Volume 6 – Geotechnical Manual, Site Investigation and Engineering ...

Chapter 7 RETAINING WALL
7.5.1 Sliding/Translational Stability
The horizontal component of all lateral pressures tends to cause the wall to slide along the base of
the wall (or along any horizontal section of a gravity and crib wall). If the passive resistance is
neglected, the sliding force along the bottom of the wall is resisted by a horizontal force which
consists of friction, adhesion or a combination of both. If the bottom of base slab is rough, as the
case of concrete poured directly on soil, the coefficient of friction is equal to tan φ', (φ' is the angle
of internal friction of the soil). Typical coefficients of friction are as follows:
Course-grained (without silt) 0.55
Course-grained (with silt) 0.45
Silt 0.35
Sound rock (with rough surface) 0.60
For cohesive soils the adhesion between the base slab and the soil is assumed to be equal to the
cohesive strength of the clay and φ is assumed to be zero. The designer should consider the
possibility of reduction in cohesive strength due to construction works such as excavation, exposure
to surface water etc. If the retaining wall is supported on piles, the entire vertical and horizontal load
should be assumed to be carried by piles. No frictional resistance and no adhesion should be
assigned along the base slab.
For checking the sliding factor of safety, the live load surcharge is usually not considered in the
stabilising forces over the heel of the wall. Also, the passive resistance of the soil in front of the wall
is commonly neglected in the stability analysis. If it is included in the computation, as in the case
where the toe of wall is covered by a large depth of soil, its value should be reduced to take care of
the high potential of the soil to be removed by erosion, future excavation, and tension cracks in
cohesive soils.
The minimum safety factor for sliding/translational stability shall be of minimum 1.5. The sliding
stability can be increase by either increasing the overall weight of the retaining wall or providing
sufficient passive lateral resistance of the wall. This can be done by introducing a wider base,
construction of structural shear key and incorporating deep foundation support.
7.5.2 Overturning Stability
The lateral pressure due to the backfill and surcharge tends to tip the retaining over about its toe.
This overturning moment is stabilised by the weight of the wall and the weight of the soil above the
base of the wall. The overturning stability of the wall is always the most critical potential mode of
failure when the walls are underlain by weak soils. The minimum factor of safety against overturning
is:
F s =
Sum of stabilizing moment
Sum of overturning moment
≥2.0
To overcome the overturning stability, normally pile foundation is recommended. For some cases,
ground improvement such as removal and replacement is adopted to increase the bearing capacity of
the ground (provided the soft bearing ground is relatively thin).
For passive resistance of the soil in front of the wall, designer should evaluate whether to ignore or
to use a reduced value basing on the reason discussed in 7.5.1 above.
March 2009 7-19