retaining wall design
DESCRIPTION
specifications walls gabionsTRANSCRIPT
Advantages of Using Gabions
LINK MIDDLE EAST's versatile range of
standard and special sizes of gabions allows
the 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
Retaining Walls
Slaggered Vertical Face Slaggered Sloping Face
Design Principles
Gabion retaining walls should be designed as mass 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 failureof
the wall
. 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.For overturningthe contributionof the soil in trontof thewall,
P , is usually ignored. The resultant force R should act within thep
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 b
W+Py
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>'to the
perpendicular to the wall, where «1>'is the design effective angle of shearing resistance.
Values of Pa and its comP9nents Pv and Pb 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 increase the resistance to 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 consider possible 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 be made to appropriate Standards and Codes ofPrac-
tice and specialist texts.
Suggested ReferencesBritish Standard BS 8002 : 1994 Code of Practice for
Earth 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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