Pore water pressure developed by a soft fine-grained soil subjected to combined monotonic and cyclic shear stresses

Abstract

The magnitude of the pore water pressure developed by a soft fine-grained soil subjected to shear stresses is what determines the occurrence of sudden failure. As part of an extensive testing programme, besides identification and classification tests, 20 monotonic simple shear tests and 139 cyclic simple shear tests with different levels of static stresses (τo ) before the cyclic phase were performed. In this research, reference is made to pore water pressure (u) developed by a soft fine-grained soil subjected to combined monotonic (τo ) and cyclic shear (τc ) stresses. Laboratory tests were carried out on undisturbed samples from the Port of Barcelona, Spain. The samples were obtained at depths between 29 m and 52 m, that correspond to an effective vertical stress of (σ’ov ) between 277 kPa and 413 kPa, respectively. In general, the results indicate that: (a) the combination of τo and τc controls the generation of pore water pressure (u); (b) it is not always true that, as a consequence of increased pore water pressure when τo/σ’ov + τc/σ’ov ≈ Su/σ’ov , the soil reaches the failure cyclically (with Su being the undrained shear strength); (c) in the monotonic case, it is possible to establish empirical functions between u/σ’ov and the monotonic shear strain (γm ); and (d) in the combined monotonic and cyclic case, it is possible to establish empirical functions among u/σ’ov , the cyclic shear strain (γc ) and the number of cycles (N), for different (τo/σ’ov ) ratios. In addition, an application was made to study the seabed stability based on the knowledge of the material under cyclic load.