Seasonal ratcheting and softening in clay slopes, leading to first-time failure

Centrifuge tests have been carried out on kaolin clay slopes subject to variations in surface rainfall and humidity corresponding, at model scale, to successive wet and dry seasons in the field. These model slopes have been instrumented with miniature high-capacity tensiometers, and the deformations of their cross-sections have been observed by digital photography and analysed by particle image velocimetry. Sequences of swelling and shrinkage have been seen to be potentially irreversible, leading to creep in the form of down-slope ratcheting, accompanied by progressive regional softening within the zone affected by the seasonal moisture movements. Ultimately, this regional softening has been seen to lead to slope failures, in which segments of soil have separated from the mass through the opening of tension cracks and the formation of a localised shear rupture. An analysis of the phase of episodic regional softening is presented here, based on a Spencer limit-equilibrium approach. These back-analyses illustrate that clay slopes which temporarily mobilise an average stress ratio in excess of the critical state stress ratio during any portion of a typical year may eventually be brought to a long-term failure under the action of seasonal variations of pore pressure. Conversely, it is hypothesised that clay slopes which can maintain their overall equilibrium without exceeding the critical state stress ratio may not experience progressive seasonal softening.