First-Time Slides in Over-Consolidated Clays

(a) After a slide has taken place the strength on the slip surface is equal to the residual value. The residual strength is associated with strong reorientation of the clay particles and is represented by an angle of shearing resistance $= which in most clays is considerably smaller than the value of 4’ at peak strength (Fig. 1). (b) First-time slides in slopes in non-fissured clays correspond to strengths only slightly less than the peak. (c) First-time slides in fissured clays correspond to strengths well below the peak. (d) Some form of progressive failure must be operative to take the clay past the peak. This could be simply the result of a non-uniform ratio of stress to strength along the potential slip surface; but probably the fissures play an important role as stress concentrators and in leading to softening of the clay mass. (e) Granted a progressive failure mechanism the limiting strength would be residual, and it is therefore reasonable to express the actual strength at the time of failure as a function of the upper (peak) and lower (residual) limits, i.e. by the residual factor. (f) In natural slopes of London CIay the strength has fallen approximateIy to the residual value. (g) The residual strength obtains on pre-existing shear surfaces, whether these are the result of tectonic shearing or old landslides.