For the soils in sloping ground, the effect of static shear stress must be considered to evaluate the cyclic behaviors of soils when subjected to seismic loading. This study aims to reveal the effect of both static shear stress magnitude and direction on the cyclic behaviors of the medium-dense sand based on a series of multi-directional cyclic simple shear tests. It is found that the effect of static shear stress on the liquefaction resistance of the medium-dense sand is detrimental in both parallel and perpendicular loading modes. The detrimental effect is more pronounced in parallel loading mode. Under the perpendicular loading mode, the full liquefaction of the specimens cannot be reached. The deformation pattern of the specimens is cyclic mobility along the cyclic loading direction, and plastic strain accumulation along the static stress direction. A modified pore pressure prediction model with two fitting parameters is further proposed to incorporate the effect of static shear stress.

Energy dissipation can macroscopically synthesize the evolutions in the microstructure of the marine clay during cyclic loading. Hence an energy-based method was employed to investigate the failure criterion and cyclic resistance of marine clay. A series of constant-volume cyclic direct simple shear tests was conducted on undisturbed saturated marine clay from the Yangtze Estuary considering the effects of the plasticity index (IP) and cyclic stress ratio (CSR). The results indicated that a threshold CSR (CSRth) exhibiting a power function relationship with IP exists in marine clay, which divides the cyclic response into non-failure and failure states. For failed specimens, the development of energy dissipation per cycle (Wi) with the number of cycles (N) exhibited an inflection point owing to the onset of serious damage to the soil structure. In this regard, the energy-based failure criterion was proposed by considering the inflection point as the failure point. Consequently, a model was proposed to quantify the relationships between failure energy dissipation per cycle (Wf) [or failure accumulative energy dissipation (Waf)], initial vertical effective stress, IP, and the number of cycles to failure (Nf,E). An evaluation model capturing the correlation among CSR, IP, and Nf,E was then established to predict the cyclic resistance, and its applicability was verified. Compared with the strain-based cyclic failure criterion, the energybased failure criterion provides a more robust and rational approach. Finally, a failure double-amplitude shear strain (gamma DA,f) evaluation method applicable to marine clay in different seas was presented for use in practical geotechnical engineering.

The amount of energy dissipated in the soil during cyclic loading controls the amount of pore pressure generated under that loading. Because of this, the normalized dissipated energy per unit volume is the basis for both pore pressure generation models and energy-based liquefaction analyses. The pattern of energy dissipation in the soil in load-controlled cyclic triaxial and load-controlled cyclic direct simple shear tests and displacement-controlled cyclic triaxial and displacement-controlled cyclic direct simple shear tests is quite different. As a result, the pattern of pore pressure generation associated with load-controlled tests is markedly different from that in displacement-controlled tests. Pore pressure generation patterns for each of the four test types were proposed based upon the manner in which the load was applied during the test and the soil's response to that loading. The results of four tests, two load controlled and two displacement controlled, were then used to verify these patterns. Pore pressure generation rates in load-controlled and displacement-controlled tests are different when plotted against their cycle ratios. Conversely, the tests produce nearly identical patterns when plotted against energy dissipation ratio. This occurs because of the relationship between energy dissipation ratio and pore pressure generation is independent of the loading pattern.

Fully liquefied soils behave like viscous fluids, and models developed within the framework of soil mechanics fail to catch their behaviour on the verge of liquefaction or after it. Several research works have shown that modelling the liquefied soil as a fluid is physically more convincing. Such an equivalent fluid can be characterised via an apparent viscosity (g) (sharply dropping when liquefaction is triggered) which can be modelled as a power law function of the shear strain rate (pseudo-plastic fluid), depending on two parameters: the fluid consistency coefficient (k) and the liquidity index (n). With this approach, it is possible to consider a simple correlation between the equivalent viscosity and pore pressure increments independent on the equivalent number of cycles, whose parameters can be calibrated from the results of stress-controlled laboratory tests. The paper investigates the effect of some relevant experimental factors (effective vertical stress, stress path, frequency and waveform of the applied cyclic load, soil fabric and pre-existing shear stress) on the apparent viscosity of soils during their transition from the solid to the liquefied state, and therefore also on the pore pressure increments generated by the stress path. To do that, the results of stress-controlled laboratory tests performed in a sophisticated simple shear apparatus, along with published data, have been interpreted in terms of the apparent viscosity. Simple correlations in terms of viscosity-based pore pressure generation and pseudo-plastic behaviour are proposed and confirmed from the results of 1D non-linear site response analysis for the (c) 2023 Production and hosting by Elsevier B.V. on behalf of The Japanese Geotechnical Society.