Civil Engineering Reference
In-Depth Information
or displacement in undrained tests, at least up to the critical state. The question is
which of these strengths - peak, critical state or residual - should be used as design
parameters.
Residual strength
After very large displacements the shear stress which can be mobilized is the residual
strength
τ
r
. For drained loading this is given by
τ
r
=
φ
r
(
σ
−
u
) tan
(18.3)
φ
r
is a material parameter.
Figure 18.3(a) shows a pile driven into the ground. Some of the load on the pile is
transferred into the ground by shear stresses between the side of the pile and the soil.
Driving the pile into the ground has generated large displacements between the pile
and the soil and so the limiting shear stress between the side of the pile and the ground
is governed by the residual strength. It will also be limited by shearing on the interface
between the soil and the pile if this is smaller. Figure 18.3(b) shows an old landslide. The
near-surface soil has slid down-slope along a clearly defined slip surface. In analyses
of the stability of the slope the limiting shear stress across this slip surface is governed
by the residual strength.
Since the residual strength of a clay soil is often very much smaller than its critical
state and peak strengths it is essential to discover the presence of pre-existing slip
surfaces in the ground. This requires careful and detailed ground investigation. Old
landslides can often be very difficult to detect as surface changes over geological time-
scales, together with vegetation, hide the evidence of the movements.
where the residual friction angle
Critical state strength
The critical state strength is reached after relatively large strains of the order of 10%,
as illustrated in Fig. 18.2. Very much larger strains and displacements are required to
Figure 18.3
Cases where the limiting shear stress is the residual strength.
