Civil Engineering Reference
In-Depth Information
to prescribed assumptions, is but one of a number of possible solutions; there is
certainly room for readjustment of designs to balance moments within boundaries
as indicated and particularly to accommodate sensibly adjusted reinforcement in the
concrete.
6.8 Structural wall design
6.8.1 General considerations
When a retaining wall has been analyzed for stability, it remains to design the structural
form of the wall, and at this stage appropriate bending moments and shear forces must
be used. This aspect of design has an important influence on the economy of the design
as a whole. Temporary and permanent states must be considered. For example, in the
case of many cantilever walls, a ground-bearing slab is cast to abut the wall and has
the effect of propping the wall in the post-excavation stage. In many circumstances this
effectively stops wall deflection and further moment development. In most instances,
therefore, the temporary state proves to be the critical one. It is unusual to design
such walls for pressures on the retained side other than the water pressure and the
(effective stress) active pressure. Even in overconsolidated clays it would appear that
the horizontal stresses in the soil mass, which are relieved at the excavation stage,
do not become restored. The force lines in the soil mass almost certainly reorientate
around and beneath the excavation (Figure 6.21).
Experience with many walls in such circumstances in stiff clay appears to indicate
that wall bending moments in the slab vicinity do not in the long-term significantly
exceed those calculated at the excavation stage, when only active and passive soil states
are normally considered, although the position of maximum moment may change a
little.
6.8.2 Determination of bending moments and shear forces
in a cantilever wall
The conventional method of determining maximum bending moment in a cantilever
wall
is to consider the active and passive pressure diagrams as used in the
Figure 6.21
Force lines indicating redistribution of pressures on wall with time.
