The stability of a silty clay under repeated loading

The object of this research programme was to investigate the stability of a silty clay under undrained repeated loading. Equipment has been designed and developed for the application of a quasi-sinusoidal compressive load form to triaxial samples. A series of monotonic strain controlled triaxial tests with pore pressure measurement was performed on normally consolidated and overconsolidated samples of saturated reconstituted Keuper Marl. These tests defined the critical state parameters, and hence the stable state boundary surface for this soil, and provided a basis for a comparison with the results of cyclic stress controlled triaxial tests. Two series of cyclic triaxial compression tests were performed: firstly, single stage tests using a constant level of repeated stress; and secondly, multistage tests. These aimed to investigate: a) stress conditions at failure under cyclic loading; b) strength of non-failed samples after cyclic loading; and c) strain and pore pressure behaviour under repeated loading. Criteria for the stability of a soil under cyclic loading are presented and a model is developed based on the pore pressure behaviour of the soil under repeated loading. This model can be used to predict pore pressures and failure for normally consolidated and lightly overconsolidated soils.