DHARMSTABILITY OF EARTH SLOPES 331
For soils with φ </ 3°, the critical slip circle is invariably through the toe. It is so irrespec-
tive of the value of φ for inclination of slopes exceeding 53°. However, when there is a stiff layer
at the base of the slope, the slip circle will be tangential to it.
For cohesive soils with a small friction angle the slip circle tends to be deeper and may
extend in front of the toe. If there is a stiff layer below the base, the depth of the slip circle
would be limited by this stiff layer.
These various possibilities are illustrated in Fig. 9.15.f<3°f >3°(a) Toe failure (b) Slip circle tangential to base (stiff layer)(c) Deep circle passing below base (d) Slip circle tangential to stiff layer below base
Fig. 9.15 Types of slip surfaces9.3.2 Effective Stress Analysis
Total stress analysis is applicable for the analysis of stability of a slope soon after construction
under undrained conditions. If pore water pressures exist in an embankment under certain
conditions of drainage or seepage, an analysis in terms of effective stress is considered appro-
priate; in fact, this method is applicable at any stage of drainage–from no drainage to full
drainage–or for any value of the pore pressure ratio, ru. Depending upon the situation, the
pore water pressure may depend upon the ground water level within the embankment or the
flow net pattern owing to the impounded water. It may also depend upon the magnitude of the
applied stresses, for example, during rapid construction of an earth dam or embankment.
Steady Seepage
The case when steady seepage is occurring at the maximum possible rate through an
earth dam or an embankment is considered as the critical condition for the stability of the
downstream slope.