The foundation soil below the structure usually bears the combined action of initial static and cyclic shear loading. This experimental investigation focused on the cyclic properties of saturated soft clay in the initial static shear stress state. A range of constant volume cyclic simple shear tests were performed on Shanghai soft clay at different initial static shear stress ratios (SSR) and cyclic shear stress ratios (CSR). The cyclic behavior of soft clay with SSR was compared with that without SSR. An empirical model for predicting cyclic strength of soft clay under various SSR and CSR combinations was proposed and validated. Research results indicated that an increase of shear loading level, including SSR and CSR, results in a larger magnitude of shear strain. The response of pore water pressure is simultaneously dominated by the amplitude and the duration of shear loading. The maximum pore water pressure induced by smaller loading over a long duration may be greater than that under larger loading over a short duration. The initial static shear stress does not necessarily have a negative impact on cyclic strength. At least, compared to cases without SSR, the low-level SSR can improve the deformation resistance of soft clay under the cyclic loading. For the higher SSR level, the cyclic strength decreases with the increase of SSR.

The mechanical properties and constitutive model of unsaturated soils under cyclic loading are crucial for understanding the behavior of foundations and slopes subjected to dynamic motions such as earthquakes and traffic loading. In this study, multilevel strain-controlled cyclic simple shear tests of unsaturated weathered red mudstone (WRM) were conducted. The detailed investigation focused on cyclic responses, including shear stressstrain behavior and volume change, strain-dependent secant shear modulus and damping ratio, and stress-dilatancy behavior. This study revealed the significant influences of the degree of saturation and vertical stress on these responses, with the initial static shear stress mainly affecting the shear stress-strain behavior and volume changes at the initial loading stage. Based on the experimental observations, a cyclic constitutive model was proposed for unsaturated WRM. The model incorporates a slightly revised Davidenkov model and Masing criterion to generate shear stress-strain hysteresis loops with or without initial static shear stress. Additionally, a stress-dilatancy equation was included to capture the volume changes during cyclic loading. The proposed model was verified by comparing representative test data and calculation results, demonstrating the excellent performance of the proposed model in modeling the main features of unsaturated WRM under cyclic loading.

Soil liquefaction response is significantly affected by soil gradation (particle size, angularity, coefficient of uniformity) and density. However, the literature on the factors affecting liquefaction resistance with initial static shear stress (e.g., sloping ground) is more limited and primarily based on clean, poorly graded sands. As a result, the influence of particle size and gradation on the liquefaction potential of soils with initial shear stress is overlooked. In this study, 223 large-size cyclic simple shear tests were conducted on poorly and well-graded sands and gravels to evaluate the effects of soil gradation on the liquefaction resistance with the presence of initial static shear stress. Sandy and gravelly soils with coefficients of uniformity ranging from 1.6 to 42 were tested in a large-scale cyclic simple shear device under constant volume conditions, and the initial static shear stress correction factor K alpha values were obtained. The results show that poorly graded sand specimens exhibit flow liquefaction, have a more significant vertical effective stress reduction as the initial static shear stress increased, but also exhibit beneficial effects of initial static shear stress even if loosely packed, mainly due to their more dilative nature. Well-graded sandy soils, on the other hand, did not have as an abrupt loss of stiffness compared to poorly graded sand specimens, but due to their higher coefficient of uniformity may be more contractive, causing more pronounced shear strain development at the last few cycles. Gravel content also affected the void ratio of sand, which influenced the onset of strain softening or hardening during cyclic loading. Dense specimens with initial static shear stress exhibit cyclic mobility, but this may not necessarily provide beneficial effects of the K alpha correction factor, especially for higher coefficients of uniformity. The experimental results suggest that the widely used K alpha correction factor approaches that were originally suggested based on poorly graded sand may be overoptimistic for both loose and dense soils when considering a broader spectrum of soils such as those encountered in engineering practice. It is proposed that the K alpha correction factor should consider not only relative density and initial static shear stress but also particle size and gradation (i.e., determining the gravel content and the coefficient of uniformity), as well as angularity.

There are many geotechnical applications involving dams, embankments and slopes where the presence of an initial static shear stress prior to the cyclic loadings plays an important role. The current paper presents the experimental results gathered from undrained cyclic simple shear tests carried out on non-plastic silty sand with fines content in the range 0-30% with the consideration of sustained static shear stress ratio (alpha). Two distinct parameters, namely the conventional state parameter Psi, and the equivalent state parameter Psi*, are introduced in the context of critical state soil mechanics (CSSM) framework to predict failure mode and undrained cyclic resistance (CRR) of investigated soils. It is proved that the failure patterns for silty sands are related to (a) the initial states of soils (Psi or Psi*) and (b) the combination of initial shear stress with respect to cyclic loading amplitude. At each alpha, the CRR-Psi (or Psi*) correlation can be well represented by an exponential trend which is practically unique for both clean sands and silty sands up to a threshold fines content (f thre congruent to 24.5%). Varying alpha from low to high levels simply brings about a clockwise rotation of the CRR-Psi (or Psi*) curves around a point. This CRR-Psi (or Psi*) platform thus provides an effective methodology for investigating the impact of initial shear stress on the cyclic strength of both clean sands and silty sands. The methodology for estimating Psi (or Psi*) state parameters from in-situ cone penetration tests in silty sands is also discussed.

Earthquake-induced lateral spreading usually takes place under a stress state governed by the presence of initial static shear stress coupled with time varying cyclic stresses. However, due to the unavailability of high-quality element test data conducted with an initial static shear bias, usually the numerical modeler ignores the presence of static shear stresses on a soil element. This paper presents the initial calibration framework and the subsequent numerical insights on the deformation mechanism for an embedded cantilever retaining wall subjected to dynamic loading, when the initial static shear stress is comprehensibly considered in the constitutive model framework. For the sake of effective comparison, calibrations were also conducted without any static shear stress bias. A cocktail glass model was used to represent the soil elements, for which the initial calibration was conducted based on the results of cyclic direct simple shear tests. The constitutive model was able to capture important features arising due to the initial static shear stress including considerable lesser degradation in the shear modulus of soil due to limited generation of excess pore pressure under the subsequent undrained cyclic loading. Post initial calibration, a system level performance was conducted to evaluate the effects of static shear in terms of excess pore pressure generation and sheet-pile deformation mechanism. The simulations revealed the occurrence of predominant dilative responses representing the soil stiffening during the cyclic shearing without the application of initial static shear bias as compared to a case with initial static shear stress leading to a different deformation mechanism.