Behavior of saturated micaceous silty sand subjected to a combination of static and cyclic load using simple shear tests

Abstract

The dynamic behavior of saturated sands has been sufficiently studied from different points of view, but in laboratory tests only the stress conditions, prior to dynamic stress, are reproduced at the level of the surface; that is, for conditions of zero static shear stresses (τ_o). For this reason, the main objective of this research is to reveal the influence that static shear stresses (τ_o) have on the response of the dynamically saturated sands. The experimental phase included: identification and classification tests and six monotonic simple shear tests and 17 cyclic simple shear tests with different combinations of monotonic (τ_o) and cyclic shear stresses (τ_c) and in undrained conditions. The tested samples were extracted from the ‘La Bocana Norte’ sector of the Port of Barcelona–Spain. The samples were taken at depths between 31 m and 55 m, with a corresponding effective vertical stress (σ’_ov) between 290 kPa and 421 kPa, respectively. Based on the results of the tests, a detailed analysis was carried out on the influence of the combination of monotonic (τ_o) and cyclic (τ_c) shear stresses on the variation of the dynamic shear modulus (G). Specifically: in the stress–strain behavior, the development of both permanent (γ_p) and cyclic (γ_c) shear strain and the reduction of the effective stresses (σ′_v) as a consequence of the generation of pore water pressure (u). The results of the laboratory tests generally show that: (a) the combination of τ_o and τ_c controls the stress–strain behavior, the degradation of the cyclic shear modulus, the development of permanent and cyclic shear strain, the generation of pore water pressures and the liquefaction risk, (b) in the monotonic case, an empirical function was established to evaluate the internal friction angle (ϕ') as a function of the shear strain (γ_m) (mobilized friction angle), (c) in the combined case of monotonic and cyclical stress, it was possible to establish an empirical function among the G/σ′_ov ratio, the cyclic strain (γ_c) and the number of cycles (N) and among the σ′_v/σ′_ov ratio, the cyclic strain (γ_c) and the number of cycles (N), (d) based on empirical functions, a simple methodology can be proposed to assess liquefaction risk and (e) it is not always true that the liquefaction phenomenon occurs when the effective vertical stress (σ’_v) is reduced to zero. Depending on the cyclic stress ratio (τ_c/σ’_ov) liquefaction can occur in the first cycle with a reduction in vertical effective stress of just 70%.

Keywords:

• Saturated sand
• shear modulus
• pore water pressure
• 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 cyclic simple shear equipment; to the Ports of the State of Spain, for supplying all the samples used during the laboratory tests and to 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.