cross slopes

8/4/2019 Cross Slopes

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Cross Slopes And Pavement Peak

It is important to enable surface water to drain from the traffic lanes and shoulders asquickly as possible. Accumulations of water cause hazards by reducing surface friction and vehiclestability. Sufficient cross slope is needed for adequate drainage, but too much slope adversely affectsvehicle operating characteristics.

The type of surface greatly influences surface drainage characteristics. Dense, smooth surfaces (PCCor densely grading asphalt concretes) require less slope for adequate cross drainage than is requiredfor a gravel or a coarse textured open graded asphalt surface. AASHTO recommends a range ofvalues for cross slope for traffic lanes. The Department has adopted the values shown below forstandard practice on tangent sections of highways.

Surface Type Cross Slope (%)

Portland Cement Concrete 2Asphalt Concrete 2Other Asphalt Surfacing 2

Gravel 3

The several different types of cross slope design are illustrated in Figure 1.1.

For two-lane highways or multilane undivided highways, the cross slope normally goes downwardtoward both edges from a crown point at the highway center line.

On high-speed facilities when 3 or more lanes are inclined in the same direction the cross slope maybe increased to 2.5% to facilitate roadway drainage.

On divided highways, the standard practice is to crown each one-way traveled laneseparately, as on two-lane highways. The other option is to have a one-direction cross slope across

the entire width of pavement, typically sloping downward to the outer edge.


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Figure 1.1 Typical Cross Slopes


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In the design of urban highways and streets adjacent property developments may dictate that the curbon one side must be higher than the curb on the other. Two options are available. The cross slopecan be in one direction for the full width of the street or the crown point can be offset from thecenterline toward the high side of the street. The latter option usually is preferable when consideringsnow melt and drainage.

For major intersections, it may be desirable to reduce the cross slope of the major highway from thenormal 2% to a 1% crown. Minimum transition distance shall be 50. Refer to Table 1.1 for situationsthat may require a greater transition distance. The transition shall end at the edge of pavement of theintersecting road. Designer will need to ensure that drainage is not compromised in the transitionarea.

DesignSpeed(mph)

Number of Lanes Rotated1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6

Transition Distance for Number of Lanes Rotated45 50 50 50 50 50 50 60 65 70 75 8050 50 50 50 50 50 55 60 70 75 80 8555 50 50 50 50 55 60 65 75 80 85 90

60 50 50 50 50 55 60 70 75 85 90 9565 50 50 50 50 60 65 70 80 85 95 10070 50 50 50 55 60 70 75 85 90 100 105

Table 1.1 Transition Distance from 2% to 1% Cross Slope at Major Intersections

The criteria for roll-over rates between adjacent roadways (i.e. interstate and ramp driving lanes)varies depending on the design speed. A desirable maximum algebraic difference at the crossoverline is 4 or 5 percent, but it may be as high as 8 percent at low speeds and where there are fewtrucks. The recommended maximum difference in cross slope rate at a crown line, related to thespeed of turning traffic is shown in Table 1.2.

ENGLISH UNITS METRIC UNITSDesign speed of exitor entrance curve(mph)

Maximum algebraicdifference in crossslope at crossover line(%)

Design speed of exitor entrance curve(km/h)

Maximum algebraicdifference in crossslope at crossover line(%)

20 and under 5.0 to 8.0 30 and under 5.0 to 8.025 to 30 5.0 to 6.0 40 to 50 5.0 to 6.035 and over 4.0 to 5.0 60 and over 4.0 to 5.0

Table 1.2 Maximum Algebraic Difference in Cross Slopeat Turning Roadway Terminals

Within a superelevated roadway section, the maximum roll-over algebraic difference in rate of crossslope, shall not exceed 8% between the traveled way and usable shoulder.


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Traffic Isolator Or Median

Medians are provided on divided multilane highways to provide a separation of opposingtraffic lanes. Besides the obvious safety benefits, medians also provide space for:

left-turn lanes

snow storage collecting surface drainage adding future lanes refuge for pedestrians at crosswalks

Median widths are always measured between the inside edges of opposing traffic lanes. There arethree basic types of medians:

flush medians curbed medians depressed medians

Flush Medians

Flush medians consist of a relatively flat paved area separating the traffic lanes with onlypaintedstripes on the pavement. This type is generally used for lower-speed urban arterials and is sometimesused as continuous two-way left-turn lanes (TWLTL).

To accommodate painted left-turn channelization, flush medians should be at least 12 feet wide.Flush medians should be either slightly crowned to avoid ponding of water in the median area orslightly depressed (with median drains) to avoid carrying all surface drainage across the driving lanes.

Curbed Medians

Curbed, raised medians are also most commonly used on lower-speed urban arterials. They have the

same basic advantages and characteristics of flush medians except the separation is more clearlydefined than for painted lines. Curbed medians may be used in areas where control of Access isdesired.

Typical widths of raised medians range from 4 to 20 feet measured from edge of opposing drivinglanes. A raised median with a 4 feet concrete island may be used on a city street, but it has limitedadvantages. Although it provides a positive separation between opposing traffic and an opportunity tocollect drainage, it offers no opportunity to channelize left turns and is too narrow to provide adesirable pedestrian refuge. When a narrow raised island must be used, it is desirable to increase thewidth between the edge of the traffic lane and the face of curb. A raised island, at least 8 feet wide,should be used if traffic signs and signals are to be located on the island.

A 20 feet median width is desirable when left-turn channelization is to be provided. This width

accommodates the left-turn lane, and an 8 feet wide island to provide space for traffic signs andsignals. When dual left-turn lanes are warranted the median width should be increased to 32 feet tomaintain the 8 feet wide island. See Figure 7-4 for examples of curbmedians.

Depressed Medians

Depressed medians are most commonly used for high-speed expressways and freeways. Normally,the widths of depressed medians are considerable greater than for either flush medians or raisedmedians. Smoother traffic operations and improved traffic safety are observed advantages of wide,depressed medians.

Designing a relative narrow depressed median creates problems. The result is that the longitudinal

drainage ditch in the center of the median is too shallow or the transverse slopes from the roadwaysto the ditch are too steep.


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Side slopes in the median should be 6:1 or flatter for a distance of at least 30 feet from the edge ofthe traffic lanes. Other median slopes (for median crossovers, ditch blocks, etc.), which might be inthe path of an out-of-control vehicle, should be 10:1 or flatter as a safety feature.

The desirable minimum width for a depressed median is 84 feet measured from centreline ofopposing directions of traffic. This distance permits adequate drainage design with flat slopes and

also permits placement of a median bridge pier with a horizontal clear distance in excess of 30 feet tothe median edge of the traffic lanes. Wider medians are desirable where the additional right-of-waycost is not prohibitive, or where there is potential need for adding lanes in the median to providecapacity for larger future traffic volumes.

Median Barriers

For divided highways with large traffic volumes and high operating speeds, a wide,depressed median is the best choice. Under some conditions this is not practicable, and a flush orraised median may be considered. With narrow medians on high operating speed facilities (i.e.interstate) consideration should be made to construct a physical barrier placed in the median toprevent out-of-control vehicles from crossing into opposing traffic lanes. Several types of physicalmedian barriers can be designed.

Median Openings

The designs of median openings and channelization for left turns are included with the discussion ofintersections.


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Figure 7-4. Typical Medians


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Curb and Gutters

Curb designs are classified as vertical or sloping. Vertical curbs are defined as those having a verticalor nearly vertical traffic face 6 inches [150 mm] or higher. Vertical curbs are intended to discouragemotorists from deliberately leaving the roadway. Sloping curbs are defined as those having a slopingtraffic face 6 inches [150 mm] or less in height. Sloping curbs can be readily traversed by a motorist

when necessary. A preferable height for sloping curbs at some locations may be 4 inches [100 mm] orless because higher curbs may drag the underside of some vehicles.

Curbs are used primarily on frontage roads, crossroads, and low-speed streets in urban areas. Theyshould not be used in connection with the through, high-speed traffic lanes or ramp areas except atthe outer edge of the shoulder where needed for drainage, in which case they should be of thesloping type.

Roadside Design

Of particular concern to the design engineer is the number of single-vehicle, run-off-the-roadaccidents which occur even on the safest facilities. About one-third of all highway fatalities are

associated with accidents of this nature. The configuration and condition of the roadside greatly affectthe extent of damages and injuries for these accidents.

Increased safety may be realized through application of the following principles, particularly on high-speed facilities:

A forgiving roadside should be provided, free of unyielding obstacles including landscaping,drainage facilities that create obstacles, steep slopes, utility poles, etc. For adequate safety, itis desirable to provide an unencumbered roadside recovery area that is as wide aspracticable for the specific highway and traffic conditions.

For existing highways, treatment of obstacles should be considered in the following order:

o Eliminate the obstacle.o Redesign the obstacle so that it can by safely traversed.o Relocate the obstacle to a point where it is less likely to be struck.o Make the obstacle breakaway.o Apply a cost-effective device to provide for redirection (longitudinal barrier) or severity

reduction (impact attenuators). Barrier should only be used if the barrier is less of anobstacle than the obstacle it would protect, or if the cost of otherwise safety treatingthe obstacle is prohibitive.

o Delineate the obstacle.o

Use of higher than minimum design standards result in a driver environment which isfundamentally safer because it is more likely to compensate for driver errors. Frequently, adesign, including sight distances greater than minimum, flattened slopes, etc., costs little

more over the life of a project and increases safety and usefulness substantially. For improved safety performance, highway geometry and traffic control devices should merely

confirm drivers' expectations. Unexpected situations, such as left side ramps on freeways,sharp horizontal curvature introduced within a series of flat curves, etc., have demonstratedadverse effects on traffic operations.

These principles have been incorporated as appropriate into the design guidelines included herein.These principles should be examined for their applicability at an individual site based on its particularcircumstances, including the aspects of social impact, environmental impact, economy, and safety.


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Embankment and Cutting Slope

General

The design and construction of road embankment and cuttings shall meet the

requirements, standards and specifications given in this Section. This Section also

covers specifications for subgrade and earthen shoulders. Where the Project Highway involvesimprovement to an existing road, efforts should be made to remove the inherent deficiencies in

plan, profile and the roadway width. It shall be ensured that the final centre line of the road and

the road levels are fixed with great care, duly considering all the relevant factors covering

structural soundness, safety and functional requirements.

Road Embankment

4.2.1 The width of the embankment shall be in accordance with the cross-sectional details

specified in Section 2 of this Manual.

4.2.2 The height of the embankment shall be based on the final road levels. The following

principles shall be kept in view while fixing the road level:

(i) For Improvement/Widening of the existing road;

(a) In case, bottom of the existing subgrade is 0.6 m above the HFL, the

existing height of embankment can be retained.

(b) In case, it is less, then the bottom of subgrade should be raised to ensure

a minimum 1 m clearance of the bottom of the subgrade from HFL. If

raising of any section (s) of the Project Highway is required, that shall

be specified in Schedule-B of the Concession Agreement.

(c) In case, Project Highway is passing through an area not affected by floods

and is free from any drainage problem/water ponding/over-topping

situations with water table being quite deep, to the extent that subgrade

is not likely to be affected by the capillary saturation, then the employer

may avoid raising in larger stretches to save cost, even though the

minimum clearance of 0.6 m from existing ground level is desirable.pc/coi35

36 IRC:SP:73-2007

MANUAL OF SPECIFICATIONS AND STANDARDS

(ii) New two lane road;

(a) Bottom of subgrade is 1.0 m above the high flood level/highest water

table/ponded water level. The HFL should be decided by intelligent

inspections, local observations, enquiries and studying the past records.

(b) Portions forming approaches to structures provide smooth vertical

profile and fulfil the minimum free board requirement.

4.2.3 Portions forming approaches to structures provide smooth vertical profile.

4.2.4 Deficiencies in the existing vertical profile of the road shall be corrected.

4.2.5 Construction of embankment, subgrade and earthen shoulders shall conform to the

requirements of Clause 305 of MOSRTH Specifications as a minimum requirement.

4.2.6Materials and Physical Requirements

4.2.6.1 Sourcing of materials for embankment and subgrade construction, as well as

compliance with environmental requirements in respect of excavation and borrow

areas under the applicable laws shall be the sole responsibility of the Concessionaire.

4.2.6.2 Borrow pits shall not be located within the right of way or along the road. Clause

305.2.2 of MOSRTH Specifications shall apply.

4.2.6.3 The materials used in embankment, subgrade and earthen shoulders shall be soil,

moorum, gravel, a mixture of these or any other material conforming to the

requirements of Clause 305.2 of MOSRTH Specifications. Pond ash can also be used

subject to requirement indicated in para 4.2.16 of this Section.4.2.6.4 The following types of materials shall be considered unsuitable for embankment


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construction and shall not be used:

(i) Materials from swamps, marshes and bogs;

(ii) Peat, log, stump and perishable material, any soil that is classified as OL, OI,

OH in accordance with IS: 1498;

(iii) Materials susceptible to spontaneous combustion;

(iv) Materials in frozen conditions;(v) Clay having Liquid Limit (LL) exceeding 70 and Plasticity Index (PI)

exceeding 45;ROAD EMBANKMENT

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(vi) Materials with salt resulting in leaching in the embankment;

(vii) Expansive clays, 'Free Swelling Index' (FSI) exceeding 50% when tested as

per IS: 2720 (Part 40).

4.2.6.5 Expansive clays/black cotton soil shall not be used for subgrade construction.

4.2.6.6 Where expansive clay with acceptable Free Swelling Index value is used as a fill

material, subgrade and top 500 mm portion of the embankment just below subgrade

shall be non-expansive in nature.

4.2.6.7 The size of the coarse material in the mixture of earth shall not exceed 75 mm whenplaced in the embankment and 50 mm when placed in the subgrade.

4.2.6.8 Only the materials satisfying the density requirements given in Table 4.1 shall be

employed for the construction of the embankment and the subgrade.

Table 4.1 : Density of Materials of Embankment and Subgrade

Maximum Dry Density with heavy

CompactionIS: 2720 (Part 8)Embankment upto 3 m height, Not less than 15.2 kN/cu. m

not subjected to extensive flooding.

Embankments exceeding 3 m height Not less than 16.0 kN/cu. m

or embankments of any height subject

to long periods of inundation.Subgrade and earthen shoulders/ Not less than 17.5 kN/cu. m

verges/backfill.

4.2.6.9 The density requirements specified in Table 4.1 shall not be applicable to light weight

material e.g. pond ash.

4.2.6.10 The material to be used, in subgrade shall satisfy the design CBR at the specified

density and moisture content.

4.2.6.11 The embankment and subgrade shall be compacted to satisfy the minimum

compaction requirements given in Table 4.2.

Type of Work38 IRC:SP:73-2007


Table 4.2 : Compaction of Embankment and SubgradeRelative Compaction as

Type of Work/Material percentage of max. laboratory

dry density - IS:2720 (Part 8)

Subgrade and earthen shoulders Not less than 97

Embankment Not less than 95

Expansive Clays (of acceptable FSI)

(a) Subgrade and 500 mm portion Not allowed

just below the subgrade.

(b) Remaining portion of Embankment Not less than 90

4.2.7 Structural Features and Design of Embankment

4.2.7.1 Embankment shall be designed to ensure the stability of the roadway and shallincorporate only those materials, which are suitable for embankment construction as


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per para 4.2.6 of this Section.

4.2.7.2 The design of side slopes shall be governed by slope stability and traffic safety

considerations.

4.2.7.3 Side slopes shall not be steeper than 2H: 1V unless soil is retained by suitable soil

retaining structures. The reinforced earth where provided, shall conform to the

requirements of Section 7 of this Manual.4.2.7.4 The type of retaining structure shall be aesthetically pleasing and compatible with

the adjoining structures.

4.2.7.5 Where the embankment is to be supported on a weak stratum, it shall be necessary

to carry out adequate soil testing and to specially design the embankment and also

adopt appropriate remedial / ground improvement measures in consultation with the

Independent Engineer.

4.2.7.6 Where the embankment is more than 3 m high and fill material consists of heavy

clay or any problematic soil, the embankment stability shall be analysed and

ascertained for safe design.

4.2.7.7 High embankments (height 6 m or above) in all soils shall be designed from stability

considerations.ROAD EMBANKMENT

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4.2.7.8 For design of high embankments IRC:75 may be referred to.

4.2.7.9 The side slopes shall be protected against erosion by providing a suitable vegetative

cover, kerbs channel, chute, stone pitching/cement concrete block pitching or any

other suitable protection measures, depending on the height of the embankment and

susceptibility of soil to erosion. Drainage arrangement shall be provided as per

Section 6 of this Manual.

4.2.7.10 Stone pitching/cement concrete block pitching shall conform to Clause 2504 of

MOSRTH Specifications.

4.2.8Embankment Construction Operations

4.2.8.1 Embankment, subgrade and earthen shoulders shall be constructed in accordancewith Clause 305.3 of MOSRTH Specifications.

4.2.8.2 Any unsuitable material occurring in the embankment foundation shall be removed

and replaced by acceptable fill material in accordance with Clause 305.3.4 of


4.2.8.3 Where construction of embankment and subgrade is required to be carried out under

the special conditions such as given in paras 4.2.7 to 4.2.13, the earthwork shall be

done in accordance with the procedure and requirements described in Clause 305.4

of MOSRTH Specifications.

4.2.9Earthwork for Widening Existing Road

Where an existing embankment/subgrade is to be widened and its slopes are steeper

than 1V: 4H, continuous horizontal benches each at least 300 mm wide shall be cutinto the old slope for each lift of earthwork, for ensuring adequate bond with the

fresh material to be added.

4.2.10Earthwork for Embankment and Subgrade to be Placed Against Sloping Ground

Requirements of Clause 305.4.2 of MOSRTH Specifications shall apply.

4.2.11Earthwork over Existing Road Surface

Requirements of Clause 305.4.3 of MOSRTH Specifications shall apply.

4.2.12Embankment and Subgrade around Structures

(i) The work shall be carried out in accordance with the requirements of Clause

305.4.4 of MOSRTH Specifications.40 IRC:SP:73-2007


(ii) Filling behind abutments, retaining walls, head walls and wing walls forstructures shall conform to the general guidelines given in Appendix 6 of


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IRC:78.

(iii) The filter medium shall conform to the requirements of Clause 2504 of


4.2.13 Construction of Embankment on Ground Incapable of Supporting Construction

Equipment

For construction of embankment on ground, which is not capable of supportingconstruction equipment such as marshy land, any of the methods given in Clause

305.4.5 of MOSRTH Specifications may be used.

4.2.14Embankment Construction Under Water

Where construction of embankment is required to be done under water, only

granular material or rock consisting of graded hard and durable particles with

maximum size not exceeding 75 mm shall be used. The material shall be nonplastic

with uniformity coefficient of not less than 10. For further details refer

IRC:34.

4.2.15Earthwork for High Embankment

Where stage construction/controlled rate of filling is required for high embankments,

the methodology together with details of necessary instrumentation and monitoring

plan shall be communicated by the Concessionaire to the Independent Engineer for

review and comments, if any.

4.2.16 Use of Pond Ash for Embankment Construction

4.2.16.1 Where Pond ash is used for embankment construction in pursuance of the instructions

of the Ministry of Environment and Forests or otherwise, the embankment shall be

designed and constructed in accordance with IRC:SP:58.

4.2.16.2 The thickness of soil cover shall not be less than 1 m for embankments up to 3m

high. For high embankments, the thickness of soil cover shall be increased as per

design.

4.2.16.3 The side slopes of the embankment shall be protected against erosion by providing

turfing or stone/cc block pitching.ROAD EMBANKMENT

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4.2.17 Surface Finish and Quality Control of Work

The surface finish and quality control of materials and works shall conform to the

requirements of Clauses 902 and 903 of MOSRTH Specifications and Para 5 of

IRC:SP:58 (relevant to Pond ash).

4.3 Roadway in Cutting4.3.1 The width of the roadway in cutting shall be in accordance with the cross section

details specified in Section 2 of this Manual.

4.3.2 The road level shall be fixed, keeping in view the following requirement:-

The difference between the bottom of subgrade and the highest water table is not less

than 1.0 m. Exceptionally, where this requirement is not satisfied, drain shall beprovided to lower down the water table.

4.3.3 Soil Investigations for Cut Sections

(i) Soil investigations shall be carried out to ascertain the type of cutting involved

and in-situ soil conditions at the subgrade level. Reference may be made to

IRC:SP:19.

(ii) The side slopes of cuttings shall be provided in accordance with the nature of

the soil encountered. The slope shall be stable for the type of strata. Where

required, benching including use of slope stability measures like pitching, breast

walls, etc. shall be adopted to make the slopes stable and safe.

(iii) In the case of rock cutting, trial pits or boreholes shall be carried out at

30-50 m intervals to assess the type of rock.

4.3.4 Excavation for roadway in soil/rock shall be carried out in accordance with Clauses

301, 302 and 303 of MOSRTH Specifications, as relevant.


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While executing excavation, adequate precautions against soil erosion, water pollution

shall be taken as per Clause 306 of MOSRTH Specifications.

Appropriate drainage measures shall be taken to keep the site free of water in

accordance with Clause 311 of MOSRTH Specifications.

No back filling shall be resorted to obtain the required slopes.

Adequate precautions shall be taken to ensure that during construction, the slopes arenot rendered unstable or give rise to recurrent slides after construction.42 IRC:SP:73-2007


In cut sections in hilly terrain, the problem of seepage flow is common. Where such

conditions exist, necessary measures including provision of deep side drains to

intercept the seepage flow and to avoid any damage to road and cut slopes shall be

provided.

4.3.5Excavation of Road Shoulders for Widening of Pavement

For widening of existing pavements or providing paved shoulders, the existing

shoulders shall be removed to their full width and to the requisite depth. Care shall

be taken to see that no portion of the existing pavement designated for retention is

loosened or disturbed.4.3.6 Preparation of Cut Formation

(i) The cut formation, which serves as a subgrade, shall be prepared as per Clause

301.6 of MOSRTH Specifications.

(ii) Where the material in the subgrade has a density less than that specified in

Table 4.1, the same shall be loosened to a depth of 500 mm and compacted in layers

in accordance with the requirements of Clause 305 of MOSRTH Specifications.

(iii) Any unsuitable material encountered in the subgrade shall be removed and

replaced with suitable material and compacted in accordance with Clause 305

of MOSRTH Specifications.

4.3.7 Finishing Operations

(i) All excavated surfaces shall be properly shaped and dressed.(ii) No point on the completed slopes shall vary from the designated slopes by more

than 150 mm (in case of soils) and 300 mm (in case of rocks) measured at

right angle to the line of slope.

(iii) The finished cut formation shall satisfy the surface tolerances specified in

Clause 902 of MOSRTH Specifications.


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cross slopes

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