Retaining walls - Piling Engineering - page 236

Figure 6.11 Lateral pressure distribution produced by point load Q p . Rigid unyielding wall leads to
double the value of σ H .
and Terzaghi. The figures given by Terzaghi (1954) are in fact a modification, based
on some experimental work, of the Boussinesq method.
The Boussinesq theory is formulated to give the stresses in a semi-infinite elastic
medium due to the application of a load at the surface, and if the Boussinesq formulae
are used with reference to loads applied near a rigid boundary the effect is to constrain
the deformation of the elastic medium and to double the horizontal stresses acting on
the boundary. Thus for a point load, and taking a Poisson's ratio of 0.5, the Boussinesq
relationship is as shown in Figure 6.11.
However, it is clear that the deformability of the wall and how it relates to elastic
theory is dependent on the wall type. Thus a cantilever wall may deform much more
than a multi-strutted wall, and the section of an anchored wall above the top anchor
may act very much as if it were rigid. It is therefore important, if using the Boussinesq
method, to consider carefully how it should be applied.
The lateral soils pressures acting on a wall due to a point load and as calculated
by Terzaghi are shown in Figures 6.12 and 6.13; similarly the lateral pressures due to
line loads are shown in Figure 6.14. Lateral pressures due to a strip load, based on
the Boussinesq method as modified by experiment and given by Teng, are shown in
Figure 6.15.
6.6 The use of berms
The passive pressure acting on the embedded part of a wall is a function of the vertical
soil pressure acting in the potential failure zone near the wall, and this can be increased
significantly by leaving a berm against the foot of the wall at the excavation level.
Since in retaining-wall design the temporary condition during construction is often
worse than the permanent condition, and since berms can often be accommodated