Civil Engineering Reference
In-Depth Information
Figures 21.3(e) and (f) illustrate mechanisms of failure of steep slopes in jointed
rock. The rock itself is very strong and slope failure occurs as blocks of rock move
on pre-existing joints. These mechanisms are not compatible because cracks have
opened. The relative spacing of the joints controls whether the failure is predomi-
nantly by sliding, as in Fig. 21.3(e), or by toppling, as in Fig. 21.3(f). Figure 21.3
illustrates only a few mechanisms, primarily related to sliding. Often combinations of
mechanisms occur with one mechanism changing into another or both taking place
simultaneously.
21.3 Parameters and factors for design of slopes
The limiting height and angle of a slope depend on the strength of the soil and there
are a number of different soil strengths. The most important distinctions are between
undrained strength and drained or effective stress strength and between peak, critical
state and residual strengths. Choices of soil parameters and factors for design of slopes
and other structures were discussed in Chapter 18.
For slopes the choice between undrained strength and drained strength is relatively
straightforward. For temporary slopes and cuttings in fine grained soils with low per-
meability you should choose the undrained strength
s
u
and do analyses using total
stresses. If you do this, remember that the analysis is valid only so long as the soil
remains undrained and the stability will deteriorate with time as pore pressures rise
and the soil swells and softens, as shown in Sec. 21.4. For a permanent slope the critical
conditions are at the end of swelling when pore pressures have reached equilibrium
with a steady state seepage flownet or with hydrostatic conditions. In this case you
can calculate the pore pressures, choose the drained strength and do analyses using
effective stresses. Analyses for slopes where the excess pore pressures have only partly
dissipated are beyond the scope of this topic.
As discussed in Chapter 18 slopes should be designed for an ultimate limit state
with a single factor of safety or with partial factors applied to each uncertainty. The
designs should use either the critical state strength (
s
u
or
φ
c
) or, if there has been pre-
φ
r
). The first thing to investigate is whether
a landslide or slope failure has occurred in a clay slope in the past, in which case
the soil in a slip plane may have already reached its residual state. New construction,
either excavation or loading, may reactivate the old movements and the appropri-
ate strength is the residual strength. Detection of old landslides, some of which may
be geologically old, requires very detailed and careful ground investigation. In the
absence of preexisting failures you should design safe slopes using the critical state
strength.
For design of safe slopes you can apply a single factor of safety or apply partial
factors to the parameters which control the stability. Usually a single factor of safety
is applied to the soil strength so the safe shear stresses in the soil
vious landsliding, the residual strength (
τ
s
are given by
τ
s
or
s
u
F
s
=
τ
=
s
us
(21.1)
s
φ
c
F
s
tan
τ
s
=
σ
=
σ
tan
φ
s
(21.2)
