Civil Engineering Reference
In-Depth Information
stability calculations. The full ultimate passive pressure is used on the excavated
wall side, rather than any reduced value. This is because maximum passive pres-
sure must be mobilized near the excavation level through wall rotation, and because
excavation will, by reduction of the vertical soil stresses at that level, have pro-
duced an ultimate or near-ultimate passive state. Hence in practice maximum
bending moment appears in most circumstances not far below the excavation
level.
This means that from the head of the wall down to the point of maximum bending
moment the design moments correspond most nearly to ultimate active and passive
conditions.
However below the point of maximum bending moment, because of the extension
of the wall to provide a rotational stability factor, the full ultimate pressures on the
passive side will no longer apply and, in consequence, if the ultimate passive state were
assumed here, the moments would be curtailed too rapidly and excessive shear forces
would be evident.
The method of determiningmoments below the maximummoment point is therefore
empirical and as shown in Figure 6.22. It should however be pointed out that where a
significant change in the stiffness of the soil occurs below the excavated surface level,
for example, if a wall toe is embedded significantly into rock, then this simple method
may have to be modified in the light of judgement and of the fixity that the rapidly
increased stiffness may provide near the wall toe.
6.8.3 Determination of bending moments and shear forces
in tied or propped walls
For walls with a single prop or tie, the earth pressures and prop or tie force can be used
simply to determine the bending moment and shear forces in the wall. It is commonly
assumed for the purpose of bending moment determination that full passive pressure
acts on the wall below the excavation level, rather than some reduced pressure that
might be derived because of the extension of the wall to provide a factor of safety
or rotational stability factor. This is because of the usually significant reduction of
Figure 6.22
Determining moments below the maximum point.
