Hoe:

Hoe is meant to excavate below the natural surface where the

machine is stationed.

- It can exert high tooth pressure and hence can excavate stiff materials

which normally can’t be excavated by dragline.



27

Activities and Techniques used in Road Construction

Following activities and techniques are used as per nature,

type of work and elements of road to be constructed;

1) Bulldozer & Scraper:

(i) Site clearance

(ii) E/W in filling for embankment (iii) Excavation for cutting

(iv) Excavation for borrow pit (v) Excavation for structural

foundations

(vi) Disposal of surplus earth

2) Drainage works:

(i) Minor bridges

(ii) Causeways

(iii) Side drains

(iv) Culverts

3) Protection works:

(i) Earth retaining structures

(ii) Slope protection works

(iii) Gully control works

(iv) Landslide stabilisation

(v) Bridge protection works

4) Pavement works:

(i) Subgrade preparation works

(ii) Sub-base works

(iii) Base works

(iv) Surface works

5) Miscellaneous works:

(i) Road ancillaries

(ii) Traffic works

(iii) Bio-engineering works

(iv) Information works for traffic & pedestrians

Various works for Road construction

Preparation of subgrade

Subgrade is the lower most portion of the highway construction. It

may be situated on embankment or excavation or at existing ground surface.

- The preparation of subgrade includes site clearance, grading (embankments

or cut) and compaction.

- In all the cases, site should be cleared off and top soil consisting grass, roots,

rubbish and other organic matter should be removed.

- Next, the grading operation is started so as to bring the vertical profile of the

subgrade to designed grade and camber.

- It is most essential to compact the top of the subgrade adequately before

placing the pavement layers.

Excavation

It is the process of cutting or loosening & removing earth including

rock from its original position, transporting & dumping it as a fill or spoil bank.

- The excavation or cutting may be needed in soil, soft rock or even in hard

rock; before preparing subgrade.

- Earth excavation work may be divided as excavation or cutting, grading and

compaction.



28

Fill / Embankment

When it is required to raise the grade line of a highway above the

existing ground level, it becomes necessary to construct embankments. The

grade line may be raised due to following reasons:

i) To keep the subgrade above the high GWT.

ii) To prevent damage to pavement due to surface water & capillary water.

iii) To maintain the design standard of highway w.r.t. the vertical alignment.

Following are the design elements of highway embankments:

a) Height b) Fill material c) Settlement

d) Stability of foundation e) Stability of slopes

Problems in the construction of high embankment

The embankment may settle after the completion of construction

either due to consolidation and settlement of the foundation or due to the

settlement of fill or due to both.

- If the embankment foundation consists of compressible soil with high moisture

content, the consolidation can occur due to the increase of load and the

embankment may be failure.

- The settlement of fill is generally due to inadequate compaction during

construction, consequently cracks may appear after application of load

through vehicles and hence it may failure.

Remedial measures:

Such problems can be eliminated by following ways:

(i) Stability of foundation (ii) Stability of slopes

i) Stability of foundation

When the embankment foundation consists of weak soil just

beneath or at a certain depth below in the form of weak stratum, it is

essential to consider the stability of the foundation against a failure.

- For this, the foundation stability is evaluated & factor of safety is estimated

by various approaches. E.g. by Swedish Circular Arc Analysis etc.

- The factor of safety, in case of compressible soil foundation should be

minimum just after the completion of the embankment.

- Also, vertical sand drain should be constructed to increase the rate of gain

in strength for compressible soil foundations.

ii) Stability of slopes

The embankment slopes should be stable enough to eliminate the

possibility of a failure under adverse moisture and other conditions.

- Hence the stability of the slope should be checked or the slope should be

designed providing minimum factor of safety of 1.5.

- Flatter slopes are preferred due to aesthetic or other reasons.



29

Compaction

It is a mechanical process by which air is expelled out from soil

mass to reduce the void and to make the soil dense.

- Compaction increases the density and stability, reduces settlement and

lowers the adverse effects of moisture.

- It is well known fact that there is an optimum moisture content (OMC) for a

soil, which would give maximum dry density for a particular type and amount

of compaction.

Factors affecting compaction

i) Moisture content ii) Amount of compaction

iii) Type of compaction

iv) Soil type

v) Addition of admixture

Compacting Equipment

Following are the various types of compacting equipment:

(i) Roller

The principle of rollers is the application of pressure, which is slowly

increased and then decreased.

(a) Smooth wheeled roller

The smooth wheeled rollers are suitable to roll wide ranges of soils,

preferably granular soils and pavement materials for the various layers.

- The gross weight of such roller ranges between 4 to 18 tons.

(b) Pneumatic tyred roller

In this type of roller, a number of pneumatic wheels are mounted on

two or more axles, under a loading platform.

- It is suitable to compact non-plastic silts and fine sands.

(c) Sheepsfoot roller

Roller consists of hollow steel cylinder with projecting feet.

- It is more suited to compact clayey soil.

- About 24 or more number of passes of the roller may be necessary to

obtain adequate compaction.

(ii) Rammers

These are useful to compact small areas and where the rollers can’t

operate such as compaction of trenches, foundations and slopes.

(iii) Vibrators

These are most suited for compacting dry cohesion less granular

material.

- There are also vibrator mounted roller to give combined effects of rolling

and vibration.

(iv) Watering (jetting & ponding)

It is considered to be an efficient method of compacting cohesion

less sand.



30

Field Control for Compaction

For adequate quality control in construction, it is essential to have

proper field control in compaction. Following are two field control tests:

(i) Measurement of moisture content

(ii) Measurement of dry density

The moisture content of the soil may be found before compaction

by any one of the rapid method suitable at the site.

- If the moisture is controlled at the OMC, then the next control is the dry

density, the desired value of which may be achieved by increasing the

number of passes for the selected equipment and thickness of each layer.

- Dry density may be found by any suitable method i.e. sand replacement

method is considered quite satisfactory.

- A certain percentage (≈ 95 or 100 %) of the standard density is generally

aimed at in the field compaction.

Moisture density relationship

Density of soil increases with increase in water content up to certain

limit. This limit is known as optimum moisture content (OMC).

- As the water content increases further beyond this limit, the density of soil

starts decreases.

The theoretical relationship between moisture content and dry density is:

Where, = unit wt. of soil solids, = unit wt. of soil

w = moisture content

Soil Stabilisation

Soil stabilisation is a method of improving the qualities of inferior

soil w.r.t. strength, stability, density, bearing capacity etc.

- If the stability of local soil is not adequate for supporting wheel loads, the

properties are improved by soil stabilisation techniques.

Following are the techniques of soil stabilisation:

(a) Proportioning technique (b) Cementing agent (c) Modifying agent (d) Water proofing agent

(e) Water repelling agent (f) Heat treatment (g) Chemical stabilisation

Moisture content →

Dry

den

sity

→

compaction curve

OMC



31

Method of Soil Stabilisation

(i) Mechanical soil-stabilisation

Correctly proportioned construction materials (aggregate and soil)

when adequately compacted to get a mechanically stable layer, the method is

called mechanical stabilisation.

- The two basic principles in this method are: (a) proportioning

(b) compaction

- Mechanical stabilisation has been successfully applied for sub-base and base

course and also used as surface course for low cost roads.

(ii) Soil-cement stabilisation

In this method, soil, cement and water are mixed and well

compacted to form a strong base course.

- By the increasing percentage of cement added, there is an increase in the

strength and durability of soil-cement and a decrease in volume change,

moisture movement and plasticity.

- Soil-cement stabilisation can be used as a sub-base or base course of all type

of pavements.

- However, as the material has poor resistance to abrasion and impact, this

can’t be used as a surface course.

(iii) Soil-lime stabilisation

Soil-lime has been widely used either as a modifier for clayey soil or

as a binder.

- When clayey soils with high plasticity are treated with lime, the plasticity index

is decreased and the soil becomes friable and easy to be pulverised, having

less affinity with water.

- Soil-lime is quite suitable as sub-base course for high types of pavements and

base course for pavements with low traffic.

- It is also quite suitable in warm regions; but it is not very suitable under

freezing temperature.

(iv) Soil-bitumen stabilisation

The basic principles in bituminous stabilisation are water proofing

and binding.

- By water proofing the inherent strength and other properties of soil could be

retained.

- Most commonly used materials are cutback and emulsion.

- Bituminous stabilised layer may be used as a sub-base or base course of

ordinary roads and even as surface course for roads with low traffic in low

rainfall regions.



32

Construction of Water Bound Macadam (WBM) Roads

1) General

The term Macadam is the present day means, the pavement course made

of crushed or broken aggregate mechanically interlocked by rolling and the

voids filled with screening and binding material with the assistance of water.

- The WBM may be used as a sub-base, base course or surfacing course.

2) Materials

a) Coarse aggregates: The coarse aggregates used in WBM generally

consists of hard varieties of crushed aggregates or broken stones. The

crushed stone aggregate should be generally hard, durable and of

acceptable shape, free from flaky & elongated particles.

b) Screenings: The screenings are used to fill up the voids in the compacted

layer of coarse aggregates. The screening consists of aggregates of

smaller size, generally of the same material as the coarse aggregates.

c) Binding materials: Binding material consisting of fine grained materials is

used in WBM construction to prevent ravelling of stones.

3) Construction procedure

(i) Preparation of foundation for receiving the WBM course: The

foundation for receiving new layer of WBM may be either the subgrade or

sub-base or base course. The foundation layer is prepared to the required

grade & camber and dust, loose materials etc. are cleaned.

(ii) Provision of lateral confinement: This may be done by constructing

the shoulder to advance, to a thickness equal to that of the compacted

WBM layer.

(iii) Spreading of coarse aggregates: The coarse aggregates are

spread uniformly to proper profile to even thickness upon the prepared

foundation. Its thickness for WBM sub-base course in normally 10cm

compacted thickness.

(iv) Rolling: After spreading the coarse aggregates, compaction is done

by three wheeled power roller of capacity 6 – 10 tonnes or alternatively by

an equivalent vibratory roller. - Rolling is started from the edges, the roller being run forward and

backward until the edges are compacted.

(v) Application of screening: After the rolling, the dry screenings are applied

gradually over the surface to fill interstices in 3 or more application. Dry

rolling is continued to spread screening and brooming is carried out.



33

(vi) Sprinkling and grouting: After the application of screening, the surface is

sprinkled with water, swept and rolled. Additional screening are applied

and rolled till coarse aggregates are well bonded and firmly set.

(vii) Application of binding materials: After then binding material is

applied at a uniform and slow rate at two or more successive thin layer. - After each application, the surface is sprinkled with water and wet slurry

swept with brooms to fill voids. - This is followed by rolling with 6 – 10 tonnes roller.

(viii) Setting and drying: After final compaction, the WBM course is

allowed to set over-night. On the next day the hungry spots are located

and are filled with screening or binding material, lightly sprinkled with

water if necessary and rolled. - No traffic is allowed till the WBM layer sets and dries out.

Construction of Bituminous Pavement

Bituminous pavements are in common use in Nepal and abroad. It is

possible to construct relatively thin bituminous pavement layers over an

existing pavement. Therefore, these are commonly adopted as wearing

course.

- Flexible pavement could be strengthened in stages by constructing

bituminous pavement layers one after another in a certain period of time

unlike the cement concrete pavement construction.

Types:

Based on the method of construction, following are the different types of

bituminous pavement construction:

(i) Interface treatment

(a) Prime coat (b) Tack coat

(ii) Bituminous surface dressing

(iii) Seal coat

(iv) Grouted or penetration type construction

(a) Penetration Macadam (b) Built-up spray grout

(v) Premix

(a) Bituminous bound macadam (b) Carpet

(c) Bituminous concrete (d) Sheet asphalt

(e) Mastic asphalt

(i) Interface treatment

The treatment with bituminous material over the existing pavement layer is

called interface treatment, which is necessary to provide the bond between

the old and the new layers.



34

a) Prime coat:

Bituminous prime coat is the first application of a low viscosity liquid

bituminous material over an existing porous or absorbent pavement

surface like WBM base course.

- The main object of priming is to plug in the capillary voids of the porous

surface and to bond the loose mineral particles on the existing surface.

- The rate of bituminous primer is @ 7.3 – 14.6 kg per 10m2 area.

b) Tack coat:

Bituminous tack coat is the application of bituminous material over

an existing pavement surface which is relatively impervious like an

existing bituminous surface or a pervious surface like WBM which has

already been treated by a prime coat.

- Its rate of application is 4.9–9.8 kg/10m2 area depending in the type of

surface.

(ii) Bituminous surface dressing (BSD)

Bituminous surface dressing (BSD) is provided over an existing pavement

to serve as thin wearing coat.

- The single coat surfacing dressing consists of a single application of

bituminous binder material followed by spreading of aggregate cover &

rolling.

- When the surface dressing is similarly done in two layers, it is called two

coat bituminous surface dressing.

(iii) Seal coat

Seal coat is usually recommended as a top coat over certain bituminous

pavements which are no impervious, such as open graded bituminous

construction like premixed carpet and grouted macadam.

- The seal coat is a very thin layer of surface treatment or a single coat

surface dressing, which is usually applied over an existing black top surface.

Function of seal coat

to seal the surfacing against the ingress of water.

to develop the skid resistant texture.

to enliven an existing dry or weathered bituminous surface.

(iv) Bituminous premixed carpet

Premixed Carpet (PC) consists of coarse aggregates of 12.5 and 10 mm

sizes, premixed with bitumen or tar binder are compacted to a thickness of

20 mm to serve as a surface course of the pavement.

- PC consists of all aggregates passing 20mm & retained on 6.3mm sieve.

- It is an open graded construction and is covered by a suitable seal coat such

as premixed sand-bitumen seal coat before opening to traffic.



35

(v) Bituminous concrete or Asphalt concrete (AC)

Bituminous concrete or Asphalt concrete is a dense graded premixed

bituminous mix which is well compacted to form a high quality pavement

surface course.

- The thickness of bituminous concrete surface course layer usually ranges

from 40 to 75 mm.

- The AC consists of a carefully proportioned mixture of coarse aggregates,

fine aggregates, mineral filler & bitumen and the mix is designed by an

appropriate method.

- Generally, Marshall Method is used to fulfil the requirements of stability,

density, flexibility and voids.

Construction procedure for surface dressing

The bituminous surface dressing (BSD) is done either in a single

coat or in two coats over the existing bituminous pavement and for renewal or

as a wearing course over a WBM road.

- The appropriate temperature for surface dressing is over 160C.

a) Specification of materials:

(i) Bitumen: grade ranges between ⁄ and ⁄ .

(ii) Aggregate:

Los Angles abrasion value 35 % max.

Impact value 30 % max.

Flakiness index 25 % max.

Water absorption 1 % max.

b) Plants and equipment:

(i) Bitumen heater (ii) Mechanical sprayer

(iii) Mechanical blower and hand brushes (iv) Roller

c) Construction Procedure: (1st coat & 2nd coat)

i) Preparation of existing surface: The existing surface is prepared

to the proper profile and ruts, depressions etc. are rectified before the

treatment is done.

- A prime coat is applied if the existing base course has a pervious surface

like WBM.

ii) Application of binder: On a prepared surface using a mechanical

sprayer, uniform spraying of the bituminous binder is done at specified

rate.

- Care should be taken that excessive binder is not applied which may

causes bleeding.

iii) Application of stone chippings: The cover material i.e. stone

chippings as per the requirement are spread to cover the surface

uniformly.



36

iv) Rolling of 1st or final coat: The rolling is done with a roller of 6–8

tonnes wt. after the cover material is spread.

- Rolling is started from the edges proceeding towards the centre

longitudinally with overlapping not less than one third of the roller tread.

This is the final rolling if the surface treatment is in single coat.

- If the 2nd coat is applied then the rolling is done again after the treatment

of 2nd coat.

v) Application of binder, stone chipping and rolling for 2nd coat: The

process ii), iii) & iv) is repeated again properly for treatment of 2nd coat.

vi) Finishing and opening to traffic: The surface is checked for

longitudinal and cross profile using a straight edge of length 3 m and

variation in surface greater than 6 mm are corrected.

- The road section is opened to traffic after 24 hrs.

Construction procedure for Bituminous Concrete

The bituminous concrete is the highest quality of construction in the

group of black to surfaces.

- The mixture contains dense grading of CA, FA and mineral filler coated with

bitumen binder and the mix is prepared in a hot-mix plant.


(i) Binder: Bitumen of grade ⁄ , ⁄ or ⁄ may be chosen

depending on climatic condition of locality.

(ii) Aggregates and filler:




(iii) Bituminous concrete mix:

Marshall Stability value, minimum 340 kg

Marshall Flow value; 0.25 mm units 8 to 16

Void in mix 3–5 %

Voids filled with bitumen 75–85 %


(i) Hot mix plant (ii) Bitumen sprayer

(iii) Bitumen distributor (iv) Mechanical finisher

(v) Pneumatic roller

c) Construction Procedure:

i) Preparation of existing base course: The existing surface is

prepared by removing the pot holes or ruts if any.

- The irregularities are filled in with premix chippings at least a week

before laying surface course.



37

ii) Application of tack coat: A tack coat of bitumen is applied @ 6 –

7.5 kg / 10 m2 area.

iii) Preparation and placing of premix: The premix is prepared in hot mix

plant of a required capacity with the desired quality control.

- The bitumen may be heated up to 150 – 177 0C and the aggregate

temperature should not differ by over 14 0C form the binder temperature.

- The hot mix is carried form mixer to the location and spread by

mechanical paver at a temp. of 121 0C – 163 0C.

iv) Rolling: The mix after placing on the base course, is thoroughly

compacted by rolling at a spread not more than 5 km/hr. Rolling can be

done by following type of roller:

Initial rolling → 8 – 12 tonnes roller

Intermediate rolling → 15 – 30 tonnes pneumatic roller

Final rolling → 8 – 12 tonnes tandem roller

- The wheels of roller are kept damp with water.

v) Quality control of bituminous concrete construction: Routine checks

are carried out at site to ensure quality of mixture & pavement surface.

- Periodical checks are made for

i. Aggregate grading

ii. Grade of bitumen

iii. Temp. of aggregate

iv. Temp. of mix during mixing & compaction

- At least one sample for every 100 tonnes of mix is tested by Marshall

Test for above requirements.

- For every 100 m2 of compacted surface, field density test is conducted to

check whether it is at least 95% of density is obtained in laboratory.

- Variation in thickness greater than 6mm per 4.5m length is not allowed.

vi) Finished surface: Longitudinal undulation should not exceed

8mm per 300m length & cross profile should not have undulation greater

than 4mm.

Construction Procedure of Plain Concrete Pavement

The construction of cement concrete pavement is divided

into two groups: (1) Construction of pavement slab

(2) Construction of joints

1) Construction Procedure of Cement Concrete Pavement Slab:

There are two modes of construction of concrete slab:

(i) Alternate bay method:

In this method, a bay or one slab in alternate succession is

constructed leaving the next or intermediate bay to follow up after a gap

of one week or so.



38

- This technique

provides additional

working convenience

for laying of slab but it

has the drawback that

large no. of transverse

joints are to be

provided.

(ii) Continuous bay method:

In this method, all the slabs or bays are laid in sequence, which has

the advantage that construction work is running on one half while

essential traffic could be diverted on other half end.

- Construction joints are provided at the end of the day’s job.

a) Materials:

Cement (OPC), Aggregate (coarse and fine)

Proportioning of concrete: - Compressive strength 280kg/cm2 at 28 days.


(i) Concrete mixture (ii) Batching device (iii) Wheel barrow (iv) Vibrating screed (v) Internal vibrators

(vi) Float (vii) Straight edge (viii) Belt (ix) Fibre brush (x) Edging tool


i) Preparation of subgrade & sub-base: The subgrade or sub-base

should have following requirements:

No soft spots are present in the subgrade or sub-base.

Uniformly compacted subgrade or sub-base extends at least 30cm on either

side of concrete width.

The subgrade is properly drained.

The subgrade is prepared & checked at least 2 days in advance of concreting.

ii) Placing of form works: The form works used may be of steel or

wooden. The forms are jointed neatly and are set with exactness to the

required grade and alignment.

iii) Batching of material and mixing: Coarse and fine aggregates are

measured by weight as per required proportion and cement is measured

by number of bags.

- The mixing of concrete is done in batch mixer which will ensure a

uniform distribution of materials throughout the mass.

- The mix should be uniform in colour and homogenous.



39

iv) Transporting & placing of concrete: The cement concrete is mixed in

quantities such that it can be used within 30 minutes. Care should be

taken to see that no segregation is occurred during transportation.

- The spreading of concrete is done uniformly.

v) Compaction and finishing: The surface of pavement is compacted

either by power driven finishing machine or by a vibrating hand screed.

The concrete is further compacted by longitudinal float to take out the

excessive water.

vi) Curing: After 24 hrs. of completion of concrete work, the whole

concreting surface is covered with jute mats and the mats are thoroughly

saturated with water.

- The concreting surface is cured regularly and uniformly for 14 days.

vii) Open to traffic: When the concrete attains the required strength or

after 28 days of curing the concrete road is opened to the traffic.

2) Construction of Joints

There are mainly two types of joints in construction of concrete roads.

(i) Longitudinal joints

(ii) Transverse joints → (a) Expansion joints (b) Contraction joints

(c) Warping joints (d) Construction joints

i. Longitudinal joints:

Longitudinal joints are provided in cement concrete roads having width

over 4.5 m.

- Such joints are provided,

To allow differential shrinkage & swelling due to rapid changes in

subgrade moisture.

To prevent longitudinal cracking in concrete pavement.

ii. (a) Expansion joints:

These joints are provided to allow for expansion of the slab due to

rise in slab temperature above the construction temp. of cement concrete.

- It is provided at interval of 50 – 60 m.

ii. (b) Contraction joints:

These joints are provided to permit the contraction of slab. These

joints are spaced closer than expansion joints.

- The maximum spacing of contraction joint is;

Unreinforced slab → 4.5 m

Reinforced slab (20 cm thickness) → 14 m.



40

ii. (c) Warping joints:

Warping joints are provided to relieve stresses included due to warping.

ii. (d) Construction joints:

It is impossible to construct the whole length of the road in a day, so

there is discontinuity in construction at certain section. The joints provided

at the end of a day work, is called construction joint.

Causes of providing joints in cement concrete pavements:

Joints are provided in cement concrete roads for expansion, contraction

and warping of the slab due to the variation in the temperature of slab.

- Changes of temp. causes expansion of slab horizontally, if there is an

increase in slab temperature above the temperature during which the slab

was laid.

- Similarly, there is contraction of slab also when the temperature falls below

the temperature that the slab was laid.

- By about mid night the temp. of the bottom slab is higher than the temperature

of the slab top, so the slab warps up during this time.

Hence, due to above reasons joints are provided in c.c. pavement.

Mass Haul Diagram

It is a graphical representation of the amount of earthwork involved in road

construction and the manner in which may be most economically handled.

- Each ordinate in the diagram given below is the balance of materials obtained

from cut and used in fill.

Haul: In earthwork calculations, the term haul has dual meaning. It is

used to describe the distance over which material is moved and also the

volumes distance of material used.



41

Free haul: It is the distance to which the contractor is supposed to move

the earth without any additional charge. The charge of free haul is covered by

the unit rate of e/w.

Overhaul: It is the distance in excess of free haul for which the

contractor will be paid extra for each unit of haulages.

Economic overhaul: It is a distance to which material from excavation to

embankment can be moved more economically than to get material form

borrow opening.

It can be determined by: a + b.L = (c + a)

⁄

Also, If haul distance be F then, F + L = F + ⁄

Where,

a = cost of roadway excavation

b = overhaul and tipping in embankment

c = cost of borrow pit material

L = economical overhaul distance



42

Construction procedure of Bituminous Bound Macadam road

Bituminous bound macadam is a premix laid immediately after

mixing and then compacted. It is suitable only as a base or binder course.


(i) Grade of bitumen: ⁄ , ⁄ & ⁄

(ii) Aggregates:





(i) Sprayer (ii) Mechanical mixer

(iii) Spreader (iv) Roller


i) Preparation of existing layer: The existing layer is prepared to a

proper profile. Pot holes are patched and irregularities are made even.

ii) Tack coat or prime coat application: A tack coat is applied of thin layer

of bitumen binder on the existing black top or WBM layer either using the

sprayer or pouring can in proper quantity.

iii) Premix preparation: The bitumen binder and aggregates are separately

heated to the specified temperature of tolerance ±100C and are then placed

in the mixer.

- The mixing is done till a homogeneous mixture is obtained.

iv) Placement: The bituminous paving mixture is then immediately placed on

the desired location and is spread with rakes to a pre-determined thickness.

- The camber profile is checked with a template.

v) Rolling and finishing the paving mix: The rolling is done with 8 – 10

tonnes tandem roller.

- The rolling is commenced from the edges of the pavement construction

towards the centre and uniform overlapping is provided.

- The roller wheels are kept damp such that the paving mix may not stick to

the wheels.

- A variation greater than 6mm per 3m length is not allowed in the cross

profile.

- Also, number of undulation (>10mm) should be less than 30 nos. in 300m

length.



43

3. HIGHWAY MAINTENANCE, REPAIR & REHABILITATION

Highway Maintenance

Highway maintenance is defined as preserving and keeping the

serviceable conditions of highway as normal as possible and practicable.

- The maintenance operations involve the assessment of road condition,

diagnosis of problems and adopting the most appropriate maintenance steps.

Causes of pavement failure:

(i) Defects in the quality of materials used.

(ii) Defects in construction method and quality control during construction.

(iii) Inadequate surface or sub-surface drainage in the locality.

(iv) Increase in magnitude of wheel loads and the number of load repetitions

due to increase in traffic volume.

(v) Settlement of foundation of embankment of the fill material itself.

(vi) Environmental factors including heavy rainfall, soil erosion, high water table,

snow fall, frost action etc.

Classification of Maintenance work

Highway maintenance work may be broadly classified as:

1) Routine maintenance 2) Periodic maintenance

3) Special repairs 4) Emergency maintenance

1) Routine maintenance: Routine maintenances are needed for any type of road, whether it is

designed and constructed with scientific bias or not. Since the highways are

exposed to the moving traffic and adverse climatic conditions, they would

positively wear out. It includes following activities:

a) Filling up pot holes and patch repair

b) Maintenance of shoulders and cross slope

c) Up-keep of road side drains

d) Clearing chocked culverts

e) Maintenance of miscellaneous items like road signs, arboriculture etc.

The maintenance schedule listed above of routine type and the repairs are

carried out at regular interval like day to day and seasonal.

2) Periodic maintenance: It includes renewal of wearing course of pavement surface and preventive

maintenance of various items.



44

3) Special repairs: Special repairs and strengthening of pavements are needed to prevent

pavement failure. It includes:

a) Strengthening of pavement structure or overlay construction b) Reconstruction of pavement c) Widening of roads d) Repairs of damages caused by floods e) Providing additional safety measures like islands etc. f) Improvement of highway geometrics

4) Emergency maintenance: This maintenance is necessary when any unfavourable conditions occur

due to landslides, road wash out due to floods such that the road is closed

to pass the traffic.

- It includes the diversion work temporarily to allow the traffic to pass around

the obstructions.

Inspection, Prioritisation and Planning of Maintenance Operation

After the construction of road, the main aim is to get reliable service at

minimum possible cost for the whole life. This is achieved by inspection,

prioritization and planning.

- During inspection, a lot of defects are found which must be maintained.

- The maintenance of these defects may or may not be possible at a time due

to limited fund. So the maintenance of roads should be done according to

priorities list without the interference of political parties.

Factors to be considered in the maintenance management system:

Following factors should be included:

i) Minimum acceptable serviceability standards for the maintenance of

different categories of roads.

ii) Field surveys for the evaluation of maintenance requirements.

iii) Various factors influencing the maintenance needs such as subgrade

soil, drainage, climate, traffic, environmental condition etc.

iv) Estimation of rate of deterioration of the pavement under the

prevailing set of condition.

v) Type & extent of maintenance requirement and their economic

valuation.

vi) Availability of funds.

vii) Maintenance cost, availability of materials, man power and

equipment.



45

Pavement Failures

The pavement failure is defined by the formation of pot holes, ruts, cracks,

localised depressions and settlements.

- The failure of any one or more components of the pavement structure

develops the waves and corrugations on the pavement surface.

A. Failures in Flexible Pavement

The flexible pavement is failed due to the failure of:

a. Subgrade b. Sub-base or base c. Wearing course

So the stability of pavement structure should be maintained as a whole such

that each layer should be stable within itself and thereby make the total

pavement maintain its stability.

1) Failure in Subgrade:

It may be fail due to following two reasons:

i) Inadequate stability:

It may be due to inherent weakness of the soil itself or excessive

moisture or improper compaction.

- Stability is the resistance to deformation under stress.

ii) Excessive stress application:

It may be due to inadequate pavement thickness or loads in excess

of design value.

- The deformation of soil subgrade is found to increase with the increase in

number of load repetitions.

2) Failure in Sub-base or Base course:

Following are the main causes of sub-base or base courses failure:

) Inadequate stability or strength

) Loss of binding action

) Loss of base course materials

) Inadequate wearing course

) Use of inferior materials and crushing of base materials

) Lack of lateral confinement for the granular base course

3) Failure in Wearing course:

This failure is observed due to following reasons:

) Lack of proper mix design

) Improper gradation of aggregates

) Inadequate binder content

) Inferior type of binder

) Inadequate compaction

) Influence of climate

) Temperature maintain for mixing and placing



46

Typical flexible pavement failures:

i) Alligator (map) cracking ii) Consolidation or pavement

layer iii) Shear failure iv) Longitudinal cracking

v) Frost heaving vi) Lack of binding (keying) to the lower

course vii) Reflection cracking viii) Formation of waves and corrugation

i) Alligator (map) cracking:

This is the most common type of failure and occurs due to relative

movement of pavement layer materials.

- This may be caused by the repeated application of heavy wheel loads

resulting in fatigue failure.

- This may also be due to moisture variations resulting swelling and shrinkage

of subgrade.

ii) Consolidation or pavement layer:

Formation of ruts is mainly due to the consolidation of one or more layers

of pavement.

- The repeated application of loads along same wheel path cause cumulative

deformation resulting in consolidation deformation or longitudinal ruts.

iii) Shear failure and cracking:

Shear failures are associated with the inherent weakness of the pavement

mixture, the shearing resistance being low due to inadequate stability or

excessive heavy loading.

iv) Longitudinal cracking

Longitudinal cracking is caused in pavement traversing through the full

pavement thickness due to frost action and differential volume changes in

subgrade.

- Settlement of fill & sliding of side slopes also would cause this type of failure.



47

v) Frost heaving:

In the case of frost heaving, there is mostly a localised heaving-up

pavement portion depending upon the ground water and climatic condition.

vi) Lack of binding (keying) to the lower course:

Slipping occurs when the surface course is not keyed / bound with the

underlying base.

- Such conditions are occurred in case when bituminous surfacing is provided

over the existing cement concrete base course of soil cement base course.

- It may result in formation of patches or pot holes.

vii) Reflection cracking:

This type of cracking is observed in bituminous overlays provided over

existing cement concrete pavements.

- The cracks reflected on bituminous surfacing and allow surface water to seep

through cracks and cause damage to the soil subgrade.

viii) Formation of waves and corrugation:

Such type of failure is also common failure and occurs due to poor

subgrade condition, defective rolling, improper gradation or mix etc.

B. Failures in Rigid Pavement

The failures of rigid pavement are mainly due to following two factors:

i) Deficiency of pavement materials:

a) Soft aggregates

b) Poor workmanship in joint construction

c) Poor joint filler and sealer material

d) Poor surface finish

e) Improper and insufficient curing

ii) Structural inadequacy of pavement system:

a) Inadequate pavement thickness

b) Inadequate subgrade support and poor subgrade soil

c) Incorrect spacing of joints

Typical rigid pavement failures:

i) Scaling of cement concrete ii) Shrinkage cracks iii) Spalling of joints

iv) Warping cracks v) Mud pumping vi) Structural cracks



48

i) Scaling of cement concrete:

The scaling is mainly attributed due to the deficiency in the mix or

presence of some chemical impurities which damage the mix.

ii) Shrinkage cracks:

During the curing operation of cement concrete pavements immediately

after the construction, the shrinkage cracks normally develop.

- The placements of cracks are in longitudinal as well as in transverse direction.

iii) Spalling of joints:

Sometimes when pre-formed filler materials are placed during casting of

pavement slabs, the placement is somehow dislocated and filler is thus

placed at an angle.

- This forms an overhang of a concrete layer on the top side.

- Due to this defect, joints show excessive cracking and subsidence on later.

iv) Warping cracks:

If the joints are not well designed to accommodate the warping of slabs at

edges, this result in development of excessive stresses due to warping and

the slab develops cracking at the edges in an irregular pattern.

v) Mud pumping:

Mud pumping is recognised when the soil slurry ejects out through the

joints and cracks of cement concrete pavement caused during the downward

movement of slab under the heavy wheel loads.

- Following are the factors which cause mud pumping:

i) Extent of slab deflection

ii) Type of subgrade soil

iii) Amount of free water

vi) Structural cracks:

Inadequate pavement thickness for the amount and type of vehicles is the

prime reason for the structural cracking.

- Largely the pavements are found to crack at the corners and edges.

Maintenance of Roads

Following maintenance techniques are used for different types of roads:

1. Maintenance of Earthen road

The damages in earthen roads which may need frequent maintenance are:

i) Formation of dust in dry weather.

ii) Formation of longitudinal ruts along wheel path of vehicles.

iii) Formation of cross ruts along the surface after monsoon due to

surface water.



49

- The dust nuisance may be remedied by frequent sprinkling of water, treatment

with calcium chloride etc.

- Formation of cross ruts may be due to excessive cross slope, which may not

be avoid in untreated earth road in heavy rainfall areas. Such ruts should be

repaired time to time during and after the monsoon or any type of surface

treatment should be provided on the top.

2. Maintenance of WBM roads

WBM roads are damaged rapidly due to the heavy mixed traffic & adverse

climatic conditions. In dry weather dust is formed & during rains mud is

formed.

- Due to the combined effects of traffic and the rain water, stone aggregates are

loosened and pot holes and ruts are also formed.

Following types of maintenance are carried out:

) To prevent the aggregate from getting loosened from surface course, it is

necessary to replace the soil binder periodically.

) Dust nuisance can be effectively prevented by providing bituminous

surface dressing course over WBM course.

) Pot holes and ruts formed should be patched up. The patch repair work is

carried out by first cutting out rectangular shape of defective area to

remove the defective materials to the affected depth. Then the same size

of fresh aggregates is filled up and compacted well by ramming such that

patch area is about 1cm above the general pavement surface.

3. Maintenance of Bituminous surface

The maintenance works of bituminous surfacing mainly consists of:

a. Patch repair b. Surface treatment c. Resurfacing

i) Patch repair:

Patch repairs are carried out on the damage or improper road surface by

cutting the pot holes to rectangular shape and the affected material in the

section is removed until the sound materials are encountered.

- A premixed material (similar new materials to replacing materials) is then

placed in the section. Generally, cut back or emulsion is used as binder.

- The material so placed in the pot hole, is well compacted by ramming having

thickness slightly above the general road surface level for future compaction

under traffic load.

ii) Surface treatment:

Excess of bitumen in the surface materials bleeds and the pavement

becomes patchy and slippery. Corrugations or rutting or shoving develop in

such pavement surfaces.

- It is customary to spread blotting materials such as aggregates chips of

maximum size about 10mm or coarse sand during summer.

- Necessary rolling is done to develop permanent bond between the existing

surface and the new materials.



50

iii) Resurfacing:

In case the pavement is of inadequate thickness due to increase in traffic

load and strengthening is necessary, then an overlay of adequate thickness

should be designed and constructed, which is called resurfacing.

4. Maintenance of Cement concrete road

It may be noted that very little maintenance such as maintenance of joints

only is needed for cement concrete roads, if they are well designed and

constructed. Main defect in this type of road is formation of cracks.

) Treatment of Cracks:

The cracks developed in cement concrete (cc) may be classified

into two groups:

a) Temperature Cracks:

These are initially fine cracks or hair cracks formed across the slab, in

between a pair of transverse or longitudinal joints, dividing the slab length into

two or more proximately equal parts due to temperature stress like shrinkage

stress, warping stress etc.

- Before these cracks get wide enough to permit infiltration of water, they

should be sealed off to prevent rapid deteriorations.

- The dirt, sand and other loose particles at the cracks are thoroughly cleaned

using a sharp tool, stiff brush and pressure blower.

- Kerosene oil is applied on cleaned cracks and the cracks are then filled by

suitable grade bituminous sealing compound, heated to liquid consistency.

b) Structural Cracks:

These cracks are formed near the edge and corner regions of the slab,

due to combined wheel load and warping stresses in the slab.

- The maintenance work in such cracks involves first remedy of the basic cause

of the failure and then re-casting the failed slab.

) Maintenance of joints:

During summer the joint sealer material is squeezed out of the

expansion joints due to the expansion of slab; subsequently as the slabs

contract during winter, the joint gap opens and cracks are formed in the old

sealer material.

- Hence, periodic maintenance of the joint sealer is essential both at expansion

& contraction joints as a part of routine maintenance work of the cc pavement.

- The opened-up joints are cleaned with brush and refilled with suitable joint

sealer material before the start of rainy seasons.

Formation of Wave & Corrugation and Its Remedies:

Following are the main causes of formation of wave and corrugation:

i) Defective rolling ii) Poor subgrade condition

iii) Poor gradation or mix iv) Compaction temp.

v) Unstable underlying layer



51

) Defective rolling: If the rolling during construction stage is improper thus

leaving the formation of waves then the process being progressive, the

wave formation would continue indefinitely.

) Poor subgrade condition: Subgrade consisting of poor soils including

highly plastic or organic soils and high water table close to subgrade surface

may cause non-uniform and inadequate subgrade stability. All these would

contribute to formation of corrugated pavement surface.

) Poor gradation or mix: Defective gradation or mix for the surface layer

is another factor which gives rise to the wave formation.

) Compaction temperature: Very high temperature during mixing and

compaction (rolling) of bituminous mix would make the resulting pavement

surface layers with low stability and wavy surface is formed during rolling.

) Unstable underlying layers: Weak underlying layers also cause the

formation of waves due to repeated plying of vehicles on such roads.

Remedial Measures:

There are no ways to improve the road surface once the waves and

corrugations are already formed. Usually another layer is laid after laying a

levelling course.

Following are some remedial measures taken for this problem:

If the instability of underlying layer is due to excessive moisture conditions,

suitable subsurface drainage system is constructed to remedy the defect

permanently.

If the failure is due to improper compaction of lower layers, this would need

complete reconstruction.

If the failure is due to subgrade soil which may be highly plastic expansive

clay, the solution may be by subgrade treatment using a modifying agent for

stabilization.

Pavement Evaluation

Pavement evaluation involves a thorough study of various factors such as

subgrade support, pavement composition and its thickness, traffic loading and

environmental conditions.

Objectives:

(i) To assess the requirements of pavement so that the maintenance and

strengthening job could be planned in time.

(ii) To investigate structural adequacy of pavement.

(iii) To provide safe and comfortable traffic operations.



52

Approaches and Methods:

Following are the various approaches & methods of pavement

evaluation:

) Structural evaluation of pavement:

The structural evaluation of both flexible and rigid pavement may be

carried out by plate bearing test.

- The structural capacity of the pavement may be assessed by the load

carried at a specified deflection of the plate or by the amount of defection at

a specified load on the plate.

- Field investigations and tests carried out in various countries have shown

that the performance of a flexible pavement is closely related to be elastic

deflection under loads or its rebound deflection.

- Among various equipment used for this purpose, Benkelman Beam is most

commonly used, as the measurements are easy and simple.

) Evaluation of pavement surface condition:

The surface condition of flexible pavement may be evaluated by the

unevenness, ruts, patches and cracks.

- The surface condition of rigid pavements may be assessed by the cracks

developed and by faulty joints affecting the riding quality of the pavement.

- The pavement unevenness may be measured using unevenness indicator,

profilograph, profilometer or roughometer and is expressed as unevenness

index (cm / km).

- In AASHO road test, profilometer was used to record the variable slope

angle of the surface formed by two probe wheels spaced 13.5 cm apart.

- The present serviceability rating (PSR) is correlated with the physical

measurements such as, longitudinal and transverse profile of the pavement,

degree of cracking and patching etc.

Strengthening of Existing pavements

For the successful maintenance of pavements it is essential that they have

adequate stability to withstand the design traffic under prevailing climatic and

subgrade conditions.

- If the pavements have to support increased wheel loads and load repetitions,

the pavements rapidly undergo the distress and no amount of routine and

periodic maintenance can help them.

- So in such conditions, the strengthening of existing pavement may be done by

providing additional thickness of the pavement of adequate thickness in one

or more layers over the existing pavement, which is called overlay.

- If the existing pavements have completely deteriorated, an overlay would not

serve the purpose, the solution would be to remove existing pavement and

then rebuild the same.



53

Overlay and its types:

The strengthening of existing pavement by providing additional

adequate thickness in one or more layers over the existing pavement; is

called overlay.

It may be of following types:

i) Flexible overlay over flexible pavements

ii) Flexible overlay over cement concrete or rigid pavements

iii) Cement concrete or rigid overlay over rigid pavements

i) Flexible overlay over flexible pavements:

The overlay thickness required over a flexible pavement may be

determined by one of the following method:

a) Conventional pavement design method

b) Non-destructive testing method (like Benkelman deflection method)

The overlay thickness required as per conventional method is given by:

ho = hd + he

Where, ho = overlay thickness required (cm)

hd = total design thickness required (cm)

he = total thickness of existing pavement (cm)

ii) Flexible overlay over cement concrete or rigid pavements:

A flexible or bituminous overlay when provided over a rigid pavement, the

wheel load is distributed through a larger area by the overlay, thus slightly

reducing the wheel load stress on the old rigid pavement.

- Further the maximum temperature differential in the rigid pavement is also

decreased due to the bituminous overlay, thus causing a substantial

reduction in the warping stress and also in the maximum combined stress.

Its thickness may be calculated as: hf = 2.5 (F × hd – he)

Where, hf = flexible overlay thickness

F = factor depending upon modulus of existing pavement

he = existing rigid pavement thickness

hd = design thickness of rigid pavement

iii) Cement concrete or rigid overlay over rigid pavements:

When a rigid or CC overlay is constructed over an existing rigid or cc

pavement, the interface between the old and new concrete can’t have

perfect bond such that the two slabs could act as a monolithic one.

Its thickness may be calculated as: (

)

Where, ho = rigid overlay thickness

hd = design thickness

he = existing pavement thickness

Values of a, b, x and n depends upon the pavement and method of overlay

construction.



54

Design of Overlay

It may be designed by one of the following methods:

i) Conventional design method

ii) Non-destructive testing method (like Benkelman beam deflection method)

(i) Conventional Design method:

The total pavement thickness requirement is designed for the design traffic

and the existing conditions of subgrade.

- The CBR method of pavement design as recommended by the IRC is

adopted for finding the total design thickness of the flexible pavement for the

design traffic volume.

- The existing thickness of the pavement is found from test pits dug along the

wheel path on the pavement.

The overlay thickness required is given by: ho = hd – he

(ii) Overlay design by Benkelman Beam deflection method:

Benkelman Beam is a device which can be conveniently used to measure

the rebound deflection of a pavement due to dual wheel load assembly or

the design wheel load.

Principle:

A well compacted pavement section or one which has been well

conditioned by traffic deforms elastically under each wheel load application

such that when the load moves away, there is an elastic recovery or

rebound defection of the deformed pavement surface. This is the basic

principle of deflection method.

- The amount of pavement deflection is a measure of the structural stability of

the pavement system.

- Larger rebound deflection indicates weaker pavement structure which may

require earlier strengthening or higher overlay thickness.

Procedure:

The stretch of road length to be evaluated is first surveyed to assess the

general condition of the pavement w.r.t. the ruts, cracks and undulations.

- The stretches are classified and grouped into different classes such as

good, fair & poor and length of each stretch should not be less than 500m.

- The loading points on the pavement are located along wheel path on a line

0.6 – 0.9 m from the pavement edge, according to width of pavement.

- A minimum of 10 deflection observations (preferably 20 nos.) may be taken

on each of the selected stretch.

overlay = ho

existing = he

hd

Fig: Overlay Design



55

- After marking the deflection observation points, following procedure is done:

i) The truck is driven slowly parallel to the edge and stopped such that

the left side rear wheel is centrally placed over the first point for

deflection measurement.

ii) The probe end of the Benkelman beam is inserted between the gaps of

the dual wheel and is placed exactly over the observation point.

iii) The initial dial gauge reading Do is noted, when the rate of change of

pavement deflection is < 0.025 mm / minute.

iv) Similarly, intermediate gauge reading Di and final gauge reading Df is

taken at a distance of 2.7m from the point and a further distance of

9.0m respectively.

v) The three deflection dial reading Do, Di and Df form a set of readings at

one deflection point. Similarly for next deflection point, above process

is repeated.

vi) The temperatures of pavement surface are recorded at interval of 1 hr.

and also the tyre pressure is checked.

The rebound deflection value D at any point is given by,

D = 2 (Do – Df) ; if Di – Df ≤ 2.5

And, D = 2 (Do - Df) + 2k (Di – Df) ; if Di – Df > 2.5

Where,

d = dist. between bearing of the beam & rear adjusting leg

e = dist. between dial gauge & rear adjusting leg

f = dist. between front and rear leg.

Overlay thickness design:

Overlay thickness design (ho) can be determined as,

1) For bituminous overlay:

(cm)

Where,

R = deflection reduction factor (= 10 ~ 15)

Da = allowable deflection (= 0.75 ~ 1.25 mm)

Dc = + σ = characteristic deflection

=

mm (= mean deflection value)

σ = √ ( )

(= standard deviation)

2) For granular or WBM overlay:

(mm)



56

Da is given by;

Value of Da Projected design traffic (A)

1.0 mm 1.25 mm 1.5 mm

1500 – 4500 450 – 1500 150 – 450

Where,

Design traffic, A = P [ 1 + r ] (n + 10)

Numerical

Benkelman Beam deflection studies were carried out on 15 selected

points on a stretch of flexible pavement during summer season using

a dual wheel load of 4085 kg, 5.6 kg/cm2 pressure. The deflection

values obtained in mm after making the necessary lag corrections are

given below. If the present traffic consists of 750 commercial vehicles

/day, determine the thickness of bituminous overlay required, if the

pavement temperature during the test was 390C and the correction

factor for subsequent increase in subgrade moisture content is 1.3.

Assume annual rate of growth of traffic as 7.5%. Adopt IRC guidance.

1.4, 1.32, 1.25, 1.35, 1.48, 1.60, 1.65, 1.55, 1.45, 1.40, 1.36, 1.46,

1.50, 1.52, 1.45

Solution:

√ ( )

Deflection after temperature correction = 1.557 – (39 – 35) × 0.0065

= 1.531 mm

Corrected deflection for subgrade moisture = 1.531 * 1.3 = 1.99 mm

Assume, no. of yrs. (n) = 2

A = P [1 + r] (n+10) = 750 [1 + 0.075] (2 + 10) = 1786 com. vehicles / day

So, allowable deflection (Da) = 1.0 mm ( A is between 1500 – 4500)

Overlay thickness of granular material,

Assume an equivalency factor of 2.0 for bituminous overlay,

Thickness of bituminous overlay = 16.5 × ½

= 8.25 cm #



57

4. TRAFFIC ENGINEERING

Intro

Traffic engineering is that branch of engineering which deals with the

improvement of traffic performance of road networks and terminals.

- Traffic engineering deals with the application of scientific principles, tools,

techniques and findings for safe, rapid, convenient and economic movement

of people and goods.

Scope of Traffic Engineering

The basic object of traffic engineering is to achieve efficient, free and rapid

flow of traffic, with least number of traffic accidents.

Following are the main scope of traffic engineering:

a) Traffic characteristics b) Traffic studies and analysis c) Traffic operation, control & regulation

d) Planning and analysis e) Geometric design f) Administration and management

- Study of traffic characteristics is the most essential prerequisite for any

improvement of traffic facilities.

- The various studies to be carried out on the actual traffic include speed,

volume, capacity, travel pattern, origin, destination, parking, accident studies.

- Installation of traffic control devices like signs, signals and islands are most

common means to regulate and control the traffic.

- The planning is separate phase for major highways like express-ways, arterial

roads and parking facilities.

- Improvement of road geometries like horizontal and vertical alignment, sight

distance, cross section, etc. all fall under the scope of traffic engineering.

Impact of Human and Vehicular characteristics on traffic engineering:

1) Human Characteristics:

The human element is involved in all actions of the road users either as

pedestrian, cyclist, car driver or motorist.

Following are the various factors which affect human (road user)

characteristics:

i) Physical Characteristics:

The physical characteristics of road users may be either permanent

or temporary.

- Permanent characteristics are the vision, hearing, strength and the

general reaction to traffic situations.

- Temporary characteristics are fatigue, alcohol or drugs and illness.

- All these reduce alertness and increase the reaction time and also affect

the quality of judgement.



58

ii) Mental Characteristics:

Knowledge, skill, intelligence, experience and literacy can affect the

road user characteristics.

- Knowledge of vehicle characteristics, traffic behaviour, driving practice,

rules of roads and traffic regulation, psychology of road users will be

quite useful for safe traffic operation.

iii) Psychological Factors:

The emotional factors such as attentiveness, fear, anger,

superstition impatience, general attitude towards traffic regulation and

maturity also come under this.

iv) Environmental Characteristics:

The various environmental factors such as fog, rain, heavy sunlight,

heat, etc. affect the behaviour of road user.

- The environmental conditions affecting the behaviour of road user are

traffic stream characteristics, facilities to the traffic, atmospheric

conditions and the locality.

- The traffic stream may consist of mixed traffic or heavy traffic whereas

the facilities to overtake faster vehicles may be limited.

- The facilities to the traffic may be location of fuel station, parking,

maintenance workshop etc.

2) Vehicular Characteristics:

It is quite important to study the various vehicular characteristics which

affect the design & traffic performance, because it is possible to design a

road for a particular vehicle having standard dimension & weight but not for

an indefinite vehicle.

The following vehicular characteristics affecting road design may be classified as:

i) Static Characteristics:

Static characteristics of vehicles affecting road design are the

dimensions, weight an maximum turning angle.

- The height of vehicle affects the clearance of the overhead structures.

- The height of driver seat affects the visibility distance.

- The height of head light affects the head light sight dist. at valley curves.

- The length of vehicle affects the capacity, over taking distance, turning

on sharp curves etc.

ii) Dynamic Characteristics:

Dynamic characteristics of vehicles affecting road design are

speed, acceleration and braking characteristics.

- The speed and acceleration depends upon the power of the engine and

the resistance to be overcome and are important in all the geometric

design elements.

- The deceleration & braking characteristics guide safe vehicle operation.



59

Traffic operation and regulations:

In order to have safe traffic operations on roads, it is essential to impose

adequate traffic regulation and traffic control devices.

Traffic regulations:

The traffic regulations should cover all aspects of control of vehicles, driver

and all other road users. The regulations should be rational.

Traffic regulations and laws cover the following four phases:

i) Driver controls:

These include driving licences for light and heavy motor vehicles,

driver tests & minimum requirements, financial responsibility & civil liability.

ii) Vehicle controls:

These include vehicle registration, requirements of vehicles,

equipment and accessories, maximum dimensions, weight, fitness and

inspection of vehicles.

iii) Flow regulations:

Regulations of traffic flow have been laid down such as directions,

turning and overtaking etc. It also includes regulatory signs like one way,

speed limit, prohibition signs, pedestrian controls etc.

iv) General controls:

Some other general regulations and provisions are made to report

accidents, recording and disposing traffic violation cases.

TRAFFIC CONTROL DEVICES

The various aids and devices used to control, regulate and guide traffic

may be called traffic control devices.

- The general requirement of traffic control devices are: attention, meaning,

time for response and respect of road users.

Following are the most common traffic control devices:

(1) Traffic signs (2) Traffic signals

(3) Markings (4) Islands

(1) Traffic signs

The traffic sings are mounted on sign posts. The signs should be placed

such that they could be seen and recognised by the road users in time.

- The edge of the sign adjacent to the road is not less than 0.6m away from the

edge of the kerb.

Types of traffic signs are: (a) Regulatory or mandatory signs

(b) Warning or cautionary signs

(c) Informatory signs



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(a) Regulatory signs:

These signs are used to inform the road users of certain laws,

regulations and prohibitions.

These are further classified as:

i) Stop and give- way signs ii) Prohibitory signs iii) No parking / stopping signs

iv) Restriction end signs v) Direction control signs vi) Speed limit signs

- The stop sign is intended to stop the vehicles on a roadway and the give-

way sign is used to control the vehicles on a road so as to assign right of

way to traffic.

- Prohibitory signs are meant to prohibit certain traffic movements, use of

horns or entry of certain vehicle class.

- No parking sign is meant to prohibit parking of vehicles at that place.

- Direction control signs indicate by arrows, the appropriate directions in

which the vehicles are obliged to proceed.

- Speed limit signs are meant to restrict the speed of all or certain classes of

vehicles on a particular stretch of a road.

(b) Warning:

Warning or cautionary signs are used to warn the road users of

certain hazardous conditions that exist on or adjacent to the roadway.

- The warning signs are in the shape of equilateral triangle with its apex

pointing upwards.

- They have a white back ground, red border and black symbols.

(c) Informatory Signs:

These signs are used to guide the road users along routes, inform

them of destination and distance, provide information to make travel

easier, safe and pleasant.

These are divided into:

Direction and place identification signs Facility information signs

Parking signs



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(2) Traffic signals

Traffic signals are control devices which could alternately direct the traffic

to stop and proceed at intersections using red and green traffic light signals

automatically.

- The main requirements of traffic signal are to draw attention, provide meaning

and time to respond with the minimum waste of time.

Advantages of traffic signals:

They provide orderly movement of traffic and increase the traffic handling

capacity at intersections.

They reduce the accidents.

Pedestrians can cross the roads safely at the signalised intersections.

Automatic traffic signal may work out to be economical compound to

manual control.

They provide all the vehicles to move approximately at same reasonable

speed along the major road traffic.

Disadvantages:

The rear-end collisions may increase.

Improper design and location of signals may lead to violations of the control

system.

Failure of the signal due to electric power or any other defect may cause

confusion to the road users.

Types of Traffic signals:

The signals are classified as follows:

i) Traffic Control Signals:

The traffic control signals have three coloured

light glows facing each direction of traffic flow.

- The red light is meant for ‘stop’, the green light

indicates ‘go’ and the amber or yellow light allows

the ‘clearance time’.



62

a) Fixed-time signals: are those in which the timings of the phase &

cycle are set to repeat regularly; a cycle of red, amber & green lights.

b) Manually operated signals: are those which are operated manually.

c) Traffic actuated (automatic) signals: are those in which the timing of

the phase & cycle are changed automatically according to traffic

demand.

ii) Pedestrian Operated Signals:

These signals are meant to give right of way to pedestrians to cross

a road during walk period when vehicular traffic shall be stopped by red.

iii) Specific Traffic Signals:

(3) Road Markings

Road or traffic marking may be called special signs intended to control,

warn, guide or regulate the traffic.

- These are made of lines, patterns, words, symbols by using paints in contrast

with colour and brightness of the pavement, kerb, sides of island etc.

These may be classified as:

i) Pavement markings ii) Kerb markings iii) Object markings iv) Reflector unit markings

v) Road delineators a) Roadway indicators b) Hazard marker c) Object marker

i) Pavement Markings:

Pavement markings may generally be white paint.

- Yellow colour markings are used to indicate parking restrictions.

- Longitudinal solid lines are used as guiding or regulating lines and are not

meant to be crossed by the driver.

- Transverse solid lines indicate the position of stop lines for vehicles.

ii) Kerb Markings:

These may indicate certain regulations like parking regulations.

- Also the markings on kerb with alternate black and white line increase the

visibility from long distance.

iii) Object Markings:

Physical obstruction on or near the roadway are hazardous and hence

should be properly marked.

- These obstructions may be supports for bridge, level crossing gates,

narrow bridges, culvert head walls etc.

iv) Reflector Unit Markings:

Reflector markers are used as hazard markers (yellow light) and guide

markers for safe driving during night.



63

(4) Traffic Islands

Traffic islands are raised area constructed within the roadway to establish

physical channels through which the vehicular traffic may be guided.

Following are the types of traffic islands based on the function:

(i) Divisional islands (ii) Channelizing islands

(iii) Pedestrian loading islands (iv) Rotary islands

(i) Divisional islands:

Divisional islands are intended to separate opposing flow of traffic

on a highway with four or more lanes.

- It eliminates the head on collisions and also reduces the accidents.

(ii) Channelizing islands:

These are used to guide the traffic into

proper channel through the intersection area.

- These are very useful as traffic control

devices for intersection at grade, particularly

when the area is large.

- These are useful when the direction of flow is

to be changed.

- These can reduce the possible conflicts

between traffic streams.

(iii)Pedestrian loading islands:

These are provided at regular bus stops and similar places for the

protection of passengers.

(iv) Rotary islands:

Rotary island is the large central island of a rotary intersection and

is much larger than the central island of channelized intersection.

Numerical s

A vehicle was stopped in 1.4 sec by fully jamming the brakes and the

skid marks measured 7 m. Determine the average skid resistance.

Solution:

We have,

(i) v = u + at, v = o, u = – at

(ii) v2 – u2 = 2as

or, 0 – a2t2 = 2as



64

Given,

Braking distance, L = (s) = 7 m

Braking time, (t) = 1.4 sec.

An isolated signal with pedestrians’ indication is to be installed on a

right angled intersection with road A, 18m wide road B, 12m wide.

The heaviest volume per hour for each lane of road A & B are 275 and

225 respectively. The approach speeds are 55 and 40 kmph, for A & B

respectively. Design the timings of traffic and pedestrian signals.

Solution:

Assume, Pedestrian walking speed = 1.2 m/s

Amber time for road A, AA = 4 sec

Amber time for road B, AB = 3 sec

Initial interval to start crossing = 7 sec (= It )

Now,

Green time,

Road A, GA = (tB + It) – AA = (10 + 7) – 4 = 13 sec

Road B, GB = (tA + It) – AB = (15 + 7) – 3 = 19 sec

Now, Based on approach volume, Green time is calculated as;

Total cycle length = GA+ AA + GB + AB = 23.2 + 4 + 19 + 3 = 49.2 sec

Hence, adopt cycle length (c) = 50 sec

Adjusting the additional period of (50 – 49.2) = 0.8 sec

GA (adjust) = 0.8 ×

=

= 0.44 sec.

GB (adjust) = 0.8 – 0.44 = 0.36 sec

GA (new) = 23.2 + 0.44 = 23.64 sec #

GB (new) = 19 + 0.36 = 19.36 sec #

Red time, RA = GB + AB = 19.36 + 3 = 22.36 sec #

RB = GA + AA = 23.64 + 4 = 27.64 sec #



65

Design of pedestrian signal:

Do not walk (DW) period,

DWA = RB = 27.64 sec #

DWB = RA = 22.36 sec #

For walk (w) period,

WA = C – (RB + tA) = 50 – (27.64 + 15) = 7.36 sec #

WB = C – (RA +tB) = 50 – (22.36 + 10) = 17.64 sec #

The average normal flow of traffic on cross roads A and B during

design period are 400 and 250 PCU/hr.; the saturation flow values on

these roads are estimated as 1250 and 100 PCU/hr. respectively. The

all-red time required for pedestrian crossing is 12 sec. Design two

phase traffic signal by Webster method.

Solution:

Normal flow, qa = 400 PCU/hr.

qb = 250 PCU/hr.

Saturation flow, Sa = 1250 PCU/hr.

Sb = 1000 PCU/hr.

Ratio, ya =

=

= 0.32

yb =

=

= 0.25

Now, Y = ya + yb = 0.32 + 0.25 = 0.57

Total lost time per cycle, L = 2n + R

= 2 × 2 + 12 = 16 sec

where, n = 2 = no. of phase

R = 12 sec = red time required

Now, Optimum signal cycle, (co) =

=

= 67.4 sec ≈ 67.5 sec

Ga =

( ) =

(67.5 – 16) = 28.9 ≈ 29 sec

Gb =

( ) =

(67.5 – 16) = 22.5 sec

Here, All-red time (R) = 12 sec

Assume, Amber time (A) = 2 sec, for each road

Total cycle time = GA + GB + R + AA + AB

= 29 + 22.5 + 12 + 2 + 2 = 67.5 sec #



66



67

5. TRAFFIC STUDIES

Intro

Traffic studies or surveys are carried out to analyse the traffic

characteristics. These studies help in deciding the geometric design feature

and traffic control for safe and efficient traffic movements.

The various traffic studies carried out are:

1) Traffic volume study 2) Speed studies

i) Spot speed study ii) Speed and delay study

3) Origin and Destination (O & P) study

4) Traffic flow characteristics 5) Parking study 6) Accident study 7) Traffic capacity study

1) Traffic volume study:

Traffic volume is the number of vehicles crossing a section of road per unit

time at any selected period.

- Traffic volume is used as a quantity measure of flow.

- The commonly used units are vehicles/day and vehicles/hour.

Following are the objects and uses of traffic volume studies:

a) To measure the relative importance of roads and in deciding the priority for

improvement and expansion.

b) This study is used in planning, traffic operation and control or existing and

new facilities.

c) This study is used in the analysis of traffic pattern and trends.

d) Useful in structural design of pavement, geometric design and computing

roadway capacity.

e) Used for planning sidewalks, cross walks, signal timing, channelization etc

Counting of traffic volume:

Traffic volume counts may be done by following ways:

a) Mechanical Counters:

The mechanical counters can automatically record the total number

of vehicles crossing a section of the road in a desired period.

- Traffic count is recorded by electrically operated counters and

recorders capable of recording the impulses.

- Other methods of working the mechanical detectors are by photo-

electric cells, magnetic detectors and radar detectors.

b) Manual Counts:

This method employs a field team to record traffic volume on the

prescribed record sheets.



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- By this method, it is possible to obtain data which can’t be collected by

mechanical counters, such as vehicle classification, turning movements

and counts.

- This method is very commonly adopted due to the specific advantages

over other methods.

2) Speed studies:

The actual speed of vehicles over a particular route may fluctuate widely

depending on several factors such as geometric features, traffic conditions,

time, place, environment and driver.

Travel time is the reciprocal of speed.

Spot speed is instantaneous speed of veh. at a specified section or location.

Average speed is the average of the spot speeds of all vehicles passing a

given point on the highway.

Running speed is the average speed maintained by a vehicle over a particular

stretch of road, while the vehicle is in motion.

Overall speed or travel speed is the effective speed with which a vehicle

traverses a particular route between two terminals.

There are two types of speed studies carried out:

i) Spot speed study:

Following are the uses of spot speed study:

a) To use in planning traffic control and in traffic regulations.

b) To use in geometric design – for redesigning existing highway.

c) To use in accident studies.

d) To study the traffic capacity and decide the speed trends.

Measurement of spot speed:

(i) By finding running speed of vehicles over a short distance of < 50 m.

(ii) By finding instantaneous speed while crossing a section.

Presentation of Spot speed data

(a) Average speed of vehicles:

From the spot speed data of the selected samples, frequency

distribution tables are prepared by arranging the data in groups

covering various speed ranges & the number of vehicles in such range.

- The arithmetic mean is taken as the average speed.

(b) Cumulative speed of vehicles:

A graph is plotted with the average values of each speed group on

the x-axis and the cumulative percentage of vehicles travelled at or

below the different speeds on the y-axis.

- The drivers exceeding 85th percentile speed are usually considered to

drive faster than the safe speed under existing conditions and hence

this speed is adopted for the safe speed limit at this zone.



69

- 98th percentile speed is

taken for the purpose of

highway geometric

design.

- The 15th percentile speed

represents the lower

speed limit.

(c) Modal Average:

A frequency distribution curve of

spot speeds is plotted with speed of

vehicles or average values of each

speed group of vehicles on the x-axis

and the % of vehicles on the y-axis.

- This graph is called the speed

distribution curve.

- This curve will have a definite peak value of travel speed across the

section and this speed is denoted as modal speed.

ii) Speed and delay study:

The speed and delay studies give the running speeds, overall

speeds, fluctuations in speeds and the delay between two stations of a

road spaced far apart.

- They also give the information such as the amount, location, duration

frequency and causes of the delay in the traffic stream.

- The studies are utilised in finding the travel time and in benefit-cost

analysis. The efficiency of the roadway is judged form the travel time.

- The delay or the time lost by traffic during the travel period may be either

due to fixed delays or operational delays.

There are various methods of carrying out speed and delay study:

(a) Floating car or riding check method (b) License plate or vehicle number method (c) Interview technique

(d) Elevated observations (e) Photographic technique

3) Origin and destination (o & d) studies:

The origin and destination (o & d) study is carried out mainly to

a) plan the road network and other facilities for vehicular traffic.

b) plan the schedule of different modes of transportation for the trip demand of

commuters.



70

- The O & D studies of vehicular traffic determine their number, their origin and

destination in each zone under study.

- O & D study gives information like the actual direction of travel, selection of

routes and length of the trip.

- These studies are most essential in planning new highway facilities and in

improving some of the existing systems.

Applications of O & D studies:

To judge the adequacy of existing routes and to use in planning new

network of roads.

To plan transportation system and mass transit facilities.

To locate expressway or major routes along the desire lines.

To locate terminals and to plan terminal facilities.

To locate new bridges as per traffic demands.

To locate intermediate stops of public transport.

Methods of collecting O & D data:

Following are the methods for collecting the O & D data:

i) Road side interview method ii) Licence plate method iii) Return post cart method

iv) Tag-on-car method v) Home interview method

i) Road side interview method:

The vehicles are stopped at previously decided interview stations,

by a group of persons and the answers to prescribed questionnaire are

collected on the spot.

- The information collected is time of origin & destination, route, purpose

of trip, type of vehicle and number of passenger in each vehicle.

- This method is simple and quick, as the data is collected quickly in

short duration.

- The main drawback of the method is that the vehicles are stopped for

interview, and there is delay to vehicular movement.

ii) Licence plate method:

The entire area under study is cordoned out and the observers are

simultaneously stationed at all points of entry and exit on all the routes

leading to and out of the area.

- The observers at all points note the license plate number (registration

number) of the vehicles entering and leaving the cordoned area and

the time.

- This method is quite easy and quick as far as field work is concerned.

- The method however involves a lot of office computation in tracing the

trips through a network of stations.



71

4) Traffic flow characteristics:

Traffic stream has

flow and counter flow

along a common route,

unless the stream is

separated into pair of

one-way flows by proper

design or regulation.

- The basic traffic

manoeuvres are

diverging, merging and

crossing; as shown in fig:

Diverging:

Diverging is the process of leaving the lane by the vehicle and gaining

either left or right of the existing lane.

- Diverging on left is easiest movement causing least problem of traffic conflicts.

Merging:

Merging is the process of mixing the vehicle form left or right of main lane.

Crossing:

Crossing is the process of passing a lane perpendicular to other lane.

Weaving:

Waving is the combination of diverging and merging.

- When a vehicle moves obliquely across the path of another vehicle moving in

the same direction, at relatively and small angle of crossing, the action is

termed as weaving.

5) Parking study:

Parking is one of the most important places in transportation system,

demanded by automobile users especially in big cities.

- In industrial, commercial and residential places with multi-storeyed building;

parking demand is particularly high.

Following are the main aspects which are studied during parking studies:

(i) Parking demand (ii) Parking characteristics (iii) Parking space inventory

(i) Parking demand:

The parking demand may be evaluated by different methods. One of the

methods is by making cordon counts of the selected area and recording

accumulation of vehicles during the peak hours by subtracting the outgoing

traffic from the traffic volume entering the cordoned area.

- Also it can be done by noting the registration number of each parked vehicle

at any desired time interval (such as 30 minutes, 1 hour etc.).



72

(ii) Parking characteristics:

The study is directed to note the present parking practices prevalent in the

area and general problems in parking.

- It is also necessary to study parking pattern, interference to smooth flow or

traffic and accidents involved during parking and unparking.

(iii) Paring space inventory:

The area under study is fully surveyed and a map is prepared showing all

places of parking facilities to meet the parking demand.

- The traffic engineer has to strike a balance between capacity and parking

demand and to design proper facilities for parking.

Types of parking facilities:

(a) On-street parking:

When the parking facility is provided on kerb, it is on-street parking.

- On-street parking is further divided into:

a. Angle parking : 300, 450, 600 or 900

b. Parallel parking

- Angle parking accommodates more vehicles per unit length of kerb.

- The maximum vehicles that can be parked, is with an angle of 90o. 450

parking is considered the best one.

- The chance of accidents in angle parking is more.

- Parallel parking is generally preferred when the widths of kerb for

parking space as well as width of the street are limited.

(b) Off-street parking:

When the parking facility is provided at a separate place away from

the kerb, it is known as off-street parking.

- The main advantage of this method is that there is no undue

congestion and delay on the road as in kerb parking.

- But the main drawback is some of the owners will have to walk a

greater distance after parking the vehicles.

6) Accident study:

Accident in traffic engineering is defined as a phenomenon which may

occur with the combination of vehicular traffic, pedestrians etc.

- It is occurring due to complex flow patterns of vehicular traffic, presence of

mixed traffic and pedestrians.

- Traffic accidents may involve property damages, personal injuries or even

casualties.

Objectives:

The following are the main objectives of the accident studies:

i) To study the causes of accidents

ii) To suggest corrective treatment at potential location.



73

iii) To evaluate the existing design.

iv) To support proposed design.

v) To make computation of financial losses.

Causes of accidents:

Following are the causes of accidents:

(i) Road users

(a) Drivers

(b) Pedestrians

(c) Passengers

(ii) Vehicle defects

(iii) Road

(a) Road condition

(b) Road design

(iv) Environmental factors

(v) Other factors

Drivers: Excessive speed and rash driving, carelessness, violation of rules

and regulation, sign or signal, temporary effects due to fatigue, sleep,

alcohol etc. fall under the causes of accidents due to drivers.

Pedestrians: Violating regulations, carelessness in using the carriageway,

no knowledge about traffic rules and regulation etc.

Passengers: Alighting from or getting into moving vehicles.

Vehicle defects: Failure of brakes, steering system, lighting system,

tyre burst or any other defect in vehicles.

Road condition: Slippery or skidding road surface, pot holes, ruts and

other damaged conditions of the road surface.

Road design: Defective geometric design like inadequate sight distance,

inadequate width of shoulders, improper curve design, improper lighting and

improper traffic control devices.

Environmental factors: Unfavourable weather condition like mist, fog,

snow, dust, smoke or heavy rainfall which restrict normal visibility and

render driving unsafe.

Other factors: Stay animals on the road, incorrect sign and signals, badly

located advertisement boards or service stations, semi-naked girl etc.

7) Traffic capacity study:

Following related terms are often used in traffic capacity study:

(i) Traffic volume

(ii) Traffic density

(iii) Traffic capacity

(iv) Basic capacity

(v) Possible capacity

(vi) Practical capacity

(i) Traffic volume:

Traffic volume is the number of vehicles moving in a specified direction on

a given lane or roadway that pass a given point or cross section during

specified unit of time.

- It is expressed as vehicles/hour or vehicles/day.



74

(ii) Traffic density:

Traffic density is the number of vehicles occupying a unit length of lane of

roadway at a given instant.

- It is expressed as vehicles/km.

- Traffic volume is the product of traffic density and traffic speed.

(iii) Traffic capacity:

Traffic capacity is the ability of a roadway to accommodate traffic volume.

- It is expressed as the maximum number of vehicles in a lane or a roadway

that can pass a given point in unit time; i.e. vehicles/hr./lane.

(iv) Basic capacity:

Basic capacity is the maximum number of passenger cars that can pass a

given point on a lane or roadway during one hour under the most nearly

ideal roadway and traffic conditions.

- It is the theoretical capacity.

(v) Possible capacity:

Possible capacity is the maximum number of vehicles that can pass a

given point on a lane or roadway during one hour under prevailing roadway

and traffic condition.

(vi) Practical capacity:

Practical capacity is the maximum number of vehicle that can pass a given

point on a lane or roadway during one hour, without traffic density being so

great as to unreasonable delay, hazard or restriction to the driver’s freedom

to manoeuvre under the prevailing roadway and traffic conditions.

Determination of theoretical maximum capacity:

An estimate of theoretical maximum or basic capacity of a single lane may

be made from the relation:

Here, C = capacity of a single lane, vehicles/hour

V = speed, kmph

S = avg. c/c spacing of vehicles OR space headway, m

- The basic capacity depends upon the speed (v) and spacing (s).

- The average spacing (s) between c/c of vehicles is equal to the average

length of vehicles plus the clear spacing between the vehicles in the stream.

i.e. S = Sg + L = v t + L or, S = 0.278 V t + L or, S = 0.278 V * 0.7 + L

S = 0.2 V + L

Where, Sg = minimum space gap in m L = avg. length of vehicle v, V = avg. speed in m/s & kmph resp. t = reaction time (taken as 0.7 sec)

(i)

(ii)



75

By putting the value of ‘S’ from eqn (ii) to the eqn (i), we get the theoretical

capacity of traffic lane with homogeneous traffic flow.

It has been observed that the increase in speed of traffic stream, time

headway decreases and after reaching a minimum value at an optimum

speed, starts increasing.

- Hence, the maximum theoretical capacity of a traffic lane may be obtained, if

the minimum time headway (Ht) is taken.

i.e.

Where, C = capacity, vehicles/hour (3600 sec)

Ht = minimum time headway in second

Passenger Car Unit (PCU)

It is quite difficult to estimate the traffic volume and capacity of roadway

facilities under mixed traffic flow, unless the different vehicles classes (such

as cars, vans, buses, trucks, motorcycles, pedal cycles, bullock carts etc.) are

converted to one common standard vehicle unit.

- Hence, it is a common practice to consider the passenger car as the standard

vehicle unit to convert the other vehicle classes and this unit is called

passenger car unit (PCU).

- Thus in mixed traffic flow, the traffic volume and capacity are expressed as

PCU/hr. or PCU/lane/hr. and traffic density as PCU/km length of lane.

Factors affecting PCU values:

Vehicles characteristics such as dimensions, power, speed, acceleration and

braking characteristics.

Transverse and longitudinal gaps or clearances between moving vehicles.

Roadway characteristics such as road geometrics including gradient, curve

etc., rural or urban roads, presence of intersections etc.

Regulation and control of traffic such as speed limit, one way traffic, traffic

control devices etc.

Environmental and climatic conditions.

Factors affecting practical capacity:

Following are some important factors that affect the practical capacity:

i) Lane width: As the lane width decreases, the practical capacity of a traffic

lane also decreases.

ii) Lateral clearance: Vertical obstructions such as retaining wall or parked

vehicles near the traffic lane reduce the effective width of lane and thus

reduction in the capacity of lane. - A minimum clearance of 1.85m from the pavement edge to the obstruction

is considered desirable so that capacity is not affected adversely.



76

iii) Width of shoulder: Narrow shoulders reduce the effective width of traffic

lanes as the vehicles travel towards the centre of the pavement, resulting in

a great reduction in the capacity of lane.

iv) Commercial vehicles: Large commercial vehicles like truck and buses

occupy greater space and also heavy commercial vehicles may travel at

lower speeds especially on grades which may affect the other traffic in the

same lane, thus reducing capacity.

v) Alignment: If the alignment and geometrics are no up to the design

standards, the capacity will decrease.

vi) Presence of intersection at grade: Intersections restrict free flow of traffic

and thus adversely affect the capacity of lane.

vii) Other factors: The other factors that affect the capacity of lane are number

of traffic lane, one or two way traffic movement, vehicular and driver

characteristics.

Presentation of Traffic volume data:

i) Annual Average Daily Traffic (AADT):

AADT is the total volume of vehicle traffic of a particular highway or road

for a year divided by 365 days.

- AADT is useful for the measurement of busyness of the road.

- One of the most important uses of AADT is for determining funding for the

maintenance and improvement of highway.

ii) Average Daily Traffic (ADT):

It is the volume of traffic counted on the roadway over a given time period

(greater than 1 day but less than 1 year) divided by the number of days in

that period.

- Average daily traffic also estimates to monitor the growth in traffic on a

roadway and for funding of major improvement.

- It is also useful for analysing the rate of traffic accidents on a roadway.

- Its measurement may be seasonal, weekly or hourly as per requirement.

iii) Thirtieth (30th

) Highest Hourly Volume:

It is also known as Design Hourly Volume (DHV). It is usually the 30th

highest hourly volume for the design year, generally taken 20 yrs. from the

time of construction completion.

- The 30th highest volume is hourly volume which is generally taken for design

purposes in both view point i.e. design facilities and economic

considerations.



77

Numerical s

Spot studies were carried out at a certain stretch of a highway and the

consolidated data collected are given below:

Speed range, kmph

No. of vehicles observed

Speed range, kmph

No. of vehicles observed

0 to 10 10 to 20 20 to 30 30 to 40 40 to 50

12 18 68 89 204

50 to 60 60 to 70 70 to 80 80 to 90 90 to 100

255 119 43 33 9

Determine:

(i) Upper & lower values of speed limits for regulation of mixed traffic.

(ii) The design speed for checking geometric design elements of highway.

Solution: Frequency distribution of spot speed data:

speed range, kmph mid speed, kmph frequency frequency % cumulative freq. %

0-10 5 12

= 1.41 1.41

10-20 15 18 2.12 3.53

20-30 25 68 8.00 11.53

30-40 35 89 10.47 22.00

40-50 45 204 24.00 46.00

50-60 55 255 30.00 76.00

60-70 65 119 14.00 90.00

70-80 75 43 5.06 95.06

80-90 85 33 3.88 98.94

90-100 95 9 1.06 100.00

total 850 100

Using the values of mid speed and cumulative frequency % column,

cumulative speed distribution curve is plotted.

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

cum

ula

tive

fre

qu

ency

%

→

mid speed, kmph →

85 % 98 %

15 %



78

i) Upper speed limit for regulation = 85th percentile speed

= 60 kmph #

ii) Lower speed limit for regulation = 15th percentile speed

= 30 kmph #

iii) Speed to check design elements = 98th percentile speed

= 82 kmph #

Two vehicles A & B approaching at right angles, A from west and B

from south, collide with each other. After the collision, vehicle A skids

in a direction 500 north of west and vehicle B 600 east of north. The

initial skid distances of the vehicles A & B are 38 and 20 m

respectively before collision. The skid distances after collision are 15

and 36 m respectively. If the weight of vehicles A & B are 4.4 & 6

tonnes; calculate the original speeds of the vehicles. The average skid

resistance of the pavement is found to be 0.55.

Solution:

Speeds of vehicles just after collision,

√ √

√ √

Speeds of vehicles just before collision,

Original speed of vehicles before application of brakes are obtained,

√ √

√ √



79

6. ROAD INTERSECTIONS

Intro

General area of road or street about which two or more roads join or cross

including the roadway and roadside facilities for traffic movement within it, is

called road intersections.

Basic requirements of intersections

i) At the intersection, the area of conflict should be as small as possible.

ii) The relative speed and particularly the angle of approach of vehicle should

be small.

iii) Adequate visibility should be available for vehicles approaching intersection.

iv) Sudden change of path should be avoided.

v) Geometric features like turning radius and width of pavement should be

adequately provided.

vi) Proper signs should be provided on the road.

vii) Good lighting at night time is desirable.

viii) If the number of pedestrians and cyclist are large, separate provision should

be made for the safe passage in intersections.

Types of Intersections:

i) Intersections at grades a) Un-channelized intersections b) Channelized intersections c) Rotary intersections

ii) Grade separated intersections a) Over-pass intersections b) Under-pass intersections

1) Intersections at grades:

All road intersections which meet at about the same level allowing traffic

manoeuvres like merging, diverging, crossing and weaving are called

intersections at grade.

a) Un-channelized intersections:

The un-channelized (all paved) intersections are the lowest class of

intersection, easiest in design; but most complex in traffic operations

resulting in maximum conflict area and more number of accidents, unless

controlled by traffic signals or police.

- When no additional pavement width for turning movement is provided, it is

called plain intersection.

- When the pavement is widened at the intersection area, by a traffic lane or

more, it is known as flared intersection.



80

b) Channelized intersections:

Channelized intersection is achieved by introducing islands into the

intersectional area, thus reducing the total conflict area available in the un-

channelized intersection.

- The islands of proper shape and size are designed, which help to

channelized turning traffic, to control their speed and angle of approach

and to decrease the conflict area at the intersection.

Advantages of channelized intersections:

Vehicles can be confined to definite paths.

Speed control can be established over vehicles entering the intersection.

Points of conflicts can be separated.

Both the major and minor conflict area within the intersection can

considerably be decreased.

Angle between intersecting streams of traffic may be kept as desired in a

favourable way.

The channelizing islands provide proper place for installation of design and

other traffic control devices.

c) Rotary intersections:

A rotary intersection or traffic rotary is an enlarged road intersection

where all converging vehicles are forced to move round a large central

island in one direction (clockwise) before they can weave out of traffic flow

into their respective directions radiating from the central island.

- The main objects of providing rotary intersection are to eliminate the

necessity of stopping even for crossing streams of vehicles and to reduce

the area of conflict.



81

Elements of Rotary Intersections:

Advantages and Disadvantages of Traffic Rotary:

Advantages

Crossing manoeuvre is converted into weaving or merging and diverging

operations. Hence the journey is more consistent and confortable when

compared with any other intersection at grade.

All traffic including those turning right or going straight across the rotary

have equal opportunity as those turning left.

The variable cost of operation of automobile is less at a traffic rotary than

at a signalized intersection.

There is no necessity of traffic police or signal to control the traffic.

It is the simplest traffic controlled intersection and its maintenance cost is

almost nil.

The possible number of accidents & severity of accidents are quite low.

These are advantageous when the number of intersecting roads is

between 4 and 7.

The capacity of rotary intersection is the highest (i.e. 3000 vehicles/hour)

than other intersections.

Limitations

These require comparatively large area of land.

Total cost may be very high where space is limited & costly built up areas.

In places where there is mixed traffic and large number of cyclists and

pedestrians, the operation and control of traffic become complex.

These are unsuitable in the intersecting roads having more than 7.



82

These are also unsuitable where the angle of intersection of two roads is

too acute.

When distance between intersections is less, rotary become troublesome.

When the traffic volume is low, its construction can’t be justified.

Design of Intersections

At the intersection there are through, turning and crossing traffic and these

traffic movements may be handled in different ways depending upon the type

of intersection and its design.

Following factors are to be considered in intersection design:

(i) Relative speed

(ii) Manoeuvre areas a) Elemental manoeuvre areas b) Multiple manoeuvre areas

(i) Relative speed:

Relative speed is the vector difference in the velocities of two

vehicles in the same flow and is the sum of the speeds of approaching

vehicles from opposite direction.

- It depends on the absolute speed of intersecting vehicles and the angle

between them.

- When the angle of merging is small, the relative speed will also be low.

- As the relative speed increases, the judgement of drivers regarding

time and distance is likely to be more inaccurate and the possibility and

severity of accident will increase.

(ii) Manoeuvre areas:

Manoeuvre areas are those areas where there is a potential

collision, channels of approach and departure is influenced.

- Elemental manoeuvre areas are those formed by only two single one-

way lanes of flows when they diverge, merge or cross. These are the

simplest manoeuvres.

- Multiple manoeuvre areas are those formed by two one-lane one-way

flows are present. Traffic operations are much more complex and

hence are to be avoided in the intersection design.

- The point where the possible path of two vehicles intersect is called

conflict points and the area containing conflict points is called conflict

area, which should be minimum.

2) Grade separated intersections

Grade separated intersection design is the highest form of intersection

treatment.

- This type of intersection causes least delay and hazard to the intersections

at grade from the point of view of traffic safety and efficient operation.



83

- A highway grade separation is achieved by means of vertical level.

- Separation of intersection roads by means of a bridge thus eliminating all

crossing conflicts at the intersections.

- The grade separation may be either by an over-bridge or under-bridge.

a) Over-pass

When the major highway is taken by raising its profile above the

general ground level by embankment and an over-bridge across another

highway, it is called over-pass.

b) Under-pass

When the highway is taken by depressing its profile below the

general GL to cross another road by means of an under-bridge, it is

known as under-pass.

Advantages and Disadvantages of Grade Separation

Advantages:

It provides maximum facility to cross the traffic and avoids accidents while

crossing. There is overall increase in comfort and convenience to the motorists and

saving in travel time. There is increased safety for turning traffic, even right turn movement is made

quite easy. Grade separation is an essential part of controlled access highway. It is possible to adopt grade separation for all likely angles.

Disadvantages

It is very costly to provide complete grade separation & interchange facilities. Construction of grade separation is costly, difficult and undesirable, where

there is limited right of way like built up or urban area. In flat or plain terrain, grade separation may introduce undesirable crests and

sags in vertical alignment.

Factors to be considered in traffic rotary

Following are various design factors to be considered in traffic rotary:

i) Design speed ii) Shape of central islands iii) Radius of rotary roadway iv) Weaving angle & weaving distance v) Width of carriageway vi) Entrance and exit curves vii) Capacity of the rotary

viii) Channelizing islands ix) Camber and super elevation x) Sight distance and grade xi) Lighting xii) Traffic signs xiii) Provision for cyclists & pedestrians



84



85

7. TRAFFIC LIGHTS

Importance of Road Lighting

One of the various causes of increased accident rate during night

may be attributed to poor night visibility.

- Highway lighting is particularly more important at intersections, bridge site,

level crossings and in places where there is restriction of traffic to movements.

- On urban roads where the density of population is also high, road lighting has

other advantages, like feeling of security and protection.

- Head lights of vehicles may be sufficient for safe night driving, but road

lighting may be considered as an added facility to the road users.

- When the brightness of object is less than that of background, i.e. when object

appears darker than road surface, discernment is principally by silhouette. If

brightness of pavement is uniformly increased, discernment by silhouette is

enhanced.

- When the brightness of the object is more than that of the immediate

background, discernment is by reverse silhouette. The objects adjacent to the

roadway, projections above the pavement surface such as island or vehicles

may be seen by this process of reverse silhouette.

Factors influencing night visibility

Following factors affect the night visibility of the object:

i) Amount and distribution of light flux from the lamps

ii) Size of the object

iii) Brightness of the object

iv) Brightness of the background

v) Reflecting characteristics of the pavement surface

vi) Glare on the eyes of the driver

vii) Time available to seen an object

Design factors of Highway Lighting

Various factors to be considered in the design of road lighting are:

i.) Lamps ii.) Luminaire distribution of light iii.) Spacing of lighting units

iv.) Height & overhang of mounting v.) Lateral placement vi.) Lighting layouts

i) Lamps

The colour of the lamp, its type, size and colour depends on several

considerations in addition to distribution of light flux.

It is economical to use the largest lamp size in a luminaire which will

provide sufficient uniformity of pavement brightness.

The various lamps used are filament, fluorescent etc.



86

ii) Luminaire distribution of light

To have the best utility of the luminaire or source of light, it is

necessary to have proper distribution of light.

The distribution should be downward so that high percentage of lamp

light is utilised for illuminating the pavement and adjacent area.

The illumination is necessary for traffic sigh and objects.

iii) Spacing of light units

The spacing of lighting units is often influenced by the electrical

distribution poles, property lines, road layout and type of side features &

their illumination.

Large lamps with high mountings and wide spacing should be preferred

from economy point of view.

iv) Height and overhang of mounting

The distribution of light, shadow and the glare effect from street

lamps depend also on the mounting height.

Usual mounting heights range from 6m to 10m, higher values being

preferred where possible.

The minimum vertical clearance required for electric power lines up to

650 volts has been specified as 6m above pavement surface (as per IRC).

v) Lateral placement

Street lighting poles should not be installed close to the pavement edge.

If they are too close to the carriageway, free movement of traffic is

obstructed, decreasing the capacity of roadway.

vi) Lighting layouts

The lighting layout may be of single side, staggered (both sides) or

central.

Special care should be taken while locating the lights on curves.

Lights are installed at closer spacing on curves than on straights.

The lights are located on outside of the curves to provide better visibility.

For single lane or narrow roads, single side lighting is sufficient but for

two way lane or wider roads, staggered lighting should be installed.

Design of Highway Lighting System

For various types of luminaire distribution, the utilisation coefficient

charts are available for determination of average lux of intensity over the

roadway surface where lamp lumen, mounting height, width of pared area and

spacing of lighting poles are known.



87

Fig: Coefficient of utilisation

The following relationship is used for computation of spacing:

- The maintenance factor is taken as 80 %.

Numerical

Design a street lighting system for the following conditions:

Street width : 15 m

Mounting height : 7.5 m

Lamp size : 6000 lumen

Luminaire type : II

Calculate the spacing between lighting units to produce avg. lux = 6.

Solution:

The ratio,

From the chart 5.46 (page 256),

Coefficient of utilization = 0.44

Assume maintenance factor = 80 % = 0.8

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 1 2 3 4 5 6

Co

eff

icie

nt

of

uti

lizat

ion

Ratio,

street side

house side



88



89

8. BRIDGE AND TUNNELING

BRIDGE

A structure constructed over an obstacle to provide the passage is known

as Bridge.

- According to NRS 2027 the cross drainage structure whose span length is

more than 6m is called bridge and less than 6m is called culvert.

Characteristics of an Ideal Bridge:

i) The line of bridge should not have serious deviation from the line of approach

road.

ii) It should be in level.

iii) The width of bridge should be sufficient to cater future traffic.

iv) Bridge should carry standard loading with reasonable factor of safety.

v) Foundation should be kept on firm ground and they should be kept at

sufficient depth to avoid damage by floods.

vi) Bridge should fit into surrounding landscape.

vii) Bridge should provide passage for services like water pipe, telephones etc.

viii) Bridge surface should be similar to road surface.

ix) Bridge should be economical in terms of construction and maintenance.

Choice of Location of Bridge Site

The characteristics of location of bridge site are as follows:

i) A straight reach of the river.

ii) Steady river flow without serious whirl and cross currents.

iii) A narrow channel with firm banks.

iv) Suitable high banks above high flood level on each side.

v) Rock or other hard erodible strata close to the river bed level.

vi) Absence of sharp curves in approaches.

vii) Absence of expensive river training works.

viii) Avoidance of expensive ground water construction.

ix) Proximity to a direct alignment of the road to be connected.

x) Economical approaches which should not be very high or long or liable to

flank attacks of the river during flood.

Classification of Bridges

A. According to the span (NRS 2027):

(i) Minor Bridge (span < 20 m but total length is < 20 m)

(ii) Medium Bridge (span < 20 m but total length is > 20 m)

(iii) Major Bridge (span > 20 m)



90

B. According to loading (NRS 2027):

(i) Major Bridge (HS 20 – 44 or IRC class AA or any other equivalent loading

or class 70R additional loading)

(ii) Medium or Minor Bridge (HS 15 – 44 or IRC class A or any other

equivalent loading)

(iii) Temporary bridge (HS 15 – 44 or IRC class B or any other equivalent

loading or class 70R additional loading)

C. According to the Structure:

(i) RCC T-bridge or RCC simply supported bridge (ii) Cantilever bridge (single or double) (iii) Arch bridge (iv) Suspended or suspension bridge

(v) Stayed cable bridge (vi) Steel bridge (vii) Movable bridge

D. According to the Materials:

(i) Timber Bridge (ii) RCC Bridge (iii) Masonry Bridge

(iv) Steel Bridge (v) Floating Bridge (pantron bridge or boat bridge)

Components of a Bridge

The components of bridge are classified into two groups:

a) Substructure: portion below the bearings

b) Superstructure: portion above the bearings

Following are the different component parts of the bridge:

i. Foundation for the abutments and piers or towers.

ii. Abutment and piers or towers.

iii. Bearing for the girder, trusses, deck slab etc.

iv. Decking consisting of girders or trusses or cables and slab.

v. Handrails, parapet walls, guard stones etc.

vi. Approaches to bridge to connect the road or railway to the bridge proper.

vii. River training works like revetment for slopes for embankment at

abutment and aprons for bed necessary at upstream & downstream side.

Hydraulic Analysis of Bridge

1. Length of Clear Span:

i. For masonry arch bridge: S = 2 H

ii. For RCC bridge: S = 1.5 H

Where, S = clear span

H = height of abutment on pier including foundation

2. Linear Waterway:

i. Linear waterway is width of stream for one edge to another.

ii. From Lacey’s formula;

Linear Waterway, W = C√ , for large alluvial deposit

Where, C = 4.5 ~ 6.5



91

3. Number of Spans:

i. If w < span length (s); single span bridge.

ii. If w > span length (s); w = N S

Where, N = number of span

4. Afflux:

*

+

Where, x = afflux in meter v = velocity of normal flow

L = width of stream c = 4.5 ~ 6.5 Note:

When a bridge is constructed across a contracted stream, water on

the upstream will rise up. This rise in water level near the bridge site is

called afflux.

5. Design Discharge:

i. From empirical relationship, Q = C A⅔

ii. From area velocity method, Q = A V

TUNNEL

A tunnel is an underground structure beneath the ground, under the water

bodies or through mountains.

Purposes

) To provide passage ways for rail, roads and vehicles through mountain

and underground water bodies.

) To avoid the long routes around the mountain.

) To relief the congestion on surface road.

) To carry water for power generation.

) To provide access for water supply, waste water collector etc.

Types of Tunnels:

The tunnels can be classified on following basis:

A. According to Purpose (i) Traffic tunnels (ii) Conveyance tunnels (iii) Mining tunnels

B. According to Pressure (i) Pressure tunnels (ii) Free flowing tunnels

C. According to Lining (i) Lined tunnels (ii) Unlined tunnels

D. According to Shape (i) Circular (ii) D-shaped (iii) Semi-elliptical (iv) Horse-shoe

E. According to Supporting Arrangement (i) Tunnel supported by shot Crete (ii) Tunnel supported by RSJ section (iii) Tunnel supported by shotcret & RSJ section (iv) Tunnel supported by RCC



92

Component parts of tunnel

Following are the components of tunnel:

(1) Rib beam (2) Rock bolting (3) Shotcrete (4) Lining

Tunnel cross-sections

Following cross sections are generally used:

i) Circular section ii) D-shaped section iii) Semi-elliptical section

iv) Horse shoe section v) Rectangular or square section

i) Circular section:

The circular section is most suitable for structural considerations.

However, it is difficult for excavation, particularly where the cross section

area is small.

- In a case where the tunnel is subjected to high internal pressure but does

not have good quality or rock, circular section is considered the best.

ii) D-shaped section:

This section is suitable for tunnels located in good quality in fact

sedimentary rocks & massive external igneous, hard, compacted

metamorphic rocks where external pressure due to rock & water are not

very large.

iii) Semi-elliptical section:

This section is more stable. As the shape of the arch nearly coincides with

the line of pressure, the arch section can be made relatively thinner keeping

stress within allowable limits.

iv) Horse shoe section:

These sections are compromise between circular and D-shaped section.

These sections are structurally strong to withstand external rock and water

pressures.

Survey of Tunnel Alignment

While selecting the alignment following points should be considered:

i) Shortest ii) Straight

iii) Easily available iv) Careful selection of Entry & Exit location



93

However, it is not always possible to follow a straight alignment because of

the following parameters affecting the design of tunnels:

(i) Topography

(ii) Geological section along the alignment

(iii) Ground and/or rock water loads along the alignment

(iv) Rock mechanics properties

(v) Creep or tectonic movement along the tunnel

(vi) Other parameters:

a) Rock temperature

b) Presence of methane gas

c) Geometric design

Tunnel Drainage

Since the tunnel is constructed below the original ground level, therefore

tunnel have drainage problem. The problem may be due to surface as well as

subsoil water.

- Attempts made in seal off the rock by grouting with cement, chemical or

concrete linings.

The drainage arrangements for keeping off and removing off water may be

classified into three systems:

i) Pre-drainage

ii) Dewatering of tunnel or drainage during construction

iii) Permanent drainage

iv) Incandescent lamps should be fixed in the centre of the roof of the tunnel.

v) The electric circuit of the lighting should be divided into a number of

independent circuits with their isolators and fuse boxes separate.

vi) Provision of flood lights should be made at suitable interval for detailed

inspection for the particular length of spot.

Ventilation of Tunnels

The process of removing used or vitiated air by fresh air from tunnel is

called ventilation.

Object of Ventilation

Following are the main objective of ventilation:

i) To replace the used air by fresh air in tunnel.

ii) To remove harmful obnoxious gases and dust for safe working space.

iii) To supply oxygen for workers in the tunnels.

iv) To remove the excessive moisture.

v) To bring down the temperature raised by diesel engines.



94

Methods of Ventilation

Ventilation can be done by following two methods:

) Natural Method:

This method is normally applicable in short tunnels. The natural draft can

be depended to renew the air inside the tunnel.

- In straight reaches where uniform grade exists, tunnel up to 100m length

would need not any artificial ventilation.

) Artificial Method:

In this method, artificial measures are done to control moisture, dust and

temperature in the tunnel. It is also known as mechanical ventilations.

Following are the three artificial method of ventilation:

a. Blowing in fresh air

b. Exhausting foul air by ducts

c. Combination of blowing in and exhaust system

Lining of Tunnels

Tunnel lining is a part of support design. It can take form of shotcrete

lining, concrete lining or steel lining.

Following are some of the methods used for tunnel lining:

i) Unsupported rock ii) Rock reinforcement lining iii) Shotcrete lining iv) Steel ribs

v) Segmental system vi) Monolithic concrete vii) Precast pipe segments

Objectives:

i) To reduce the losses in system.

ii) To protect steel ribs from deterioration.

iii) To protect the turbine form loose rock particle falling into the water.

iv) To take part of internal pressure induced by water.

Method of Tunnelling:

The choice of a particular method depends upon the nature of the strata

and the geometry of the tunnel section. It may broadly be divided in following categories:

1. Tunnelling in firm ground

i) Traditional methods

a) Full face method

b) Top beading and benching method

c) Drift method

ii) Excavation by tunnelling method

2. Tunnelling in soft ground

3. Tunnelling in rock



95

1. Tunnelling in firm ground:

The methods to be adopted depend upon the shape, size of available

equipment. The methods adopted fall under the following categories:

i) Traditional methods: (Drilling and Blasting)

Depending upon the type of strata, size of tunnel and method

adopted; excavation is supported by temporary wooden supports or

permanent steel supports. Different methods under this category are:

a) Full face method:

This method is suitable for comparatively firm soils

where the excavated portion can hold itself for sufficient time

to permit mucking and supporting operations to be completed.

- The method is recommended for tunnel or small size.

- Excavation is done in three section i.e. top, middle & bottom.

b) Top beading and benching method:

The beading is excavated and

supported to the full length or part

length of the tunnel before benching

is started.

- The heading is always ahead of the

benching by a convenient length and

may be formed by excavation the full

width of the tunnel above the springing line.

c) Drift method:

In the case of large tunnels a pilot tunnel or

drift is made in the side or at the centre of the

tunnel.

- The drift is then widened by drilling holes on its

face as shown in figure.

ii) Excavation by tunnelling method:

This method is suitable for large tunnel in soft soil.

- In this method excavation is done and support is also arranged

simultaneously.

2. Tunnelling in soft ground:

In case of soft soil requiring instantaneous support; drilling and blasting is

not done.

- In this method the board driven ahead to support the ground ahead of the

last rib are known as spiles.

- The fore poles act as cantilevers beyond breasting and carry the weight of

the ground till the steel rib supports their forward ends.



96

- The soil should be excavated out after removing the breast boards and the

new rib is created in position.

3. Tunnelling in rock:

Tunnels are driven in rock by repeating in sequence the operation of

drilling hole in the rock face, loading the holes, with explosive, blasting,

removing and disposing off the broken rock.

The following are commonly adopted method of tunnelling in rock:

i) Full face method

ii) Top heading and benching method

iii) Drift method

Hydraulic Analysis of River:

The hydraulic analysis is required for new locations, proposed facility

replacements and widening of existing facilities. So, flood frequency for

design and checks must be considered for new location, replacement or

modification of facility.

- The intent of design flood is to establish conditions under which the highway

facility will provide uninterrupted service with minimal damage to the highway.

The design flood must not overtop the highway.

- A check flood must be applied on proposed highway or stream crossing

facilities to determine whether a proposed crossing will cause significant

damage to the highway or to any other property.

- Analysis should include a comparison of existing conditions for interim and

estimated future watershed characteristics.

- Occasionally, flood control system may have been constructed that

significantly reduce runoff rates at the highway site.

- Hydraulic analysis is required not only for the selection of proper stable and

durable site conditions to implement the proposed facilities (i.e. highway,

bridge etc.) replacement and widening of existing facilities but also for the

replacement or relocation of any kind, bridge superstructure replacement if

the hydraulic opening of the bridge is changed in anyway, channel

modifications including the placement of bank stabilization material, scouring

action of bridge foundation by high current of flood, fill placed in flood plain,

excavation in flood plain, overtopping of bridge due to HFL etc.

- Occasionally abridge or culvert will be inundated by backwater from a

downstream river. Hence, in this case hydraulic analysis is conducted to

control tail water.

- Hydraulic analysis is required to provide channel restoration plan which help

the avoidance of barriers for fish/aquatic movement, maintain or improve

water quality, recreation, aesthetics and flow capacity.



97

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