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 (τ_o) 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 (τ_o) and cyclic shear (τ_c) 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 τ_o and τ_c controls the generation of pore water pressure (u); (b) it is not always true that, as a consequence of increased pore water pressure when τ_o/σ’_ov + τ_c/σ’_ov ≈ S_u/σ’_ov, the soil reaches the failure cyclically (with S_u 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 (γ_m); and (d) in the combined monotonic and cyclic case, it is possible to establish empirical functions among u/σ’_ov, the cyclic shear strain (γ_c) and the number of cycles (N), for different (τ_o/σ’_ov) ratios. In addition, an application was made to study the seabed stability based on the knowledge of the material under cyclic load.

Keywords:

• Pore water pressure
• soft fine-grained soil
• monotonic simple shear
• cyclic simple shear
• empirical functions

Acknowledgements

The authors are grateful for the José Entrecanales Ibarra Foundation, for the donation of the necessary equipment to carry out this investigation, mainly the donation of the cyclic simple shear equipment. It is also necessary to thank the entity Ports of the State of Spain, for the supply of all the samples used to carry out the laboratory tests, and the late Professor Antonio Soriano Peña for his unconditional support for research at the UPM.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was funded by Fundación José Entrecanales Ibarra and Ports of the State, Government of Spain.