cd-17-major project report.pdf

1

Project Report

On

DESIGN OF RETAINING WALL FOR A MINOR BRIDGE

A project report submitted in the partial fulfillment of

Requirement for the award of the degree of

Bachelor of Technology

In

Civil Engineering

By

SIRIPURAM ANUSHA (09241A0157)

M.BALAKOTESHWARI (09241A0160)

R.SABITHA RAJ (09241A0195)

A.SWETHA (09241A01A7)

Under the guidance of

Mrs.C.Lavanya

Assistant Professor, Department of Civil Engineering.

GOKARAJU RANGARAJU INSTITUTE OF ENGINEERING AND TECHNOLOGY

BACHUPALLY, KUKATPALLY, HYDERABAD-500090

2012-2013

2

GOKARAJU RANGARAJU INSTITUTE OF ENGINEERING AND

TECHNOLOGY, HYDERABAD

CERTIFICATE

This is to certify that the dissertation entitled “DESIGN OF RETAINING WALL FOR A

MINOR BRIDGE” is a bonafide project work done under the guidance of Mrs. C.LAVANYA ,

Assistant Professor in the Department Of Civil Engineering, GOKARAJU RANGARAJU

INSTITUTE OF ENGINEERING AND TECHNOLOGY, Hyderabad.

Project done by


M.BALAKOTESHWARI (09241A0160)



Mrs.C.Lavanya Prof.Dr.G.Venkata Ramana

Guide HOD Civil Engineering External Examiner

3

STUDENT DECLARATION

We hereby declare that the project entitled “DESIGN OF RETAINING WALL FOR A MINOR

BRIDGE” is the work done by us during the academic year 2012-13. This project report is

submitted in partial fulfillment of the requirements for the award of degree of bachelor of

technology in CIVIL ENGINEERING from JNTU, Hyderabad.


M.BALAKOTESHWARI (09241AO160)



4

ACKNOWLEDGEMENT

Salutations to our beloved and highly esteemed institute “Gokaraju Rangaraju Institute of

Engineering and Technology” for having well qualified staff and labs furnished with necessary

equipment and computers.

Firstly we would like to express our deep sense of gratitude towards Dr. G.Venkata Ramana,

Head of the Department, Civil Engineering for giving us the opportunity to do an industrial

oriented project work.

We would also like to thank our project guide, Mrs.C.Lavanya, Assistant Professor, Department

of Civil Engineering at Gokaraju Rangaraju Institute of Engineering and Technology for always

being available when we required her guidance.

Special thanks to all our team members for this team work and other friends for their support.

5

CONTENTS

PAGE NUMBERPAGE NUMBERPAGE NUMBERPAGE NUMBER

1. Introduction 1

2. Objective of the project

3

3. Site location

3.1 Bridge definition

3.2 Classification of bridges

3.3 Bridge specifications

4

4

5

6

4. Soil strata analysis

4.1 Type of soil

4.2 Properties of soil

7

8

8

5. Effect of wall movement on earth pressure

12

6. Retaining wall

6.1 Types of retaining walls

13

13

7. Tests

7.1 Standard compaction test

7.2 Direct shear test

7.3 Unconfined compressive test

17

17

25

34

8. Estimation of safe bearing capacity

8.1 Factors influencing bearing capacity

8.2 Terzaghi’s bearing capacity theory

8.3 Effect of shape of Foundation

8.4 Summary of Shape factors

8.5 Factor of safety

8.6 Calculation of safe bearing capacity

42

43

43

48

49

49

50

6

9. Design considerations for a retaining wall

9.1Wall Deformations And Earth Pressures

9.2 Backfill Material

9.3 Gravity Retaining Walls

9.4 Design

52

52

52

53

54

10. Designing of retaining wall

Existing section

Trial section 1

Trial section 2

Trial section 3

57

58

59

64

69

11. Conclusion 75

12. Bibliography

76

7

LIST OF FIGURES

Fig.3

Existing Minor Bridge

4

Fig 4.1

Moorum soil

8

Fig. 5

Effect of Wall Movement on Earth Pressure

13

Fig 6.1.1 Gravity Retaining Wall

14

Fig 6.1.2 Cantilever Retaining Wall

15

Fig. 6.1.3 Counter-fort Retaining Wall

15

Fig. 6.1.4 Piling Wall

16

Fig. 6.1.5 Anchored Retaining Wall

16

Fig. 7.1.1

Compaction Mould and Rammer

18

Fig. 7.1.2

Compacting the soil sample

20

Fig. 7.1.3

Scraping

20

Fig.7.2.1

Direct Shear Apparatus

25

Fig. 7.2.2

Components of Direct Shear Mould

26

8

Fig. 7.2.3

Components of Direct Shear Apparatus

28

Fig.7.2.4

Proving Ring

28

Fig.7.2.5

Load applied on Soil Sample

29

Fig.7.2.6

Sample conditions before and after the test

30

Fig.7.3.1

Unconfined Compression Test Apparatus

35

Fig.7.3.2

Sample Extractor

36

Fig. 7.3.3

Cylindrical Mould

37

Fig.7.3.4

Extracted soil sample

37

Fig. 7.3.5

Load applied on soil specimen

38

Fig.7.3.6

Soil Specimen after failure

39

Fig. 8.1

Main components of a structure including soil

42

Fig. 8.2.2

Terzaghi’s concept of Footing with five distinct failure

zones in foundation soil

45

9

Fig. 8.2.3

Terzaghi’s Bearing Capacity Factors for different ϕ

48

Fig.9.3.1

Gravity Retaining wall

53

Fig. 10.1

Existing section of Retaining wall

58

Fig. 10.2

Trial section1

59

Fig. 10.3

Trial Section2

64

Fig. 10.4

Trial section 3

69

Fig.10.5

Retaining wall being constructed

74

10

LIST OF TABLES

Table No.1 Standard Compaction Test Readings

22

Table No.2 Direct Shear Test Readings 31

Table No.3 Unconfined Compression Test Readings

40

Table No. 4 Bearing Capacity Factors For Different Φ

47

Table No.5 Load Details Of The Trial Section 1

60


65


70

11

LIST OF GRAPHS

Graph 1

Graph showing the variation

of moisture content and dry

density in compaction test

24

Graph 2

Graph showing variation of

normal load and shear stress in

direct shear test.

33

12

ABSTRACT

Structures which are used to hold back a soil mass are called retaining structures. Our

project is to design retaining wall for a minor bridge. As the metro rail project is running through

Miyapur, there is a need for the extension of 4 lane carriage way to 6 lane carriage way. But the

carriage way includes a minor bridge, which too has to be extended for facilitating the traffic

flow. Thus, this project focuses on the design of retaining wall for the modified section of the

minor bridge.

Retaining walls are the structures designed to restrain soil to unnatural slopes. They are

used to bound soils between two different elevations in areas of terrain possessing undesirable

slopes. They are also used in areas where the landscape needs to be shaped severely and

engineered for more specific purposes like hillside farming or roadway overpasses. They are also

used in bridge abutments and wing walls.

The design of structures like retaining wall requires the knowledge of the earth pressure

acting on the back of the wall because of the soil backfill in contact with it. Hence relation

between the earth pressure on the retaining wall and strains within a backfill is a prerequisite.

The project also includes the estimation of safe bearing capacity of soil and its properties, earth

pressure calculations and design criteria of a modified section of a retaining wall. The design

criteria includes: check for stability against sliding, overturning and bearing capacity.

13

1. INTRODUCTION

A soil mass is stable when the slope of the surface of the soil mass is flatter than the

safe slope. At some locations where the space is limited, it is not possible to provide flat slope

and the soil is to be retained at a slope steeper than the surface one. In such cases, a retaining

structure is required to provide lateral support to the soil mass. Retaining walls are relatively

rigid walls used for supporting the soil mass laterally so that the soil can be retained at different

levels on the two sides. Generally, the soil masses are vertical or nearly vertical behind the

retaining structure. Thus, a retaining wall maintains the soil at different elevations on its either

side. In the absence of a retaining wall, the soil on the higher side would have a tendency to slide

and may not remain stable. However for a minor bridge of span 15 m a retaining wall is

constructed without considering the slope factor but only the soil properties.

The project concentrates on the designing of a retaining wall located on National

Highway-9; Pune- Hyderabad via Miyapur. The road in this region is extended for two lanes,

from four lane road way to six lane road way to accommodate free traffic flow because of metro

railway construction process. The minor bridge located in this region is also to be extended.

Thus, our project is been cleared with the design of a retaining wall to this bridge on one side and

hence replicating it to remaining.

Structures which are used to hold back a soil mass are called retaining structures.

Retaining walls, sheet pile walls, crib walls, sheeting in excavations, basement walls, etc., are

examples of retaining structures. A retaining wall helps in maintaining the ground surface at

different elevations on either side of it. Without such a structure, the soil at higher elevation

would tend to move down till it acquires its natural, stable configuration. Consequently, the soil

that is retained at a slope steeper than it can sustain by virtue of its shearing strength, exerts a

force on the retaining wall. This force is called the earth pressure and the material that is retained

by the wall is referred to as backfill. The gravity retaining wall is the simplest type of retaining

wall along with other common types of retaining walls such as the cantilever, and the counter-

fort walls.

The design of structures like a retaining wall requires the knowledge of the earth

pressure acting on the back of the wall because of the soil backfill in contact with it.

14

The magnitude of earth pressure itself, on the other hand, is a function of the

magnitude and nature of the absolute and relative movements of the soil and the structure.

Problems such as these, where structure is in contact with the soil mass and the behavior of each

one is influenced by that of the other, are classed as soil-structure interaction problems. Often,

the earth pressures are statistically indeterminate and hence pose problems in evaluation. In some

cases, the desired accuracy in determination cannot be achieved. But knowledge of the

relationship between the earth pressure on the retaining wall and strains within a backfill is a

prerequisite for the solution of earth pressure problems.

The design of the retaining structure requires determination of the magnitude and line

of action of the lateral earth pressure. The magnitude of the lateral earth pressure depends upon a

number of factors, such as the mode of the movement of the wall, the flexibility of the wall, the

properties of the soil, and the drainage conditions. It is a soil-structure interaction problem, as the

earth pressure depends on the flexibility of the wall.

The design has been accomplished with few tests which we have done in the college

laboratory. The results extracted were used in the design. The site being in a hard rocky soil

strata, we got very high safe bearing capacity. Hence, two more sections apart from actual

section according to standard specifications were also evaluated for economic reasons with

increased safety factor.

The definition of minor bridge, retaining wall and their types, tests and tests results

with brief procedure were explained in this report in addition to the safe bearing capacity of the

ground, designed sections and their evaluations for safety conditions.

15

2. OBJECTIVE OF THE PROJECT

Our Project focuses on design of retaining wall for a minor bridge. The project, designing

of retaining wall for a minor bridge is the consequence of road widening due to prestigious metro

rail project. In the construction course of metro railway, a four lane road way has to be extended

to six lanes, to avoid the occurrence of traffic problems. As there is a minor bridge existing there

it should also be extended as a part of road widening.

The idea behind our project is to design a modified section of existing retaining wall for

the extension of the bridge. Before designing part, all the soil properties which include dry

density, optimum moisture content, cohesion, shear strength parameters were evaluated by

conducting corresponding tests. The safe bearing capacity of the ground is then calculated with

the test results.

Taking existing retaining wall as an example each modified section has been analysed for

stability against bearing capacity, sliding and overturning with the reduced dimensions according

to the area of the total project so as to make it economical.

16

3. SITE LOCATION

The site is located opposite to Talkie Town, Miyapur on Pune-Hyderabad section of NH-9.

The figure here shows the existing minor bridge.

cc

Fig 3 Existing Minor Bridge

3.1 BRIDGE

A bridge is a structure built to span physical obstacles such as a body of water, valley, or road,

for the purpose of providing passage over the obstacle.In other words Bridge is a structure

having a length of above 6 m between the inner faces of dirt walls for carrying traffic or other

moving loads over a depression or obstruction such as channels,raods,railways etc. There are

17

many different designs that all serve unique purposes and apply to different situations. Bridges

can be categorized in several different ways. Common categories include the type of structural

elements used, by what they carry, whether they are fixed or movable, and by the materials used

and by the span of bridge..In this project we carried out work for designing of retaining wall for a

minor bridge.

3.2 CLASSIFICATION OF BRIDGES

Classifications based on span of bridge are categorized as follows.

3.2.1 MINOR BRIDGE

A Major Bridge is a bridge having a total length of upto 60 m.This is a cross drainage structure

consisting of 4 piers and 2 abutments currently in this project.Centre to centre span between piers

is 3m.Total length of the bridge is 15m and hence called minor bridge.Retaining walls are

otherwise called Wing walls which are supported to the two abutments.Type of retaining wall

existing at this site is gravity retaining wall or gravity wing wall.

3.2.2 MAJOR BRIDGE

A Major bridge is a bridge having a total length upto 60m.These can also be a cross drainage

structures. It includes all other bridge structures including large or complex culverts. Major

bridges are typically built from site-specific drawings, but can also be built from standard girder

drawings. Typically major bridges are river crossings, highway interchanges or railway

crossings.

3.2.3 CULVERT

Culverts are cross drainage structures having a total length upto 6m or less between the inner

faces of dirt walls or extreme vent way boundaries measured at right angles thereto. A large

number of culverts are needed for taking care of the streams crossing the railway lines. A culvert

is a cutting under or beside a road which allows water to drain, rather than pooling and making

road conditions hazardous.

18

3.3 SPECIFICATIONS

• The work shall be carried out as per M.O.R.T & Hs (Ministry of Road Transport and

Highways)

• The work will be governed by design considerations & specifications contained in IRC

codes of practice for roads/bridges.

• The materials of construction shall be governed as per relevant I.S codes.

• The grading, size, quality of coarse aggregates shall be strictly as per “Specifications for

Road and Bridge works” M.O.R.T & Hs and relevant I.R.C codes.

• The size, quality of aggregates and mixing etc for plain concrete and R.C.C works shall

be as per “Specifications for Road and Bridge works” and relevant I.S codes

19

4. SOIL STRATA ANALYSIS

For any type of retaining wall design it is necessary to evaluate the engineering properties of

soil mainly the strength parameters as all retaining walls serve to hold back a vertical or near

vertical face of soil that would, without adequate retention, cave, slump or slide to a more natural

slope. Therefore the following parameters need to be assessed

4.1 Type of soil

4.2 Properties of soil

4.2.1. Safe bearing capacity

4.2.2. Dry density

4.2.3. Optimum moisture content

4.2.4. Shear Strength parameters

4.2.4.1 Cohesion

4.2.4.2 Angle of internal friction

4.2.5. Unconfined Compressive strength of soil

4.2.6. Lateral Earth Pressures

20

4.1. TYPE OF SOIL

Soil is composed of particles of broken rock (parent materials) which have been altered by

physical, chemical and biological processes that include weathering (disintegration) with

associated erosion (movement).Soil existing at our site is Moorum. Moorum means powdered

rock. It consists of small pieces of disintegrated rock or shale, with or without boulders. These

are products of decomposition and weathering of the pavement rock. Visually these are similar to

gravel except presence of higher content of fines. Site is completely a rocky area with this type

of granular soil. Generally Moorum soil possess maximum dry density and therefore good

strength.

Fig 4.1 Moorum

4.2. PROPERTIES OF SOIL

4.2.1. SAFE BEARING CAPACITY

In geotechnical engineering, bearing capacity is the capacity of soil to support the loads applied

to the ground. The bearing capacity of soil is the maximum average contact pressure between

the foundation and the soil which should not produce shear failure in the soil. Ultimate bearing

21

capacity is the theoretical maximum pressure which can be supported without failure; allowable

bearing capacity is the ultimate bearing capacity multiplied by a factor of safety. Sometimes, on

soft soil sites, large settlements may occur under loaded foundations without actual shear failure

occurring; in such cases, the allowable bearing capacity is based on the maximum allowable

settlement. Terzaghi’s Bearing Capacity theory is used in this project to calculate safe bearing

capacity of soil which can be further used to evaluate stability analysis of retaining wall.

4.2.2. DRY DENSITY

This is one of the important strength parameters need to be calculated. It can be determined from

Standard Compaction Test. The safe and economic design of retaining wall depends on the dry

density of surrounding soil. Bearing capacity of soil in turn depends on dry density of the soil. If

the soil is not having sufficient dry density the structure built on it couldn’t carry loads. To make

it strengthened soil is stabilized which will be expensive. Hence dry density of oil is the

important parameter for any structure. Dry density of soil can be achieved by Compaction.

Compaction is the process of increasing the density of the soil by packing the solid particles

closer together by reducing the volume of the air. The degree of compaction is measured in terms

of dry density, γd. The dry density after compaction depends on;

• type of soil

• water content of the soil

• amount of effort supplied by the compactive equipment

4.2.3. OPTIMUM MOISTURE CONTENT

The optimum moisture content for a specific compactive effort is the moisture content at which

the maximum density of soil is obtained. The optimum moisture content is an important control

parameter of the soil base construction. This is also an outcome of Standard Compaction Test.

Dry density and Optimum moisture content results are required to conduct further tests like

direct shear and unconfined compression test. The optimum moisture content is an important

control parameter of the soil base construction. Compaction is a process that brings about

an increase in soil density or unit weight, accompanied by a decrease in air volume. There is

usually no change in water content. The degree of compaction is measured by dry unit weight

22

and depends on the water content and compactive effort (weight of hammer, number of impacts,

weight of roller, number of passes). For a given compactive effort, the maximum dry unit weight

occurs at an optimum water content.

4.2.4. SHEAR STRENGTH PARAMETERS

The shear strength of a soil is a function of:

� The particle shape -- (Angular vs. rounded)

� The gradation of the soil (Poor vs. well graded)

� The soil density (Loose vs. dense)

� The amount and types of fines

� The pore pressure u and how the rate of loading affects it

4.2.4.1. COHESION

This is the shear strength parameter which is determined from direct shear test. Shear

strength is a term used in soil mechanics to describe the magnitude of the shear stress that a

soil can sustain. The shear resistance of soil is a result of friction and interlocking of

particles, and possibly cementation or bonding at particle contacts. Soil derives its shear

strength in two forms - Cohesion and angle of internal friction. Cohesion is the interlocking

strength between the soil particles. It is stress independent component.

4.2.4.2. ANGLE OF INTERNAL FRICTION

Internal Friction angle (φ) is the measure of the shear strength of soils due to friction. It is

stress dependent component. This is due to frictional resistance due to particles. The

mechanical strength of soil is an important concept in considering (and predicting) soil

behavior. We use strength to represent the reaction of a soil to an applied force. High-

strength soils resist deformation (compaction especially), break-up (shearing and shattering),

and slippage. However, high-strength soils also resist root penetration and exploration.

Strength is imparted to a soil by virtue of:

23

• cohesive forces between particles; and

• frictional resistance met by particles that are forced to slide over one another, or move

from interlocked positions.

4.2.5. COMPRESSIVE STRENGTH OF SOIL

The unconfined compressive strength (qu) is defined as the compressive stress at which an

unconfined cylindrical specimen of soil will fail in a simple compression test. In addition, in this

test method, the unconfined compressive strength is taken as the maximum load attained per unit

area, or the load per unit area at 15% axial strain, whichever occurs first during the performance

of a test.

4.2.6. LATERAL EARTH PRESSURES

Retaining walls shall be designed to withstand lateral earth and water pressures, the effects of

surcharge loads; the self-weight of the wall Lateral earth pressure is the pressure that soil exerts

against a structure in a sideways, mainly horizontal direction. The common applications of

lateral earth pressure theory are for the design of ground engineering structures such as retaining

walls, basements, tunnels, and to determine the friction on the sides of deep foundations. These

are of three types:

• At rest Earth pressure

• Active Earth Pressure

• Passive Earth Pressure

The above specified earth pressures are used according to the situation i.e according to the

movement of the wall and the soil pressure acting on it.

5. EFFECT OF WALL MOVEMENT ON EARTH PRESSURE

When the wall is rigid and unyielding, the soil mass is in a state of rest and there are no

deformations and displacements. The earth pressure corresponding to this state is called the earth

pressure at rest. If the wall rotates about its toe, thus moving away from the backfill, the soil

mass expands, resulting in a decrease of the earth pressure. This is a consequence of the

24

mobilization of shearing resistance in a direction opposing the movement of the earth mass.

When the wall moves away from the backfill, a portion of the backfill locates next to the

retaining wall tends to break away from the rest of the soil mass and tends to move downwards

and outwards relative to the wall. Since the shearing resistance is mobilized in directions away

from the wall, there is a resultant decrease in earth pressure which continues, until, at a certain

amount of displacement, failure will occur in the backfill and slip surfaces will be developed. At

this stage, the entire shearing resistance has been mobilized. The forces acting on the wall at this

stage does not decrease anymore beyond this point even with further wall movement. This force

is called the active earth pressure.

On the other hand, if the wall is pushed towards the backfill, the soil is compressed and

the soil offers resistance to this movement by virtue of its shearing resistance. Since the shearing

resistance builds up in directions towards the wall, the earth pressure gradually increases. If this

force reaches a value which the backfill cannot withstand, failure again ensues and slip surfaces

develop. The pressure reaches a maximum value when the entire shearing resistance has been

mobilized and does not increase any more with further wall movement. This pressure is called

the passive earth pressure.

The active earth pressure and passive earth pressure develop corresponding to two

limiting states of equilibrium. The soil mass is said to be in a state of plastic equilibrium at these

two stages. A small increase in stress at this stage will cause a continuous increase in the

corresponding strain- a condition known as plastic flow.

25

Fig. 5 Effect of Wall Movement on Earth Pressure

6. RETAINING WALL

A retaining wall is a structure that retains holds back any material usually earth and prevents it

from sliding or eroding away. It is designed so that to resist the material pressure of the material

that it is holding back.

6.1 TYPES OF RETAINING WALLS

There are four common types of retaining walls. They are

6.1.1 Gravity Retaining wall

6.1.2 Cantilever Retaining Wall

6.1.3 Counter-fort Retaining Wall

6.1.4 Piling Retaining wall

6.1.5 Anchored Retaining Wall

26

6.1.1 GRAVITY RETAINING WALL

It is that type of retaining wall that relies on their huge weight to retain the material behind it

and achieve stability against failures. Gravity Retaining Wall can be constructed from concrete,

stone or even brick masonry. Gravity retaining walls are much thicker in section. Geometry

of these walls also help them to maintain the stability. Mass concrete walls are suitable for

retained heights of up to 3 m. The cross section shape of the wall is affected by stability, the use

of space in front of the wall, the required wall appearance and the method of construction.

Fig 6.1.1 Gravity Retaining Wall

6.1.2 CANTILEVER RETAINING WALL

A cantilever retaining wall is one that consists of a wall which is connected to foundation.

A cantilever wall holds back a significant amount of soil, so it must be well engineered. They are

the most common type used as retaining walls. Cantilever wall rest on a slab foundation. This

slab foundation is also loaded by back-fill and thus the weight of the back-fill and surcharge also

stabilizes the wall against overturning and sliding.

27

Fig 6.1.2 Cantilever Retaining Wall

6.1.3 COUNTER-FORT RETAINING WALL

Counter fort walls are cantilever walls strengthened with counter forts monolithic with the back

of the wall slab and base slab. The counter-forts act as tension stiffeners and connect the wall

slab and the base to reduce the bending and shearing stresses. To reduce the bending moments in

vertical walls of great height, counter forts are used, spaced at distances from each other equal to

or slightly larger than one-half of the height Counter forts are used for high walls with heights

greater than 8 to 12 m.

Fig. 6.1.3 Counter-fort Retaining Wall

6.1.4 PILING RETAINING WALL

Sheet pile retaining walls are usually used in soft soils and tight spaces. Sheet pile walls are

made out of steel, vinyl or wood planks which are driven into the ground. For a quick estimate

the material is usually driven 1/3 above ground, 2/3 below ground. Taller sheet pile walls will

28

need a tie-back anchor placed in the soil a distance behind the face of the wall that is tied to the

wall, usually by a cable or a rod. Anchors are then placed behind the potential failure plane in the

soil.

Fig. 6.1.4 Piling Wall

6.1.5 ANCHORED RETAINING WALL

An anchored retaining wall can be constructed in any of the aforementioned styles but also

includes additional strength using cables or other stays anchored in the rock or soil behind it.

Usually driven into the material with boring, anchors are then expanded at the end of the cable,

by mechanical means. This method is very useful where high loads are expected, or where the

wall itself has to be slender.

Fig. 6.1.5 Anchored Retaining Wall

29

7. TESTS CONDUCTED TO EVALUATE SOIL PROPERTIES

The following tests have been conducted in order to evaluate the given soil properties.

7.1 Standard Compaction Test

7.2 Direct Shear Test

7.3 Unconfined Compression Test

7.1 STANDARD COMPACTION TEST

Purpose:

This laboratory test is performed to determine the relationship between the moisture content and

the dry density of a soil compacted in a mould of given size.

Significance:

Mechanical compaction is one of the most common and cost effective means of stabilizing soils.

An extremely important task of geotechnical engineers is the performance and analysis of field

control tests to assure that compacted fills are meeting the prescribed design specifications.

Design specifications usually state the required density (as a percentage of the “maximum”

density measured in a standard laboratory test), and the water content. In general, most

engineering properties, such as the strength, stiffness, resistance to shrinkage and imperviousness

of the soil, will improve by increasing the soil density.

The optimum water content is the water content that results in the greatest density

for a specified compactive effort. Compacting at water contents higher than (wet of) the

optimum water content results in a relatively dispersed soil structure (parallel particle

orientations) that is weaker, more ductile, less pervious, softer, more susceptible to shrinking,

and less susceptible to swelling than soil compacted dry of optimum to the same density. The

soil compacted lower than (dry of) the optimum water content typically results in a flocculated

soil structure (random particle orientations) that has the opposite characteristics of the soil

compacted wet of the optimum water content to the same density.

30

Equipment:

1. Proctor mould having a capacity of 944cc with an internal diameter of 100mm and

effective height of 127.3mm. The mould shall have a detachable collar assembly and a

detachable base plate.

2. Rammer

3. Sample extruder

4. Straight edge

5. Graduated cylinder

6. Mixing tools such as mixing pan, spoon, spatula, etc.

7. Moisture tins

Fig. 7.1.1 Compaction Mould and Rammer

Procedure:

1. About 3kg of air-dried, pulverized soil passing 4.75mm sieve is taken. Thoroughly mix

the sample with sufficient water to dampen it.

2. The mould is cleaned, dried and greased lightly.

3. The mass of the empty mould with the base plate, without collar, is taken.

4. The collar is then fitted to the mould.

31

5. The mould is placed on a solid base and filled with fully matured soil to about one-third

its height.

6. The soil is compacted by 25 blows of the rammer, weighing 2.6kg, with a free fall of

310mm.

7. The blows are evenly distributed over the surface. The soil surface is scratched with a

spatula before the second layer is placed.

8. The mould is fitted to about two-thirds height with the soil and compacted again by 25

blows.

9. Likewise, the third layer is placed and compacted.

10. The third layer should project above the top of the mould into the collar by not more than

6mm.

11. The collar is rotated to break the bond between the soil in the mould and that in collar.

12. The collar is then removed, and the soil is trimmed off flush with the top of the mould.

13. The mass of the mould, base plate and compacted soil is taken and thus the mass of

compacted soil is determined.

14. The bulk density of the soil is computed from the mass of the compacted soil and the

volume of the mould.

15. Representative soil samples are taken from the bottom, middle and top of the mould for

determining the water content.

16. The soil is removed from the mould.

17. More water is added to the soil so as to increase the water content 2-3%. It is thoroughly

mixed and allowed to mature.

18. The test is repeated and the dry density and water content are determined.

19. Continue this series of determination until there is either a decrease or no change in the

wet unit weight of compacted soil.

32

Fig. 7.1.2Compacting the soil sample

Fig. 7.1.3 Scraping

33

Analysis:

1. Calculate the moisture content of each compacted soil specimen.

2. Compute the wet density in grams per cm3 of the compacted soil sample by dividing the wet

mass by the volume of the mold used.

3. Compute the dry density using the wet density and the water content determined in step 1. Use

the following formula:

Where w = moisture content in percent divided by 100, and ρ = wet density

Formulae:

Wet density (gm/cc) = weight of compacted soil/944

Dry density = Wet density/ (1+w)

Where “w” is the moisture content of the soil.

Observations:

Diameter of the cylinder = 10 cm

Height of the cylinder = 12 cm

Volume of the cylinder = 944 cc

Weight of the empty cylinder = 2219 gm

γd= γ / (1+w)

34

TABLE.NO.1 : STANDARD COMPACTION TEST READINGS

Details 1 2 3

Water to be added (%)

8

10

12

Weight of water to be added (gm)

240

300

360

Weight of cylinder + compacted

soil(gm)

4151.36

4250.48

4294.85

Weight of compacted soil (gm)

1932.36

2031.48

2075.86

Container No.

6

12

14

Weight of Container + wet soil

(gm)

99

105

98

Weight of container + dry soil (gm)

94

100

92

Weight of empty container (gm)

49

52

48

Water content (%)

6.8

9.2

12.8

Wet density (gm/cc)

2.047

2.152

2.199

Dry density (gm/cc)

1.915

1.97

1.949

35

Calculations:

1. For 8% water,

Wet density = Weight of compacted soil / 944

= 1932.36/944

= 2.047 gm/cc

Dry density = Wet density / (1+w)

= 2.047 / (1+0.068)

= 1.915 gm/cc

2. For 10% water,


= 2031.48 /944

= 2.152 gm/cc


= 2.152 / (1+0.092)

= 1.97 gm/cc

3. For 12% water,


= 2075.86/944

= 2.199 gm/cc


= 2.199 / (1+0.128)

= 1.949 gm/cc

36

Graph 1:

1. Plot the dry density values on the y-axis and the moisture contents on the x-axis.

2. Draw a smooth curve connecting the plotted points.

Fig Graph showing the variation of moisture content and dry density in compaction test

Result:

From the graph,

Maximum Dry Density of the Moorum soil = 1.97 gm/cc

37

Optimum Moisture Content = 10.8 %

7.2 DIRECT SHEAR TEST

Purpose:

This test is performed to determine the consolidated-drained shear strength of the given soil. The

shear strength is one of the most important engineering properties of a soil, because it is required

whenever a structure is dependent on the soil’s shearing resistance. The shear strength is needed

for engineering situations such as determining the stability of slopes or cuts, finding the bearing

capacity for foundations, and calculating the pressure exerted by a soil on a retaining wall.

Fig.7.2.1 Direct Shear Apparatus

Significance:

38

The direct shear test is one of the oldest strength tests for soils. In this laboratory, a direct shear

device will be used to determine the shear strength of a soil (i.e. angle of internal friction (φ)).

From the plot of the shear stress versus the horizontal displacement, the maximum shear stress is

obtained for a specific vertical confining stress. After the experiment is run several times for

various vertical-confining stresses, a plot of the maxi mum shear stresses versus the vertical

(normal) confining stresses for each of the tests is produced. From the plot, a straight-line

approximation of the Mohr-Coulomb failure envelope curve can be drawn and φ is determined.

Equipment:

1. Direct shear device

2. Load and deformation dial gauges

3. Balance

4. Spatula

Fig. 7.2.2Components of Direct Shear Mould

Procedure:

1. A soil specimen of size 60x60x25 mm is taken.

2. The base plate is attached to the lower half of the box. A porous stone is placed in the

box.

39

3. The upper grid, porous stone and the pressure pad are placed on the specimen.

4. The box is placed inside the large container and mounted on the loading frame.

5. The upper half of the box is brought in contact with the proving ring.

6. The loading yoke is mounted on the steel ball placed on the pressure pad.

7. The dial gauge is fitted to the container to give the shear displacement.

8. The other dial gauge is mounted on the loading yoke to record the vertical movement.

9. The locking pins are removed and the upper half of the box is slightly raised with the

help of spacing screws.

10. The space between two halves is adjusted, depending upon the maximum particle size.

11. The normal load is applied to give a normal stress of 25kN/m2.

12. Shear load is then applied at a constant rate of strain. For undrained tests, the rate is

generally between 1.0 mm to 2.0 mm per minute.

13. The sample shears along the horizontal plane between the two halves.

14. The readings of the proving ring and the dial gauge are taken every 30 seconds.

15. The test is continued till the specimen fails. The failure is indicated when the proving ring

dial gauge begins to recede after having reached the maximum.

16. At the end of the test, the specimen is removed from the box and its water content is

found.

17. The test is repeated under the normal stress of 50,100, 200 and 400 kN/m2. The range of

the normal stress should cover the range of loading in the field problem for which the

shear parameters are required.

18. Readings are noted down carefully. Set the dial gauges zero, before starting the

experiment.

Mechanism:

1. The direct shear test uses a box split into top and bottom halves.

2. The bottom half is fixed while the top half can slide.

3. A constant normal force P is applied to the top of the box while a shearing force T is

gradually applied to one side.

4. As the shear force increases, the lateral displacement is measured.

40

5. The normal force divided by the cross sectional area of the box is the normal stress and

the shear force T divided by the area is the shear stress.

Fig. 7.2.3Components of Direct Shear Apparatus

Fig.7.2.4 Proving Ring

41

Fig.7.2.5 Load applied on Soil Sample

Analysis:

1. Calculate the density of the soil sample from the mass of soil and volume of the shear

box.

2. Convert the dial readings to the appropriate length and load units and enter the values on

the data sheet in the correct locations. Compute the sample area A, and the vertical

(Normal) stress sv.

s v = N v / A

Where Nv = normal vertical force, and sv = normal vertical stress

3. Calculate shear stress (t) using

t = Fh / A

Where Fh= shear stress (measured with shear load gage)

4. Plot the horizontal shear stress (t) versus horizontal (lateral) displacement (∆H).

5. Calculate the maximum shear stress for each test.

6. Plot the value of the maximum shear stress versus the corresponding vertical stress for

each test, and determine the angle of internal friction (φ) from the slope of the

approximated Mohr-Coulomb failure envelope.

42

Fig.7.2.6 Sample conditions before and after the test

Formulae:

Shear stress = (Proving Ring Reading * calibration) / area of container

1 division of proving ring reading= 0.3797 kg

Observations:

Dimensions of shear box = 6x6 cm

43

TABLE.NO.2: DIRECT SHEAR TEST READINGS

S. No.

Normal load

(kg)

Normal stress

(kg/cm2)

Proving ring

reading

Shear stress (kg/ cm2)

(proving ring reading

x calibration/ area of

container)

1.

0.5

0.5

6.4

0.33

2.

1

1

14.4

0.78

3.

1.5

1.5

18.8

1.03

Calculations:

Maximum Dry Density, γd = 1.97 gm/cc

Optimum moisture content = 11%

Volume of the mould, V = 6*6*2.5

= 90 cc

1.97= weight/ 90

Weight of sample to be taken = 1.97*90

= 178.3 gm

Water to be added = 178.3 * 0.11

= 19.6 ≈ 20 ml

(1) Normal stress = 0.5 kg/cm2

Proving ring reading = 6.4

= (6*5) + 4

= 34

Area of mould = 6*6 = 36 cm2

44

Shear stress (τ1) = (34*0.3797)/36

= 0.33 kg/cm2

(2) Normal stress = 1 kg/cm2


= (13*5) + 6

= 71


Shear stress (τ2) = (71*0.3797)/36

= 0.75 kg/cm2

(3) Normal stress = 1.5 kg/cm2


= (18*5) + 8

= 98


Shear stress (τ3) = (98*0.3797)/36

= 1.03 kg/cm2

45

Graph 2:

Fig: ; Graph showing variation of normal load and shear stress in direct shear test.

Result:

Shear strength of the given soil sample is 0.03 kg/ cm2

46

7.3 UNCONFINED COMPRESSIVE STRENGTH TEST

Purpose:

The primary purpose of this test is to determine the unconfined compressive strength, which is

then used to calculate the unconsolidated undrained shear strength of the clay under unconfined

conditions. According to the ASTM standard, the unconfined compressive strength (qu) is

defined as the compressive stress at which an unconfined cylindrical specimen of soil will fail in

a simple compression test.

Significance:

For soils, the undrained shear strength (su) is necessary for the determination of the bearing

capacity of foundations, dams, etc. The undrained shear strength (su) of clays is commonly

determined from an unconfined compression test. The undrained shear strength (su) of a cohesive

soil is equal to one-half the unconfined compressive strength (qu) when the soil is under the φ = 0

condition (φ = the angle of internal friction). The most critical condition for the soil usually

occurs immediately after construction, which represents undrained conditions, when the

undrained shear strength is basically equal to the cohesion (c).This is expressed as:

Su = c = qu/2

Then, as time passes, the pore water in the soil slowly dissipates, and the Inter-granular stress

increases, so that the drained shear strength (s), given by s = c + s’tanφ , must be used. Where s’

= inter-granular pressure acting perpendicular to the shear plane; and s’ = (s - u), s = total

pressure, and u = pore water pressure; c’ is drained shear strength parameter.

Equipment:

1. Compression device

2. Load and deformation dial gauges

3. Sample trimming equipment

4. Balance

47

5. Moisture can.

Fig.7.3.1 Unconfined Compression Test Apparatus

Procedure:

1. Sample is prepared at the desired water content and dry density.

2. It is compacted in three layers in a cylindrical tube of standard dimensions.

3. In this type of unconfined compression testing machine, a proving ring is used to measure

the compressive force.

4. There are two plate, having cone seating for the specimen.

5. The specimen is placed on the bottom plate so that it makes contact with the upper plate.

6. The dial gauge and proving ring are set to zero.

7. Compressive load is applied to the specimen by turning the handle.

8. As the handle is turned, the upper plate moves downward and causes compression.

9. The handle is turned gradually so as to produce an axial strain of ½ % to 2 % per minute.

10. The shearing is continued till the specimen fails.

48

11. The compressive force is determined from proving ring reading and axial strain from dial

gauge reading.

Fig.7.3.2 Sample Extractor

49

Fig. 7.3.3Cylindrical Mould

Fig.7.3.4 Extracted soil sample

50

Analysis:

1. Convert the dial readings to the appropriate load and length units, and enter these values

on the data sheet in the deformation and total load columns. (Confirm that the conversion

is done correctly, particularly proving dial gage readings conversion into load)

2. Compute the sample cross-sectional area A0 = π d2/4

3. Compute the strain, e =∆ L / L0

4. Computed the corrected area, A' = A0 / (1- e)

5. Using A’, compute the specimen stress, sc = P/ A’ (Be careful with unit conversions and

use constant units).

6. Compute the water content, w%.

7. Plot the stress versus strain. Show qu as the peak stress of the test. Be sure that the strain

is plotted on the abscissa.

8. Draw Mohr’s circle using qu from the last step and show the undrained shear strength, su

= c (or cohesion) = qu/2.

Fig. 7.3.5 Load applied on soil specimen

51

Fig.7.3.6 Soil Specimen after failure

Formulae:

• 1 Proving Ring Reading = 0.3839 kg

• Strain = (Compression Dial Reading) / Initial Height of sample

• Axial load = Calibration x 1 Proving Ring Reading

• Compressive stress = (Axial load/Area)

Observations:

Diameter of the sample = 3.8 cm

Initial height of the sample = 7.8 cm

Area of cross-section = 11.34 cm2

52

TABLE.NO.3: UNCONFINED COMPRESSION TEST READINGS

Compression

dial reading

(mm)

Strain

c

Area

A/(1-c2)

(cm2)

Proving

ring

reading

(div)

Axial load

(kg)

Compressive stress

(kg/cm2 )

50 0.641 11.413 0.7 0.26873 0.02354

100 1.282 11.487 0.8 0.30712 0.02673

150 1.923 11.562 1.8 4.9907 0.4316

200 2.564 11.638 2.6 6.1424 0.5277

250 3.205 11.715 3.3 6.9102 0.5898

300 3.846 11.79 3.6 8.0619 0.6837

350 4.487 11.872 4.4 9.2136 0.776

400 5.128 11.95 5.4 11.1331 0.9316

450 5.769 12.034 5.6 11.9009 0.9888

500 6.41 12.11 6.2 12.2848 1.0144

550 7.051 12.2 6.6 13.8204 1.1328

600 7.692 12.28 7.2 14.2043 1.1567

650 8.33 12.37 7.7 16.1238 1.3034

53

Calculations:

Maximum Dry Density, γd = 1.97 gm/cc

Optimum moisture content = 11%

Cross-sectional Area of sample = (π*d2)/4

= (π*3.82)/4

= 11.34 cm2

Volume of the sample = ((π*d2)/4)*h

= ((π*3.82)/4)*7.8

= 11.34*7.8

= 88.46 cm3

Density = weight / volume

1.97 = weight / 88.46

Weight of sample to be taken = 1.97*88.46

= 175 gm

Water to be added = 175 * 0.11

= 19.36 ≈ 20 ml

Result:

• Maximum unconfined compressive strength of given soil (qu ) = 1.303 kg/cm2

• Shear strength of given soil (qu/2) = 0.6515 kg/cm2

54

8. ESTIMATION OF SAFE BEARING CAPACITY

Foundation is that part of the structure which is in direct contact with soil. Foundation

transfers the forces and moments from the super structure to the soil below such that the stresses

in soil are within permissible limits and it provides stability against sliding and overturning to the

super structure. It is a transition between the super structure and foundation soil. The job of a

geotechnical engineer is to ensure that both foundation and soil below are safe against failure and

do not experience excessive settlement. Footing and foundation are synonymous.

Bearing capacity is the power of foundation soil to hold the forces from the

superstructure without undergoing shear failure or excessive settlement. Foundation soil is that

portion of ground which is subjected to additional stresses when foundation and superstructure

are constructed on the ground. The following are a few important terminologies related to

bearing capacity of soil.

Fig. 8.1 Main components of a structure including soil

Super Structure

Foundation

Foundation Soil

Ground Level

55

8.1 Factors influencing Bearing Capacity

Bearing capacity of soil depends on many factors. The following are some important ones.

1. Type of soil

2. Unit weight of soil

3. Surcharge load

4. Depth of foundation

5. Mode of failure

6. Size of footing

7. Shape of footing

8. Depth of water table

9. Eccentricity in footing load

10. Inclination of footing load

11. Inclination of ground

12. Inclination of base of foundation

8.2 Terzaghi’s bearing Capacity Theory

Terzaghi (1943) was the first to propose a comprehensive theory for evaluating the safe bearing

capacity of shallow foundation with rough base.

8.2.1 Assumptions

1. Soil is homogeneous and Isotropic.

2. The shear strength of soil is represented by Mohr Coulombs Criteria.

3. The footing is of strip footing type with rough base. It is essentially a two dimensional plane

strain problem.

4. Elastic zone has straight boundaries inclined at an angle equal to Φ to the horizontal.

5. Failure zone is not extended above, beyond the base of the footing. Shear resistance of soil

above the base of footing is neglected.

56

6. Method of superposition is valid.

7. Passive pressure force has three components (PPC produced by cohesion, PPq produced by

surcharge and PPγ produced by weight of shear zone).

8. Effect of water table is neglected.

9. Footing carries concentric and vertical loads.

10. Footing and ground are horizontal.

11. Limit equilibrium is reached simultaneously at all points. Complete shear failure is mobilized

at all points at the same time.

12. The properties of foundation soil do not change during the shear failure

8.2.2 Limitations

1. The theory is applicable to shallow foundations

2. As the soil compresses, Φ increases which is not considered. Hence fully plastic zone may

not develop at the assumed Φ.

3. All points need not experience limit equilibrium condition at different loads.

4. Method of superstition is not acceptable in plastic conditions as the ground is near failure

zone.

57

Fig. 8.2.2 : Terzaghi’s concept of Footing with five distinct failure zones in foundation soil

8.2.3 Concept

A strip footing of width B gradually compresses the foundation soil underneath due to the

vertical load from superstructure. Let qf be the final load at which the foundation soil experiences

failure due to the mobilization of plastic equilibrium. The foundation soil fails along the

composite failure surface and the region is divided in to five zones, Zone 1 which is elastic, two

numbers of Zone 2 which are the zones of radial shear and two zones of Zone 3 which are the

zones of linear shear. Considering horizontal force equilibrium and incorporating empirical

relation, the equation for ultimate bearing capacity is obtained as follows.

Ultimate bearing capacity, γγγ BNDNcNq qcf 5.0++=

If the ground is subjected to additional surcharge load q, then

γγγ BNNqDcNq qcf 5.0)( +++=

58

Net ultimate bearing capacity, DBNDNcNq qcn γγγ γ −++= 5.0

γγγ BNNDcNq qcn 5.0)1( +−+=

Safe bearing capacity, [ ] DF

BNNDcNq qcs γγγ γ ++−+=1

5.0)1(

Here, F = Factor of safety (usually 3)

c = cohesion

γ = unit weight of soil

D = Depth of foundation

q = Surcharge at the ground level

B = Width of foundation

Nc, Nq, Nγ = Bearing Capacity factors

“Net safe bearing capacity” is the net soil pressure which can be safely applied to the soil

considering only shear failure. It is obtained by dividing the net ultimate bearing capacity by a

suitable factor of safety.

qns = qnu / F

Net ultimate bearing capacity is the net increase in pressure at the base pf the foundation that

causes shear failure of the soil. It is equal to gross pressure minus overburden pressure.

qnu = qu - γDf

59

TABLE NO.4

BEARING CAPACITY FACTORS FOR DIFFERENT Φ

ϕ Nc Nq Ng N'c N

'q N

'g

0 5.7 1.0 0.0 5.7 1.0 0.0

5 7.3 1.6 0.5 6.7 1.4 0.2

10 9.6 2.7 1.2 8.0 1.9 0.5

15 12.9 4.4 2.5 9.7 2.7 0.9

20 17.7 7.4 5.0 11.8 3.9 1.7

25 25.1 12.7 9.7 14.8 5.6 3.2

30 37.2 22.5 19.7 19.0 8.3 5.7

34 52.6 36.5 35.0 23.7 11.7 9.0

35 57.8 41.4 42.4 25.2 12.6 10.1

40 95.7 81.3 100.4 34.9 20.5 18.8

45 172.3 173.3 297.5 51.2 35.1 37.7

48 258.3 287.9 780.1 66.8 50.5 60.4

50 347.6 415.1 1153.2 81.3 65.6 87.1

60

\

Fig. 8.2.3 : Terzaghi’s Bearing Capacity Factors for different φ

8.3 Effect of shape of Foundation

The shape of footing influences the bearing capacity. Terzaghi and other contributors have

suggested the correction to the bearing capacity equation for shapes other than strip footing

based on their experimental findings. The following are the corrections for circular, square and

rectangular footings.

Circular footing

γγγ BNDNcNq qcf 3.03.1 ++=

61

Square footing

γγγ BNDNcNq qcf 4.03.1 ++=

Rectangular footing

γγγ BNL

BDNcN

L

Bq qcf 5.0)2.01()3.01( −+++=

8.4 Summary of Shape factors

Table 7.2 gives the summary of shape factors suggested for strip, square, circular and

rectangular footings. B and L represent the width and length respectively of rectangular footing

such that B < L.

Table 7.3 : Shape factors for different shapes of footing

Shape sc sq sγ

Strip 1 1 1

Square 1.3 1 0.8

Round 1.3 1 0.6

Rectangle )3.01(L

B+ 1 )2.01(

L

B−

8.5 Calculation of safe bearing capacity

• For a square footing,

qu = 1.2 c Nc + q Nq + 0.4 γ B Nγ

Nc , Nq, Nγ are bearing capacity factors.

62

Nq =

a = e

Nc =

Nγ =

Kp = = coefficient of passive earth pressure.

8.6 Factor of Safety

It is the factor of ignorance about the soil under consideration. It depends on many factors such

as,

1. Type of soil

2. Method of exploration

3. Level of Uncertainty in Soil Strength

4. Importance of structure and consequences of failure

5. Likelihood of design load occurrence, etc.

Assume a factor of safety F = 3, unless otherwise specified for bearing capacity problems.

63

From direct shear test graph,

Cohesion, c = 300 kg/ m2

Angle of shear resistance Ø= 330 49’

For Ø= 330 49’

Nc = 52.6

Nq = 36.5

Nγ = 30.0

Result

Ultimate bearing capacity,

qu = 202.18 t/ m2

Net ultimate bearing capacity,

qnu = 198 t/ m2

Net safe bearing capacity,

qns = 198/3

= 66.06 t/ m2

64

9. DESIGN CONSIDERATIONS FOR RETAINING WALLS

9.1 WALL DEFORMATIONS AND EARTH PRESSURES

Earth pressures are a function of both the type and magnitude of wall deformations. In

cohesion less soils, deflections of the order of 0.001H and 0.05H are required to develop active

and passive pressure, respectively, and the order of0.04H to develop active pressure in cohesive

soils. Deflections needed to develop passive pressure in cohesive soils are not exactly defined.

Most retaining walls of the gravity type are normally designed for active pressure. However,

certain categories of retaining walls are restrained at the top and are not free to tilt, as for

example, basement walls restrained by floor slabs and bridge abutments restrained by the deck

structure. In such cases, the earth pressure at rest should be used in the design. Compacting of a

backfill against a rigid wall will result in an increase in the K0 value. In the design of a gravity

wall, the passive resistance in front of the toe is usually ignored unless the wall base is founded

at a considerable depth below grade level.

9.2 BACKFILL MATERIAL

Wherever there is a choice, clean, granular backfill material should be preferred because

the computed active earth pressures are more reliable in such cases and there is less likelihood of

hydrostatic pressure build-up under adequate provision of drainage. Some forms of filter material

should be used just behind a retaining wall to preclude pore water pressure development within

the backfill. The water collecting in the filter is led away weep holes which are provided within

the section of the retaining wall.

It may be noted that when the nature of backfill changes from a free drainage to one

having water table at the top of the wall, the lateral pressure will more than double. Even a

change from a simple ‘wet’ condition to a fully saturated condition may increase the lateral

pressure by 20 to 30 percent. Hence, it is much more desirable to provide suitable drainage than

to design a retaining wall for the larger pressure which will be induced if the backfill does not

readily drain.

Clay backfills should be avoided as far as possible, essentially because they are

susceptible to swelling and shrinkage during rainy and summer seasons, respectively. Swelling is

65

likely to cause unpredictable earth pressures and wall movements while shrinkage may lead to

tension cracks in soil. The cracks may subsequently get filled with water, adding considerably to

the lateral pressure.

9.3 GRAVITY RETAINING WALLS

As in design of all other structures, a trial section is first chosen and analyzed. If the stability

checks yield unsatisfactory results, the section is changed, and rechecked. Figure shows the

general proportions of a gravity retaining wall of overall height H. the top width of the stem

should be at least 0.3m for proper placement of the concrete in the stem. The depth, D, of the

foundation below the soil surface should be at least 0.6m. The base width of the soil is generally

between 0.5H to 0.7H; with an average height of 2H/3.

Fig.9.3.1 Gravity Retaining wall

66

The earth pressure can be computed using Rankine’s theory or Coulomb’s theory. For using

Rankine’s theory a vertical line AB is drawn through the heel point A. it is assumed that Rankine

active condition exist along vertical line AB. However, the assumption for the development of

Rankine’s conditions along AB is theoretically justified only if the shear zone bounded by the

line AC is not obstructed by the stem of the wall, where AC makes an angle η with the vertical,

given by

η= (450 + i/2) - φl/2 - sin-1(sin i/ sin φl)

where i is the angle of surcharge.

The angle α which the line AC makes with the horizontal is given by,

α= (450+ φl/2) – i/2 + sin-1(sin i/ sin φl)

when i=0, the value of α is equal to (450+ φl/2)

While checking the stability, the weight of soil (Ws) above the heel in the zone ABC

should also be taken into consideration, in addition to the earth pressure on the back face (Pa), the

forces to be considered are only Pa (coulomb) and the weight of the wall (Wc). In this case, the

weight of the soil (Ws) is not to be considered separately.

Once the forces acting on the wall have been determined, the stability is checked using

the procedure shown in the preceding section. For convenience, the section of the retaining wall

is divided into rectangles and triangles for the computation of weight and determination of the

line of action of the weights.

9.4 DESIGN

The earth pressure is computed using Rankine’s theory on the vertical plane AB, provided the

shear zone bounded by the line AC is not obstructed by the stem of the wall. The line AC makes

an angle η with the vertical given by the equation,

η= (450 + i/2) - φl/2 - sin-1(sin i/ sin φl)

Figure below shows the forces acting on the wall. The Rankine pressure Pa acts at an angle i with

the horizontal. It is resolved into vertical and horizontal components Pv and Ph, as shown. The

67

passive pressure Pp is generally neglected. For convenience, the weight of soil (Ws) over the slab

is divided into two parts. Likewise, the weight of stem is divided into two parts.

a) Factor of safety against sliding

The factor of safety against sliding may be expressed as

Fs = Σ FR /Σ Fd

Where Σ FR =sum of the horizontal resisting forces,

and Σ Fd =sum of horizontal driving forces

hence the final equation is, Fs= ((ΣV) tan φ2 + bc2 + Pp) / (Ph)

where b = base width,

ΣV = sum of all vertical forces; Wc, Ws, Pv

Pv = Pa sini, Ph = Pa cosi

Pp = passive force in the front of the wall

= ½ Kp2γ2D2 + 2c2 Kp2

1/2 D

where c2, γ2 and φ2 are parameters of the foundation soil.

The factor of safety can also be determined from the equation

Fs = µRV / RH

Where RV and RH are vertical and horizontal reaction forces

µ = coefficient of friction between base of the wall and the soil

= tan 2φ/3

A minimum factor of safety of 1.5 against sliding is generally recommended.

68

b) Factor of safety against overturning

The wall must be safe against overturning about toe.

The factor of safety against overturning is given by

F0 = ΣMR / ΣM0

Where ΣMR = sum of the resisting moments about toe,

ΣM0 = sum of the overturning moments about toe.

The only overturning force is Ph, acting at a height of H/3

M0 = Ph * H/3

The resisting moments (MR) are due to weights of the soil and the concrete. The vertical

components of pressure Pv also helps in resisting moment. Its resisting moment is given

by

Mv = Pv * b

The factor of safety against overturning is usually kept between 1.5 and 2.

c) Factor of safety against bearing capacity failure

The sum of vertical forces acting on the base is equal to ΣV. The horizontal force is Ph.

the resultant force(R) is given by

R = ((ΣV)2 + (Ph)2)0.5

The net moment of these forces about toe B is given by

ΣM = ΣMR – ΣM0

The distance xl of the point E, from the toe, where R strikes the base is given by

xl= ΣM / ΣV

hence, the eccentricity e of R is given by

69

e = b/2 - xl

if e > b/6, the section should be changed as it indicates tension.

The pressure distribution under the base slab is determined as

Pmax = (ΣV/b) * (1+ (6e/b))

Pmin = (ΣV/b) * (1- (6e/b))

The factor of safety against bearing failure is given by

Fb = qna / Pmax

Where qna= allowable bearing pressure.

Generally, a factor of safety of 3 is usually specified, provided the settlement is also within the

allowable limit.

70

10. DESIGN OF THE RETAINING WALL

By considering the above design procedure and keeping in view of soil analysis, the existing

section should be modified and reconstructed. The Existing section of Retaining wall is shown in

the figure below.

EXISTING SECTION

Fig. 10.1 Existing section of Retaining wall

3.05 m

0.5 m 1.5 m

3.68 m

0.55 m

0.55 m

1 m

71

TRIAL SECTION 1

Fig. 10.2 Trial section1

2.6 m

0.5 m 1.6 m

0.5 m 0.5 m

1 m 0.5 m

2.245 m

0.55 m

72

TABLE NO. 5 : LOAD DETAILS OF THE TRIAL SECTION 1

s. no Components Load Lever

arm at sill

Moment at

sill

Lever

arm at

floor level

Moment at

floor level

I Above sill level:-

1 Heel side triangular portion

0.50*0.5*2.245*2.3

1.291

0.717

0.926

1.767

2.281

2

Rectangular portion

0.55*2.245*2.3 =

2.84 0.275 0.781 1.325 3.763

3

Toe side triangular portion

0.5*0*2.245*2.3 =

0 0 0 1.05 0

Total

4.131 1.707 6.044

II

Below sill level:-

4

Trapezoidal footing

a Rectangular portion

1.05*1*2.3

2.415 1.575 3.804

b

Triangular portion

1*0.5*1*2.3

0.633 0.867 0.549

5 Final footing

2.6*0.5*2.3 2.99

1.3 3.887

6

Weight of earth

0.5*3.245*2

3.245 2.123 6.89

Total

13.414

21.174

73

Check for the stresses at sill level:-

xl= ΣM / ΣV

=1.707/ 4.131

= 0.413 m

e = (b / 2) - xl

= (1.05/2) – 0.413)

= 0.112 m

b / 6 = 0.175 m

e < b / 6

Thus, safe against tension

Maximum stress, Pmax = (ΣV/b) * (1+ (6e/b))

= (4.131/1.05)* (1+6*0.112/1.05)

= 6.5 t/m2

Minimum stress, Pmin = (ΣV/b) * (1- (6e/b))

= (4.131/1.05)* (1-6*0.112/1.05)

= 1.32 t/m2 < 53 t/m2 for M20 grade concrete

Factor of safety against bearing capacity,

Fb = qna / Pmax

= (66 / 6.5)

= 10.15 > 3

Thus, safe against bearing failure

Where qna = allowable bearing pressure.

Check for stresses at foundation level:-

xl = ΣM / ΣV

= 15.13 / 9.283

74

= 1.6

e = -((b / 2) - xl)

= -((2.6/2) – 1.6)

=0.3

b/6 = 2.6/6

=0.433

e < b / 6



= (10.169/2.6) * (1+(6*0.3/2.6))

= 6.042 t/m2


= (10.169/2.6) * (1+(6*0.3/2.6))



Fb = qna / Pmax

= (66 / 6.042)

= 10.92 > 3



Check for sliding failure:-

Fs = µRV / RH

= (0.674*10.169) / 3.245

= 2.11 >1.5

Thus, safe against sliding

75



= tan φ (where φ = 340)

Check for overturning failure:-

The wall must be safe against overturning about toe. The factor of safety against overturning is

given by

F0 = ΣMR / ΣM0

= 14.248 / 6.89

= 2.073> 2

Thus, safe against overturning

Where ΣMR = sum of the resisting moments about toe.

ΣM0 = sum of the overturning moments about toe.

76

TRIAL SECTION 2

Fig. 10.4 Trial section 2

2.5 m

0.5 m

0.25 m

0.7 m

0.5m

0.55m

0.5 m

2.245 m

1 m

77

TABLE NO.7 : LOAD DETAILS OF TRIAL SECTION 2

S. no Components Load Lever

arm at sill

Moment at

sill

Lever

arm at

floor level

Moment at

floor level


1 Heel side triangular portion 0.5*0.5*2.245*2.3=

1.29 0.716 0.923 1.66 2.15

2

Rectangular portion 0.55*2.245*2.3 =

2.839 0.275 0.78 1.225 3.47

Total 4.129 1.703 5.62

II

Below sill level:-

4

Trapezoidal footing


1.05*1*2.3

2.415 1.475 3.56

b

Triangular portion

0.7*0.5*1*2.3

0.805 0.716 0.576

5 Final footing

2.5*0.5*2.3 2.875

1.25 3.593

6

Weight of earth

0.25*3.245*2

1.6225 2.122 3.44

Total

11.846

16.789

78


xl = ΣM / ΣV

= 1.703/4.129

= 0.412 m

e = xl – b/2

= 0.412-(1.05/2)

= 0.113 m

b / 6 = 1.05/6

=0.175 m

e < b / 6



= (4.129/1.05)* (1+(6*0.113/1.05))

= 6.47 t/m2


= (4.129/1.05)* (1+(6*0.113/1.05))



Fb = qna / Pmax

= (66 / 6.47)

= 10.2> 3



79


xl = ΣM / ΣV

=16.789/10.223

=1.642 m

e = - ((b / 2) - xl)

= - ((2.5/2) – 1.642)

=0.392 m

b/6 = 2.5/6

= 0.416

e < b/6



= (10.223/2.5)*(1+6*0.392/2.5)

= 7.93 t/m2


= (10.223/2.5)*(1-6*0.392/2.5)



Fb = qna / Pmax

= (66 / 7.93)

= 8.32> 3; safe against bearing failure



Fs = µRV / RH

80

= (0.674*10.223) /1.6225

= 4.24 >1.5







given by

F0 = ΣMR / ΣM0

= 16.78/3.44

= 4.87> 2


Where ΣMR = sum of the resisting moments about toe

ΣM0 = sum of the overturning moments about toe

81

TRIAL SECTION 3

Fig. 10.3 Trial Section 3

0.4m

2.4 m

0.5 m 0.7 m

0.25 m 1 m

0.5 m

0.55m

2.245 m

82

TABLE NO.6 : LOAD DETAILS OF TRIAL SECTION 3

s. no Components Load Lever

arm at sill

Moment at

sill

Lever

arm at

floor level

Moment at

floor level


1 Heel side triangular portion

0.5*0.5*2.245*2.3 =

1.29 0.716 0.923 1.66 2.15

2

Rectangular portion

0.55*2.245*2.3 =

2.839 0.275 0.78 1.225 3.47

Total

4.129 1.703 5.62

II

Below sill level:-

4

Trapezoidal footing


1.05*1*2.3

2.415 1.475 3.562

b

Triangular portion

0.5*0.7*1*2.3

0.805 0.716 0.576

5 Final footing

2.4 *0.5*2.3 2.76

1.2 3.312

6

Weight of earth

0.25*3.245*2

1.622 2.123 3.44

Total

11.731

16.51

83


xl = ΣM / ΣV

= 1.703/4.129

= 0.412 m

e = (b / 2) - xl

= (1.05/2) – 0.412

= 0.525 – 0.412

= 0.113 m

b / 6 = 0.175 m

e < b / 6



= (4.129/1.05) * (1+(6*0.113/1.05))

= 6.47 t/m2


= (4.129/1.05) * (1-(6*0.113/1.05))



Fb = qna / Pmax

= (66 / 6.47)

= 10.2 > 3



84


xl = ΣM / ΣV

= 16.512/11.731

= 1.407 m

e = -((b / 2) - xl )

= 1.407 – 1.2

=0.207 m

b/6 = 2.4/6

= 0.4 m

e < b / 6



= (11.731/2.4)*(1+(6*0.207/2.4))

= 7.417 t/m2


= (11.731/2.4)*(1-(6*0.207/2.4))

= 2.35 t/m2< 53 t/m2 for M20 grade concrete

Factor of safety against bearing capacity

Fb = qna / Pmax

= (66 / 7.417 )

= 8.898 > 3




Fs = µRV / RH

85

= (0.674*10.109)/1.622

= 4.205 >1.5







given by

F0 = ΣMR / ΣM0

= 13.07/3.44

= 3.79 > 2


Where ΣMR = sum of the resisting moments about toe

ΣM0 = sum of the overturning moments about toe

86

Result

The width of the foundation for the third trial section is less when compared to other two

sections. Hence, among the three trial sections, the third trial section is economical.

Fig.10.5 Retaining wall being constructed

87

CONCLUSION

The project, designing of retaining wall for a minor bridge is the consequence of

prestigious metro rail project. In the construction course of metro railway, a four lane road way

has to be extended to six lanes, to avoid the occurrence of traffic problems. The minor bridgein

this course also has to be extended.

We took existing four lane retaining wall as an example and designed retaining wall

for the six lane road way. Each section has been analyzed for failure against sliding, overturning,

tension and bearing capacity. After doing trials for many sections, we got a section satisfying all

the safety conditions, approximating the standard dimensions of a gravity retaining wall.

The location of minor bridge being in rocky strata, with moorum soil, we got a high

bearing capacity value. Taking this as a reference, we also designed two economical sections-

with reduced dimensions. Hence, apart from the main modified section other two sections can

also be considered to make the project economical, which is the main philosophy behind the

project.

88

BIBLIOGRAPHY

1. Soil Mechanics and Foundation Engineering by “Dr.K.R.Arora”

2. Irrigation Engineering And Hydraulic Structures by “S.K.Garg”

3. www.nptel.iitm.ac.in

4. www.wikipedia.com

5. www.aboutcivil.com

6. http://elearning.vtu.ac.in

7. http://engineering.sdsu.edu

cd-17-major project report.pdf

Documents