Civil Engineering Reference
In-Depth Information
It is clear therefore that there is no single simple answer to the questions that may
be asked regarding predicted wall movements but at the same time there are avail-
able methods of analysis which range from an advanced finite element type to simple
reliance on extensive experience. However good and thorough the analysis, any failure
to comply with the design concepts in final construction can completely invalidate
conclusions that might have been drawn from calculation.
A simple model for assessing the magnitude of general ground movements due to
excavation may be based on the whole mass acting elastically. Movements may be
estimated by treating the excavation as negative load as for an embankment with
vertical sides (Poulos and Davis, 1974). The analysis will then be based on Young's
modulus
E
and Poisson's ratio
ν
u
, the undrained parameters if short term conditions
are being considered, or
E
and
ν
where long-term analysis is sought.
It is useful to remember that where
G
is the shear modulus,
E
2(1
E
u
G
=
+
ν
)
=
G
u
=
2
1
+
ν
u
For undrained conditions
ν
u
=
0
.
5, whereas
ν
is in the range 0.1 to 0.33 and may be
taken as about 0.2 for stiff clays.
In so far as the local movements in the vicinity of the wall are concerned the general
pattern of behaviour is as indicated in Figure 6.23.
The interaction between the soil and a retaining wall is complex. Rigorous analysis
is not feasible at the present time. The conventional calculations for stability are based
on lumped 'factors of safety' and provide no explicit information regarding deflection.
It has been shown by Bica and Clayton (1989) that there are some 25 variants on the
normal stability calculation method, so it is hardly surprising that deflection is difficult
to determine even on an empirical basis.
Provided the deflected form of a wall can reasonably be established, either by cal-
culation or experience of similar cases, then reasonable deductions may be made
concerning the deformations that occur in the retained soil, using the method proposed
Figure 6.23
Local movements in the vicinity of (
a
) a cantilever wall, and (
b
) a propped wall.
