Stability of Slopes - Elements of Soil Mechanics - page 391

Fig. 13.6 Example of two possible slip surfaces.
Fig. 13.7 Types of slip failures.
and (c) repeating the process until the worst slip surface is found as illustrated in Fig. 13.6. The worst slip
surface is the surface which yields the lowest factor of safety, F, where F is the ratio of the restoring
moment to the disturbing moment, each moment considered about the centre of rotation. The methods
of assessing stability using this moment equilibrium approach are described in the next few sections.
Alternatively, if stability assessment is to be performed in accordance with Eurocode 7, the strength
parameters of the soil are first divided by partial factors, and stability is then confirmed by checking the
GEO limit state (see Section 13.5) .
Regardless of the approach, the critical slip circle is found by considering several trial circles, each dif-
fering by the location of their centre, and identifying the one that returns the lowest measure of safety.
This is achieved nowadays by using specific slope stability software that can perform repeated analyses
in seconds and rapidly find the location of the centre of the critical slip circle.
In the case of soils with angles of shearing resistance that are not less than 3°, the critical slip circle is
invariably through the toe - as it is for any soil (no matter what its φ ′ value) if the angle of slope exceeds
53° (Fig. 13.7a ). An exception to this rule occurs when there is a layer of relatively stiff material at the
base of the slope, which will cause the circle to be tangential to this layer (Fig. 13.7b ).
For cohesive soils with little angle of shearing resistance the slip circle tends to be deeper and usually
extends in front of the toe (Fig. 13.7c ); this type of circle can of course be tangential to a layer of stiff
material below the embankment which limits the depth to which it would have extended (Fig. 13.7 d).