Civil Engineering Reference
In-Depth Information
effective overburden pressure at and below the final excavation level, which initiates
the passive condition.
For multi-propped or tied walls, where the moments have been derived from one
of the available Finite Element or similar computer programs, the significant stiffness
difference between anchor ties and, for example, floor props will already have been
taken into account and bending moments corresponding to the assumptions will be
available. While steel reinforcement may be varied and curtailed to some extent in
reinforced concrete walls, the temptation to believe in bending moments being exactly
certain in value at given levels should be resisted as discussed above, since very often
the variation of, say, the stiffness of one prop relative to another can alter the diagrams
in a significant way. It is often the case, however, that the spacing between floors that
prop a wall in the final condition is less than the spacing between the props or ties in
the temporary state, and consequently bending moments and shear forces can be at
their most severe in the temporary state.
It will be noted that, for relatively flexible walls, pressures are increased at the
propping points and diminished midway between propping points, the soil effectively
arching between the props. This feature will be apparent in the analysis if carried out
by one of the appropriate computer programs.
Most reinforced concrete diaphragm walls are not designed in the structural sense
in accordance with conventional codes applicable to above ground conditions. His-
torically this is because concrete for use in piles and walls has to be very free flowing
because, if stiff, it would not flow properly around steel reinforcement. Shear does not
appear to be a problem in such cases. It is notable that to the authors' knowledge,
no case of shear failure in such walls has ever been reported. This may be because
shear in a wall would pre-suppose shear in the soil behind it, and it would appear
that, in this respect, the soil and the wall may therefore act in a composite manner.
Alternatively, and particularly in sandy soils, arching between props may mean that
soil loads concentrate locally to prop positions with the result that shear elsewhere is
diminished.
6.9 Retaining wall deflection and associated
soil movements
Although the act of excavation to install a bored pile or diaphragm wall panel must
in itself lead to some small ground deformation, it is the excavation that subsequently
takes place adjacent to the wall, and the following changes in the general soil profile,
which account for most of the movement that occurs.
When a deep excavation is carried out, this affects the stresses in the whole immediate
area, both within and without the plan area of the works involved. Such stress changes
lead to general movements in the soil mass, and for large excavations, particularly in
clay soils, this is a time-dependent and significant feature.
Secondly, relief of lateral pressure due to excavation adjacent to a retainingwall leads
to lateral wall movements, and the magnitude and timing of these are very dependent
on soil properties, the way in which excavations are staged, the stiffness, position and
timing of installation of props or ties, the pre-stress if any that is provided in the lateral
support system, the presence of berms and how and when they are removed, and the
stiffness, general dimensions and plan arrangement of the wall.
