The following critical conditions must be analysed (see Fig. 2.14):
- end of construction (both slopes);
- steady state, reservoir full (downstream slope critical);
- rapid drawdown (upstream slope critical);
- seismic loading additional to 1, 2 and 3, if appropriate to the location.
Analysis is in terms of effective stress shear strength parameters cand,
with porewater pressures, uw, or the pore-pressure ratio, ru. Total stress
parameters,cand, are suitable only for a short-term and approximate
analysis, e.g. stability at an intermediate stage during construction.
Design parameters to be employed in stability analysis may be
summarized as follows.
- During and at end of construction.Earthfill is compacted in a par-
tially saturated state, i.e. initial values of uware negative. For an
upper bound, and assuming no dissipation of porewater pressure,
pore-pressure ratio ruB ̄, with pore pressure coefficient B ̄(Section
2.3.1) set by the stress state within the dam. If high values of ruare
anticipated excess porewater pressures may be relieved by horizontal
drainage layers (Fig. 2.10). The design and spacing of such layers,
which should have a permeability of 10^5 –10^6 times that of the fill, is
discussed in Gibson and Shefford (1968).
- Steady-state seepage, reservoir full.Effective stress analysis should
always be employed. Values of ruin excess of 0.45 may occur in old
homogeneous clay dams; effective internal drainage blankets may
reduceruvalues to 0.10–0.20.
84 EMBANKMENT DAM ENGINEERING
Fig. 2.13 Stability analysis: failure surface schematics
