Environmental Engineering Reference
In-Depth Information
11.4.3
Drawdown pore pressures and the analysis of stability
under drawdown conditions
11.4.3.1
Some general issues
When the reservoir level behind an embankment dam is lowered, the stabilizing influence
of the water pressure on the upstream slope is lost. If the water level is dropped suffi-
ciently quickly that the pore pressures in the slope do not have time to reach equilibrium
with the new reservoir water level, the slope is less stable.
Many failures of dam slopes (and natural slopes around reservoirs) have occurred dur-
ing drawdown. Duncan et al. (1990) give some examples.
It is often assumed, for design purposes, that the drawdown is rapid or even instanta-
neous. This assumption imposes severe loadings and it is often the controlling case for the
design of the upstream slope.
Terzaghi and Peck (1967) summarized the problems in estimating pore pressures dur-
ing drawdown in these words:
“… in order to determine the pore pressure conditions for the drawdown state, all the
following factors need to be known: the location of the boundaries between materials
with significantly different properties; the permeability and consolidation characteristics
of each of these materials; and the anticipated maximum rate of drawdown. In addition,
the pore pressures induced by the changes in the shearing stresses themselves … need to be
taken into consideration. In engineering practice, few of these factors can be reliably
determined. The gaps in the available information must be filled by the most unfavorable
assumptions compatible with the known facts.”
In view of the difficulty in estimating the pore pressures, particularly those developed
on shearing, in low permeability, clayey soils it is common practice to use an essentially
total stress analysis to assess the stability under drawdown. The recommended procedure
is detailed in Section 11.4.3.3. For free draining rockfill or gravels, the pore pressures will
dissipate as the reservoir level drops, so there are no residual pore pressures.
For well compacted, medium permeability soils, e.g. silty sands, pore pressures can
be estimated using flow nets. In these cases any shearing induced pore pressures will
be negative, due to dilation of the dense soil, so they can be conservatively ignored.
If the silty sands are loose and drawdown is in fact rapid, undrained strengths should
be used.
Duncan (1992) gives guidance on how to assess whether drainage will occur. This is
covered in Section 11.4.3.3.
11.4.3.2
Estimation of drawdown pore pressures, excluding the effects of
shear-induced pore pressures
Pore pressures in the upstream slope of a dam under drawdown conditions are deter-
mined by the change in water level, geometry and zoning of the slope, the relative perme-
abilities of the zones and, importantly, layers within each zone and the relative
permeability of the foundation.
Some of these points are illustrated in
Figure 11.20
and discussed below:
(i) In most cases the assumption is made that drawdown is “instantaneous”, and hence
the flownet changes from the steady state case to the drawdown case instantaneously.
This is somewhat conservative but sufficient for most analysis;
(ii) The pore pressures in the embankment are immediately reduced on drawdown due to
the unloading effect of removing the water. Hence pore pressures on the surface of
earthfill in Figures 11.20a to 11.20d are all zero;
