ABSTRACT
CLAY SLOPES Clays are the weakest, most unstable, slope material. Undisturbed clays can stand in steep temporary slopes rendered stable by cohesion, pore water suction and peak frictional strength. Disturbed clays, and those that have been restructured through creep over time, commonly have internal realignment of the clay particles so that they are roughly parallel. This reduces their internal friction and also eliminates cohesion; at the same time drainage equilibrates and eliminates suction. Natural slopes, with long-term stability, depend on internal friction only. Saturated clay soils have nearly half their weight carried by pore water pressure; so slope is generally stable at angle close to φr/2, with residual value of φr (section 26). London Clay has φr 20; slopes are stable at 10, and do not exist at 12. Old slides are at residual strength; they are reactivated mainly by head loading or toe removal. Failure surface in homogeneous soil is a slip circle. Critical slip circle, of lowest safety factor, is found by tedious calculation of all possible circles, summing data on slices and using iterative methods; this is now always done by computer. Back analysis obtains soil strength parameters by stability analysis of failed slopes when safety factor 1.
