Cyclic strength of loose anisotropically-consolidated calcareous sand under standing waves and assessment using the unified cyclic stress ratio

Abstract

Progressive and standing waves can produce instability of the seabed and seabed-supported marine structures. In this paper, the stress paths induced by standing waves were deduced to provide a theoretical basis for laboratory element tests to establish the cyclic behavior of marine sediments, which can serve to improve the understanding of seabed liquefaction triggering and its consequences. The cyclic response of loose, isotropically- and anisotropically-consolidated (IC and AC, respectively) calcareous sand under the stress paths derived for standing waves was examined using hollow cylinder torsional shear (HCTS) tests. The stress paths corresponding to standing waves are a vertical line, horizontal line, and ellipse, in a coordinate system consisting of the axial stress difference and the horizontal shear stress, for a soil element located at the nodes, antinodes, and intermediate horizontal positions, respectively. The phase difference, θ, between axial stress difference and horizontal shear stress appears to range from −45° ~ 7° for wave and seabed characteristics representative of the North Sea. The ratio of axial stress difference and horizontal shear stress amplitudes, λ, varies in the real number field. The experimental results indicated that the failure modes of IC and AC specimens are cyclic mobility and residual deformation failure, respectively, irrespective of the stress paths imposed. With the same consolidation state and cyclic stress ratio, CSR, the number of cycles to failure, Nf, is strongly affected by θ and λ. The unified cyclic stress ratio, USR, is proposed to establish a single USR-Nf curve that can suitably describe the general cyclic strength of the calcareous sand specimens under different stress paths for a given consolidation stress ratio. The proposed USR enables estimation of the cyclic strength of IC and AC soils under complex stress paths based on the CSR-Nf curve obtained from conventional cyclic torsional shear or triaxial tests.