ABSTRACT
34 Soil Failures and Flowslides CLAY SLOPES Clays are the weakest, most unstable, slope material. Undisturbed ciays can stand in steep temporary slopes held by cohesion, pore water suction and peak frictional strength. Disturbances or restructuring through creep over time causes realignment of the clay particles to parallel. This reduces internal friction and eliminates cohesion; at the same time drainage equilibriates 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 stable at angle close to <)>r/2, with residual value of 4>r (section 26). London Clay has <|>r = 20°; slopes are stable at < 10°, and do not exist at > 12°. Old slides are at residual strength; 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; now always by computer. Back analysis obtains soil strength parameters by sta bility analysis of failed slopes when safety factor = 1.
