Undrained Cyclic Simple Shear Tests Research Articles

Liquefaction behavior of inherently anisotropic sand under cyclic simple shear: Insights from three-dimensional DEM simulations

Inherent fabric anisotropy has been widely recognized to have significant effects on the liquefaction behavior of sand. A discrete element method (DEM) study was conducted to investigate the liquefaction behavior of inherently anisotropic sand under cyclic simple shear conditions. In the DEM model, the under-compaction method was modified to generate uniform and inherently anisotropic specimens. A series of DEM simulations of undrained cyclic simple shear tests were performed to investigate the effects of bedding plane angles with a full spectrum ranging from −90° to +90°. The relative magnitudes of the three effective normal stress components were compared and analyzed. The internal structure evolution was quantified through a contact-normal-based fabric tensor that could describe the load-bearing structure of granular specimens. The simulation results show that the effect of bedding plane angle on the liquefaction resistance of anisotropic sand is not monotonic. For example, as the bedding plane angle of specimen increases from 0° to 90°, the liquefaction resistance first decreases and then increases, with the most detrimental scenario occurs at 45°. This could be explained by correlating the magnitudes of shear modulus with the discrepancy between the loading direction and the direction of fabric during each cycle.

Influence of sloping ground conditions on cyclic liquefaction behavior of sand under simple shear loading

An experimental program of undrained cyclic simple shear tests was undertaken on Ticino sand, covering a broad range of initial relative densities and static stress levels, to investigate the effect of an initial static shear stress on failure mechanism, cyclic resistance, and pore water pressure generation. The results indicated that two types of failure mechanisms occurred, namely cyclic mobility and plastic strain accumulation, depending on the magnitude of the initial static shear stress with respect to the applied cyclic shear stress. Test results showed that the cyclic resistance of Ticino sand may either increase or decrease with an increasing initial static shear stress depending on the density state of the sand. The Kα correction factors measured under cyclic simple shear loading were compared with the values reported in the literature based on both cyclic triaxial and simple shear tests, suggesting that Kα could be dependent on the particle gradations and grain shape for a given depositional mechanism. Finally, it was verified that the initial static shear stress has a remarkable impact on the development of cyclic pore-water pressures (PWP) of the sands so that a modified PWP generation model has been effectively proposed in the present study.

Undrained Seismic Compression of Unsaturated Sand

Unsaturated soils in engineered geostructures like embankments or retaining walls may experience seismic compression during earthquakes due to particle rearrangement associated with large-strain cyclic shearing. Although previous studies have investigated volume changes during drained cyclic shearing of unsaturated soils, undrained cyclic shearing presents a more complex situation. During undrained cyclic shearing, changes in total volume, matric suction, degree of saturation, effective stress, shear modulus, and damping ratio may occur that have complex coupling effects that affect seismic compression. To better understand these coupling effects, this study performed a series of undrained cyclic simple shear tests on unsaturated sand specimens. Contractile volumetric strain after 200 cycles in these tests was found to vary nonlinearly with degree of saturation. The largest volumetric contraction occurred at a degree of saturation of 0.4 and coincided with the largest decrease in mean effective stress. The sand specimens followed a wetting-path scanning curve during shearing, with small changes in matric suction. The decrease in mean effective stress during cyclic shearing for all specimens followed the same linear relation with the accumulation of volumetric strain.

Modeling cyclic shearing of sands in the semifluidized state

SummaryLiquefaction is associated with the loss of mean effective stress and increase of the pore water pressure in saturated granular materials due to their contractive tendency under cyclic shear loading. The loss of mean effective stress is linked to loss of grain contacts, bringing the granular material to a “semifluidized state” and leading to development and accumulation of large cyclic shear strains. Constitutive modeling of the cyclic stress‐strain response in earthquake‐induced liquefaction and post‐liquefaction is complex and yet very important for stress‐deformation and performance‐based analysis of sand deposits. A new state internal variable named strain liquefaction factor is introduced that evolves at low mean effective stresses, and its constitutive role is to reduce the plastic shear stiffness and dilatancy while maintaining the same plastic volumetric strain rate in the semifluidized state. This new constitutive ingredient is added to an existing critical state compatible, bounding surface plasticity reference model, that is well established for constitutive modeling of cyclic response of sands in the pre‐liquefaction state. The roles of the key components of the proposed formulation are examined in a series of sensitivity analyses. Their combined effects in improving the performance of the reference model are examined by simulating undrained cyclic simple shear tests on Ottawa sand, with focus on reproducing the increasing shear strain amplitude as well as its saturation in the post‐liquefaction response.

Shear wave velocity-based evaluation of liquefaction resistance for calcareous sands of different origin

This paper presents the results of an experimental research carried out in laboratory aimed at establishing a relationship between cyclic liquefaction resistance (CRRfield) and corrected shear wave velocity (VS1) of carbonate sands. Two carbonate skeletal sands, namely Quiou and Dubai, were investigated together with a carbonate non-skeletal sand from Kenya. The programme included also tests on two silica sands prepared and tested in similar conditions for comparison purposes. The research was based on undrained cyclic simple shear tests and bender element tests in triaxial cell performed on reconstituted specimens of sand at different initial density and stress states. The results obtained highlight that data points for carbonate sands are all located to the right of the currently used cyclic liquefaction resistance vs. shear wave velocity curves suggested for silica sands, implying that the latter curves should be considered non-conservative when applied to carbonate crushable sands. An insight into the effect of the initial effective vertical stress on liquefaction susceptibility of carbonate sands was presented and a stress normalization procedure was also proposed.

Modified Bounding Surface Hypoplasticity Model for Sands under Cyclic Loading

AbstractA modified bounding surface hypoplasticity model is developed to capture distinct dilatancy behaviors of loose and dense sandy soils during various phases of undrained cyclic loading. Based on observations from laboratory tests, new modulus formulations are proposed to improve the simulation of cyclic mobility and postliquefaction behaviors of both loose and dense sands. The state-dependent dilatancy and effects of accumulated plastic strains on the plastic moduli are also incorporated in this model. The model demonstrates excellent capabilities through systematic comparison between the model predictions and a series of undrained cyclic simple shear tests on Fraser River sand.

Effects of boundary conditions and partial drainage on cyclic simple shear test results—a numerical study

AbstractOwing to imperfect boundary conditions in laboratory soil tests and the possibility of water diffusion inside the soil specimen in undrained tests, the assumption of uniform stress/strain over the sample is not valid. This study presents a qualitative assessment of the effects of non‐uniformities in stresses and strains, as well as effects of water diffusion within the soil sample on the global results of undrained cyclic simple shear tests. The possible implications of those phenomena on the results of liquefaction strength assessment are also discussed. A state‐of‐the‐art finite element code for transient analysis of multi‐phase systems is used to compare results of the so‐called ‘element tests’ (numerical constitutive experiments assuming uniform stress/strain/pore pressure distribution throughout the sample) with results of actual simulations of undrained cyclic simple shear tests using a finite element mesh and realistic boundary conditions. The finite element simulations are performed under various conditions, covering the entire range of practical situations: (1) perfectly drained soil specimen with constant volume, (2) perfectly undrained specimen, and (3) undrained test with possibility of water diffusion within the sample. The results presented here are restricted to strain‐driven tests performed for a loose uniform fine sand with relative density Dr=40%. Effects of system compliance in undrained laboratory simple shear tests are not investigated here. Copyright © 2004 John Wiley & Sons, Ltd.

Fundamentals of Liquefaction under Cyclic Loading

The mechanism of progressive pore pressure increase during undrained cyclic simple shear tests on saturated sands has been examined in detail. The concepts developed provide a better understanding of the physical processes leading to liquefaction of horizontal sand deposits during earthquakes. A quantitative relationship between volume changes occurring during drained cyclic tests and the progressive increase of pore water pressure during undrained cyclic tests has been developed. The use of this relationship enables the build-up of pore water pressure during cyclic loading to be computed theoretically using basic effective stress parameters of the sand. The application of the theory to the case of undrained cyclic simple shear tests with uniform stress controlled loading is illustrated. Values of progressive pore water pressure increases predicted theoretically agree reasonably well with the nature of pore water pressure increases observed experimentally.