Civil Engineering Reference
In-Depth Information
means appropriate reduction in the upper part of the passive zone and probably in the
active zone down to the dredge level. This practice has been successfully followed in
many designs in combination with appropriate selection of factors against wall failure
by rotation.
It is suggested that on the passive side of a wall the value of
c
should be taken as
zero at the dredge level where, in the case of both cantilever and propped walls, the
strain will be at its maximum value due to both swelling and wall movement, and that
it may gradually be increased to the peak laboratory value at the toe of the wall or
over a depth of 1 to 2m below the dredge level whichever is the lesser depth.
On the active side of the wall because of the wall deflection and consequent soften-
ing, it is suggested that above the dredge level
c
should be reduced to about half its
laboratory value.
It is sometimes argued that for temporary works in stiff clay, the undrained triaxial
test results may be used in design, perhaps modified or reduced by some arbitrary
factors to take into account the partial reduction of strength with time. The rate of
change of intergranular stress in the soil is dependent on the soil fabric: fine sandy
or silty partings, fissures, laminations and joints. Failed cuttings and trenches in stiff
clay are often seen to have a blocky appearance. The material beneath the surface of a
block may have a strength near that which would be measured in an undrained triaxial
test, so that comparatively small volumes of absorbed water only may be involved in
softening along joints, and yet a face has failed that would not have done so at normal
undrained strengths. For this reason the time taken for the transition between short-
and long-term strength is very unpredictable, and in designing walls for a temporary
state it seems wiser to use realistic long-term effective stress parameters together with
a low factor on stability, rather than undrained strengths and a much higher but
uncertain factor.
As in the case of soft clays, it is usually undesirable to rely on loose sands for
wall support in the passive zone because of the large strains that may occur. In order
to arrive at an appropriate angle of internal friction for a sand, the current practice
is usually to carry out Standard Penetration Tests, bearing in mind the corrections
that may be necessary in the
N
value for tests at shallow depth and for fine soils in
submerged conditions. The general quality and consistency of Standard Penetration
Tests should also be considered bearing in mind possibilities of 'blowing' conditions
in boreholes or the presence of larger soil particles obstructing the test tool.
Various proposals have been made for shallow depth correction but it is suggested
that the corrections given on Figure 2.8 are appropriate.
The correction of
N
for submerged conditions applies only to very fine sand or silt
below the water table, and is due to the excess pore water pressures set up during
driving of the sampler. In this case
1
2
(
N
measured
−
N
corrected
=
15
+
15)
(6.1)
The approximate relationship given by Peck
et al
. (1974) may then be used to determine
the angle of internal friction
φ
, but as this does not fully take into account various
features of cohesionless soils it is probably better to use the system proposed by Stroud
(1989) (Chapter 2, section 2.5.4).
