Civil Engineering Reference
In-Depth Information
1.0
Tentative Upper Limit for Loose
Upper Limit for Medium Dense
0.1
Upper Limit for Dense
0.0
1.0
10.0
Breadth (m)
Figure 13.2
Observed settlements of footings on sand of various densities as suggested by Burland
et al
. (1977). Courtesy of J. B. Burland
16
Normally-Consolidated
Sands
Over-Consolidated
Sands and Gravels
In the case of a propped wall, as the excavation depth
increases below the prop level, deeper wall movements into
the site occur. The maximum displacement within the profile
of the deflected wall is sometimes described as the 'belly' and
can influence deeper soil movements that may be observed
at surface level at a distance approximating this movement
below ground level. A typical wall profile is presented in
Figure 13.4(b
).
There have been many published articles discussing meas-
urements and patterns of retaining wall deflections together
with the associated ground displacements behind them. These
include: Peck (1969); Clough and O'Rourke (1990); St John
et al
. (1992); Fernie and Suckling (1996); Long (2001). Of
these, the proposals by Clough and O'Rourke (1990) are gen-
erally used most often. The data set used to form this approach
is taken from retaining walls founded within stiff clays. This
method defines an envelope of vertical and horizontal ground
deformations behind a retaining wall, related to the excavation
depth. This envelope is presented in
Figure 13.5
.
Burland
et al
. (2001) suggest that in most cases there is
a negligible risk of damage to a building suffering less than
10 mm settlement and a deformed slope (i.e. rotation) of less
than 1:500. The limiting horizontal strains of a building should
be reviewed separately to settlements with strains taken as the
building extension over a given length, generally defined by
column spacings. Burland suggests that negligible damage will
occur at strain levels below 0.05%. By using the ground dis-
placement envelopes in
Figure 13.5
it is therefore possible to
predict whether a building will fall outside these limits. Should
this be the case further analysis will normally be required
which may include some form of numerical modelling.
14
12
10
8
6
4
2
0
0
0.1
0.2
0.3
qnet/qult
Figure 13.3 Adapted from Stroud's correlation of soil stiffness and
SPT to magnitude of foundation load (Stroud, 1989)
The short-term settlement can also be estimated by relating the
drained stiffness of the clay to its undrained value for an iso-
tropic material using appropriate Poisson's ratio:
E
u
/E'= (1+v
u
)/(1+v')
13.2.1.2 Substructure and retaining wall movement prediction
During the initial stages of basement excavation it is common
to allow the retaining walls to cantilever either the full depth
of excavation or part of this depth prior to props being intro-
duced. During this stage the soil movements behind the wall
can be assumed to be at a maximum directly behind the wall
with magnitudes reducing linearly with distance away from
the wall (
Figure 13.4(a)
).
