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

Chapter 7 RETAINING WALL
7.5.3 Bearing Capacity Failure
The computed vertical pressure at the base of the wall footing must be checked against the ultimate
bearing capacity of the soil. The generalized distribution of the bearing pressure at the wall base is
illustrated in Figure 7.15. Note that the bearing pressure at the toe is greater than that at the heel.
The magnitude and distribution of these pressures are computed by using the applied loads shown in
Figure 7.15. The equivalent uniform bearing pressure, q eq , should be used for evaluating the factor
of safety against bearing capacity failure. The procedures for determining the allowable bearing
capacity of the foundation soils can be found in Chapter 5 (Bearing Capacity) of this Volume.
Generally, the factor of safety against bearing failure is defined as
Where
F s = q ult
q eq
≥ 2.0
q ult = ultimate bearing pressure
q eq = equivalent uniform bearing pressure (as computed according to Figure 10.15)
7.5.4 Global Stability
The overall stability shall be checked to avoid deep seated failure due to circular rotational or noncircular
failure beyond the retaining wall. It must be checked with respect to the most critical failure
surface. The minimum factor of safety for the overall stability shall be of minimum 1.5. A typical
mode of circular rotational stability condition is illustrated in Figure 7.16
If global stability is found to be a problem, deep foundations or the use of lightweight backfill may be
considered. Alternatively, measures can be taken to improve the shear strength of the weak soil
stratum. Other wall types, such as an anchored soldier pile and lagging wall or tangent or secant
pile wall, should also be considered in this case.
Figure 7.16 Typical Mode of Global Stability
7-20 March 2009