RETAINING WALL

Dr. Hassan Irtaza, Professor

Department of Civil Engineering, A.M.U., Aligarh – 202002, India

Retaining Wall

Retaining wall is a structure used to retain earth or other

loose materials which would not be able to withstand

vertically by itself. In other words a retaining wall is a

structure designed and constructed to resist the lateral

pressure of soil when there is a desired change in ground

elevation that exceeds the angle of repose of the soil.

The retained materials exert pressure or push on the

structure and this tends to overturn and slide it.

The weight of the retaining wall is of considerable

significance in achieving and maintaining stability of the

entire system.

Conventional Retaining Walls

Gravity Retaining Structures

It may be of plain concrete or masonry

Stability depends upon the self weight of the wall.

Not economical for the design

Semi-gravity Retaining Structures

It is of standard concrete.

Minimum amount of

reinforcement may be

used in the wall to reduce

the size of the wall.

Not economical for the

design

Semi-gravity retaining wall

Cantilever Retaining Wall

Reinforced concrete isused in wall design withthin stem and slab base.

Each of the threecomponents acts acantilever beam.

Stability is provided byweight of the earth on thebase and weight of theretaining wall.

Relatively economical fordesign.

Cantilever retaining wall

Counterfort/buttressed Retaining Walls

Similar to cantilever retaining walls, but thin slab stemsmay be used at some interval to tie the base slab and stem inorder to reduce the shear force and bending moment formore economical design.

(a) Counterfort wall (b) Buttressed wall

Counterfort: Counterforts are transverse walls spaced at certain

intervals and act as tension ties to support the vertical wall.

Stability is provided by weight of the earth on the base slab and

weight of the retaining wall.

Buttress wall: A buttress wall is similar to a counterfort wall

except that transverse walls are located on the side of the vertical

wall opposite to the retained material and act as compression

struts.

Retained

Earth

A buttress is a compression member is more economical

than the tension counterfort, still the latter is more widely

used than a buttress because the counterfort is hidden

beneath the retained material. Moreover, buttress

occupies more space in front of the wall which otherwise

could be utilized more efficiently

Bridge Abutment

A wall type bridge abutment acts similar to a cantilever

retaining wall except that the bridge deck provides

additional horizontal restraint at top of the vertical slab.

This type of abutment is designed as a beam fixed at the

bottom and simply supported or partially restrained at the

top.

Bridge abutment Bridge abutment with piles bankseats

Box Culvert

A box culvert is a box like structure having either single cell

or multiple cells. It acts as a closed rigid frame that not only

resists lateral earth pressure but also vertical load from soil

above it or from both soil and highway vehicles.

Reinforced Earth Wall

Construction of a Reinforced Earth wall is straightforward and simple. Merely

place a layer of facing panels, bolt on the reinforcing strips then backfill and

compact. Repeat this cycle until the appropriate wall height has been reached.

Properly compacted to a uniformly high density, the earth combines with the

reinforcement to produce a strong, durable structure with predictable

performance characteristics.

Basic Components of Retaining Structures

Facing unit – not necessary but usually used to maintain

appearance and avoid soil erosion between the reinforces.

Reinforcement – strips or rods of metal, strips or sheets

of geotextiles, wire grids, or chain link fence or geogrids

fastened to the facing unit and extending into the backfill

some distance.

The earth fill – usually select granular material with 15%

passing the 200 No. sieve.

Temporary retaining structures during excavation

Sheet Pile Wall

Gabion Wall

Counterfort Wall

Crib Wall

Another type of gravity retaining structure

It consists of precast concrete members linked together to

form a crib

The zone between the member is filled with compacted soil

Retaining Wall Design: Proportioning

Firstly the approximate

dimensions are chosen

for the retaining wall.

Stability of the wall is

checked with these

dimensions.

Section is changed if it

is undesirable from the

stability or economy

point of view.

Retaining Wall Design: Proportioning

Forces on Retaining Wall

The main force that acts on a retaining wall is pressure due tothe retained material.

Typical plane of rupture behind a retaining wall is shown inthe next slide.

Plane of rupture behind a retaining wall

The earth pressure try to overturn and slide a retaining wall.

The magnitude and direction of the earth pressure can be

determined by applying the principles of soil mechanics.

The pressure exerted by the retained material is

proportional to its density and to the distance below the

earth surface.

Design Considerations

In order to calculate the pressure exerted at any point on

the wall, the following must be taken in account:

height of water table

nature & type of soil

subsoil water movements

type of wall

materials used in the construction of wall

The following two forms of earth pressure need to be

considered during the process of designing a retaining wall :

(a) Active Earth Pressure

“It is the pressure that at all times are tending to move or

overturn the retaining wall”

(b) Passive Earth Pressure

“It is reactionary pressures that will react in the form of a

resistance to movement of the wall.

Active Earth Pressure

It is composed of the earth wedge being retained together

with any hydrostatic pressure caused by the presence of

ground water. This pressure can be reduced by:

(i) The use of subsoil drainage behind the wall

(ii) Inserting drainage openings called weep holes through

the thickness of the stem to enable the water to drain

away.

Earth pressure behind a retaining wall is generally

calculated by Rankine’s theory for homogeneous

incompressible cohesionless soil assuming a hydrostatic

pressure distribution along the depth.

a a

2 2

2 2

p = C γ h

where, p = Active earth pressur

C coefficient of active earth pressure

cos cos cos = cos

cos cos cos

= density of the retained materi

a

a

al

h = depth of the section below the earth surface

= angle of surcharge

= angle of repose of soil

If there is no surcharge, i.e. δ = 0, the pressure Pa acts parallel

to the top surface of the material retained, the coefficient of

active earth pressure becomes

The total force behind the wall is given by the area of the

pressure triangle, that is

per unit length of the wall and acts at a depth of 2/3 H below

the top.

a

1-sinC =

1+sin

2a a

1P = C γ H2

h a

v a

P = P cosδ

P = P sinδ

Passive Earth Pressure

This pressure build up in front of the toe to resist the

movement of the wall if it tries to move forward.

This pressure can be increased by enlarging the depth of

the toe or by forming a rib on the underside of the base.

Rankine’s equation for passive pressure is given as:2

p p

p

2 2

2 2

1P = C γ H2

where C = Coefficient of passive earth pressure

cosδ + cos δ - cos f =cosδ

cosδ - cos δ - cos f

Some important points

The passive earth pressure (more in error) is quite high ascompared with active earth pressure. Its contribution inthe design of retaining wall is neglected which is on theconservative side.

Retaining walls are seldom designed to retain saturatedearth fill, hence proper drainage must be ensured.

Many times vehicles may travel over the retainedmaterial near the wall and cause the dynamic staticpressures. The IRC and the IR Bridge rules prescribe anequivalent static surcharge for road and railway vehicles.

Equivalent static surcharge is also added in case thebuilding is constructed near the top of the retaining wall.

Stability Requirements of RW

The designed retaining wall must be able to ensure the

following (IS:456-2000) :

Overturning doesn’t occur

Sliding doesn’t occur

The soil on which the wall rests mustn’t be overloaded,

i.e. the base width must be adequate to distribute the

load to the foundation soil without exceeding its bearing

capacity.

Check against overturning

Clause 20.1 of IS 456 requires that the resisting moment isnot less than the sum of 1.2 times the maximumoverturning moment due to characteristic dead load and 1.4times the maximum overturning moment due tocharacteristic imposed loads. If the dead load provides therestoring moment, only 0.9 times the characteristic deadload should be considered. It further requires that restoringmoment due to imposed loads should not be considered.

The factor of safety against overturning may be computedby neglecting the contribution of vertical component of theactive earth pressure. If W is the sum of the vertical loadsmade up of weight of backfill on the inner slab plus weightof the wall and base slab plus weight of the front fill, ifused, the factor of safety is given as follows:

FS =resisting moment

overturning moment

1

h

1

h

1

h

0.9Wx 1=

1.4P H 3

Wxor, 1.55=

P H 3

where, x = centre of gravity of vertical loads from toe

H = depth of bottom of base below the earth surface

P = horizontal component of earth pressure

Check against sliding

Sufficient frictional resistance

must be available against sliding

caused by the horizontal

component of the earth pressure.

h

h

reistingforce FOS =

slidingforce

0.9μW 1.4 =

P

μWor, 1.55=

P

where, μ = coefficient of friction between

soil and footing

Coefficient of friction between soil and concrete

Base width must be adequate to distribute the load to the foundation soil without exceeding its bearing capacity.

SOIL µ

Coarse grained soil (no silt) 0.55

Coarse grained soil (with silt) 0.45

Silt 0.35

Sound rock (rough surface) 0.60

Shear Keys

The sliding resistance of retaining walls is derived from thebase friction between the wall base and the foundationsoils. To increase the sliding resistance of retaining walls,other than providing a large self-weight or a large retainedsoil mass, shear keys are to be installed at the wall base.The principle of shear keys is as follows:

The main purpose of installation of shear keys is to increasethe extra passive resistance developed by the height ofshear keys. However, active pressure developed by shearkeys also increases simultaneously. The success of shearkeys lies in the fact that the increase of passive pressureexceeds the increase in active pressure, resulting in a netimprovement of sliding resistance.

In case wall is unsafe against

sliding

2p p

p

p

P = p tan 45+ 2 = p k

where, P = unit passive pressure on soil

above shearing plane AB

p = Earth pressure at BC

R = Total passive resistance = P × a

Design of shear key – contd..

p

a

If W = total vertical force acting at the key base¨

Ø = shearing angle of passive resistance

R = total passive force = P × a

P = active horizontal pressure at key base for H+a

μ W

A

A

= totalfrictionalforceunderflatbase

for equilibrium, R + μ W = FOS × P

FOS = R+μ W P 1.55

Maximum pressure at the toe

a

min

Let the resultant R due to W and P lie at a distance x from the toe

x = M W,

M = sum of all moments about toe

b beccentricity of the load = e = -x < 2 6

W 6eMinimum pressure at heel = P = 1- >

b b

max

zero

for zero pressure, e = b 6, resultant should cut the base within the middle third

W 6eMaximum pressure at toe = P = 1+

b b

< Safe Bearin

g Capacity of soil

Depth of Foundation

Rankine’s Formula

2

2

1 sin

1 sin

f

a

SBCD

SBCk

Preliminary Proportioning

T Shaped wall

Stem top width 200 mm to 400 mm

Base slab width b = 0.4H to 0.6H

= 0.75 H for surcharged wall

Base slab thickness = H/10 to H/14

Toe Projection = 1/3 to 1/4 of Base width

Structural Behavior of Retaining Wall

Design of Stem, heel

and toe slabs are same

as that of any cantilever slab.

Stem design

Stem design3

u a

2

u u,lim

y st

u ck st

ck

M = k γH /6

Determine the depth d from M = M = Qbd

Design as balanced section

or underreinforced and find steel

σ AM =0.87σ A d-

σ b

Check for shear at the junction

Provide enough developmen

t length

Provide distribution steel

Curtailment of bars

Analysis of Counterfort Retaining Wall

When H exceeds about 6 m

Stem and heel thickness is more

When more bending is acting at the

stem base, more steel will be required,

hence, Cantilever - T type retaining

wall is Uneconomical

Counterforts - Trapezoidal section is

provided at spacing of 1.5 m - 3 m c/c

Parts of CRW

Same as that of the Cantilever Retaining Wall plus Counterfort

Design of Stem

The stem acts as a continuous slab.

Soil pressure acts as the load on the slab.

Earth pressure varies linearly over height.

The slab deflects away from the earth face between the counterforts.

The bending moment in the stem is maximum at the base and reduces towards top.

But the thickness of the wall is kept constant and only the area of steel is reduced.

Maximum Bending Moments for Stem

Maximum +ve B.M. = pl2/16 (occuring

mid-way between counterforts)

and Maximum -ve B.M. =

pl2/12(occuring at inner face of

counterforts)

where, l is the clear distance between

counterforts and p is the intensity of

soil pressure

Design of Toe Slab

The base width = b = 0.6 H to 0.7 H

The projection = 1/3 to 1/4 of base

width.

The toe slab is subjected to an upward

soil reaction and is designed as cantilever

slab fixed at the front face of the stem.

Reinforcement is provided on earth face

along the length of the toe slab.

In case the toe slab projection is large i.e.

> b/3, front counterforts are provided

above the toe slab and the slab is

designed as continuous horizontal slab

spanning between the front counterforts.

Design of Heel Slab

The heel slab is designed as a

continuous slab spanning over the

counterforts and is subjected to

downward forces due to weight of

soil plus self weight of slab and an

upward force due to soil reaction.

Maximum +ve B.M. = pl2/16

(midway between counterforts)

Maximum negative B.M. = pl2/12

(occuring at counterforts)

Design of Counterforts

The counterforts are subjected to

outward reaction from the stem.

This produces tension along the outer

sloping face of the counterforts.

The innerface supporting the stem is in

compression. Thus counterforts are

designed as a T-beam of varying depth.

The main steel provided along the

sloping face shall be anchored properly

at both ends.

The depth of the counterfort is measured

perpendicular to the sloping side.

Behaviour of Counterfort Retaining Wall

Important Points

Loads on Wall

Deflected shape

Nature of BMs

Position of steel

Counterfort details

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