002-retaining wall design

8/6/2019 002-Retaining Wall Design

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Advantages of Using Gabions

LINK MIDDLE EAST's versatile range of

standard and special sizes of gabions allowsthe design of many different wall sections to

suit the architectural demands of a project as

well as its engineering requirements. Just a

few examples of different wall appearances

are shown below.

.Foolinglo r ialoverturning

SlopedtoundaUon

Slrelghl Sloping Face

RetainingWalls

Slaggered Vertical Face Slaggered Sloping Face

Design Principles

Gabion retaining walls should be designed asmass gravity walls, using standard soil mechanics principles. No allow-

ance should be made for the strength or mass of the wire mesh, and the density of the filled gabions should be taken as

60% of the density of the solid rock used.

Analysis

The stability should be analysed for:

. overturning

. sliding

. bearingfailure. localiseddeformationor failureofthewall

. deep seated failure of the retained slope.

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e

w

Overturning

The figure shows a simplified diagram of the forces acting on a

wall.Foroverturningthecontributionof thesoilin trontof thewall,P , is usually ignored.The resultant force R shouldact withinthe

middle third of the base of the wall. This is checked by taking mo-

ments about the toe.

Equation 1

d = W x a + Py x e - Ph X bW+P

y

where (W + P) is the vertical reaction

of the ground beneath the wall

The thrust, Pa' exerted by the backfill and any surcharge on a gabion wall, is assumed to act at an angle 1>' o the

perpendicular to the wall, where 1>' s the design effective angle of shearing resistance.

Values of P a and its comP9nents P v and P b are obtained trom an analysis of earth pressures behind the wall. The force

W is the weight of the wall and, if it has a footing or heel, includes the weight of soil above the footing.

Sliding

Resistance to sliding is checked by comparing the horizontal thrust trom the backfill with the resisting mction and

adhesion forces at the base of the wall. The factor of safety against sliding is given in Equation 2. The angle of base

mction should be taken as that of the foundation soil rather than that between the gabions and the soil. It is normal to

ignore the resisting force of the earth in tront of the wall hence P = O.A minimum factor of safety of 1.5 is normallyp .

acceptable if thrust P is ignored, and 2.0 if thrust P is accounted for.p p

Equation 2

Factor of Safety = (W + Py) tan Ob+ Cb x B + P"

Ph

Where Cb = base adhesion

Ob= angle of base mction

Codes of Practice using the ultimate limit state philosophy require

sliding equilibrium to be checked using design values for Cband Ob'

Design values are obtained by factoring the representative soil

strengths found trom soil tests (Reference BS 8002).

In order to increasethe resistanceto sliding a gabionwall can be

built on a slopedfoundation.

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Bearing

The foundation of the wall should be designed for Ultimate Bearing Capacity, quit.The most commonly used method

for calculating this is Terzaghi's Method using bearing capacity factors.

Allowance should be made for the inclination and eccentricity of the forces on the base of the wall. In addition,

where bearing pressures vary across the base, the maximum pressure should not exceed the Allowable Bearing

Pressure, qallwhich is given below.

qall =

Local Failure

Equation 3

2.5 or 3

Checks should be made at selected levels above the base of the gabion retaining wall for:

. sliding, to prevent shear failure through the wall;

. overturning, to prevent rotational failure of the wall.

Deep Seated Failure

The location of the gabion retaining wall should be ana-

lysed to considerpossible failure of the adjacent ground.

Drainage

Depending on the height and particular application of

the retaining wall, drainage at the toe of the wall may be

required to collect groundwater, or runoff from adja-

cent roads, for example.

The above guidelines are intended to be a summary of the

design procedures. Owing to the wide variation in soil prop-

erties general design rules cannot be given. Reference

should bemade to appropriate Standards and Codes ofPrac-

tice and specialist texts.

Suggested References

British Standard BS 8002 : 1994 Code of Practice forEarth Retaining Structures.

Soil Mechanics in Engineering Practice, Terzaghi I G

and Peck R B, 1967.

Soil Mechanics, Lambe and Whitman, 1979.

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002-retaining wall design

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