A group of earthquakes typically consists of a mainshock followed by multiple aftershocks. Exploration of the dynamic behaviors of soil subjected to sequential earthquake loading is crucial. In this paper, a series of cyclic simple shear tests were performed on the undisturbed soft clay under different cyclic stress amplitudes and reconsolidation degrees. The equivalent seismic shear stress was calculated based on the seismic intensity and soil buried depth. Furthermore, reconsolidation was conducted at the loading interval to investigate the influence of seismic history. An empirical model for predicting the variation of the accumulative dissipated energy with the number of cycles was established. The energy dissipation principle was employed to investigate the evolution of cyclic shear strain and equivalent pore pressure. The findings suggested that as the cyclic stress amplitude increased, incremental damage caused by the aftershock loading to the soil skeleton structure became more severe. This was manifested as the progressive increase in deformation and the rapid accumulation of dissipated energy. Concurrently, the reconsolidation process reduced the extent of the energy dissipation by inhibiting misalignment and slippage among soil particles, thereby enhancing the resistance of the soft clay to subsequent dynamic loading.

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.

This study explores the effectiveness of soft viscoelastic biopolymer inclusions in mitigating cyclic liquefaction in loosely packed sands. This examination employs cyclic direct simple shear testing (CDSS) on loose sand treated with gelatin while varying the gelatin concentration and the cyclic stress ratio (CSR). The test results reveal that the inclusion of soft, viscoelastic gelatin significantly reduces shear strain and excess pore pressure during cyclic shear. Liquefaction potential, defined as the number of cycles to liquefaction (NL) at an excess pore pressure ratio (ru = Delta u/sigma ' vo) of 0.7, is substantially improved in gelatin-treated sands compared to gelatin-free sands. This improvement in liquefaction resistance is more pronounced as the inclusion stiffness increases. Furthermore, the viscoelastic pore-filling inclusion helps maintain skeletal stiffness during cyclic shearing, resulting in a higher shear modulus in gelatin-treated sand in both small and large-strain regimes. At a grain scale, pore-filling viscoelastic biopolymers provide structural support to the skeletal frame of a loosely packed sand. This pore filler mitigates volume contraction and helps maintain the effective stress of the soil structure, thereby reducing liquefaction potential under cyclic shearing. These findings underscore the potential of viscoelastic biopolymers as bio-grout agents to reduce liquefaction risk in loose sands.

In practical engineering, earthquake-induced liquefaction can occur more than once in sandy soils. The existence of low-permeable soil layers, such as clay and silty layers in situ, may hinder the dissipation of excess pore pressure within sand (or reconsolidation) after the occurrence of liquefaction due to the mainshock and therefore weaken the reliquefaction resistance of sand under an aftershock. To gain more mesomechanical insights into the reduced reliquefaction resistance of the reconsolidated sand under aftershock, a series of discrete element simulations of undrained cyclic simple shear tests were carried out on granular specimens with different degrees of reconsolidation. During both the first (mainshock) and second (aftershock) cyclic shearing processes, the evolution of the load-bearing structure of the granular specimens was quantified through a contact-normal-based fabric tensor. The interplay between mesoscopic structure evolutions and external loadings can well explain the decrease in reliquefaction resistance during an aftershock.

Gassy clay, commonly encountered in coastal areas as overconsolidated deposits, demonstrates distinct mechanical properties posing risks for submarine geohazards and engineering stability. Consolidated undrained triaxial tests combined with cyclic simple shear tests were performed on specimens with varying overconsolidation ratios (OCRs) and initial pore pressures, supplemented by SEM microstructural analysis. Triaxial results indicate that OCR controls the transitions between shear contraction and dilatancy, which govern both stress-strain responses and excess pore pressure development. Higher OCR with lower initial pore pressure increases stress path slope, raises undrained shear strength (su), reduces pore pressure generation, and induces negative pore pressure at elevated OCR. These effects originate from compressed gas bubbles and limited bubble flooding under overconsolidation, intensifying dilatancy during shear. Cyclic tests reveal gassy clay's superior cyclic strength, slower pore pressure accumulation, reduced stiffness softening, and enhanced deformation resistance relative to saturated soils. Cyclic pore pressure amplitude increases with OCR, while peak cyclic strength and anti-softening capacity occur at OCR = 2, implying gas bubble interactions.

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.

In biopolymer-soil stabilization, biopolymers function in the soil either as viscous fluids or rigid gels. However, the influence of these hydrogel states on soil liquefaction resistance and their underlying mechanisms remain insufficiently understood. This study examines the seismic response of sand treated with biopolymers under small-to-medium strain cyclic loading, with a focus on the efficacy of Cr3+-crosslinked xanthan gum (CrXG) in mitigating liquefaction. Liquefaction resistance and dynamic properties of CrXG-treated soil were compared against thermogelation and non-gelling viscous biopolymer treatments using cyclic direct simple shear and resonant column tests. CrXG treatment at 1 % content improved liquefaction resistance (CRR10) from 0.088 to 0.687 by preventing shear strain accumulation and pore pressure buildup, with enhancing dynamic shear stiffness and delaying stiffness degradation and damping ratio changes to higher strain levels. In contrast, soils treated with non-gelling viscous XG exhibited limited reinforcement under large strain cyclic loading, showing earlier liquefaction and lower CRR10 compared than untreated sand, alongside reduced dynamic shear modulus and rapid stiffness degradation. Comparisons across varying earthquake moment magnitudes revealed that CrXGtreated soil achieved liquefaction resistance comparable to other soil stabilization methods and demonstrated greater improvement efficiency than thermogelation biopolymers requiring thermal treatment. These findings highlight the potential of CrXG as a sustainable and practical solution for improving liquefiable soil stability under seismic loading.

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.

Loess exhibits typical water sensitivity and dynamic vulnerability. The combination of rainfall and earthquakerelated issues presents a complex disaster process, posing a significant threat to the infrastructure in the loess region. A cyclic simple shear test was conducted on undisturbed loess under a constant vertical stress ranging from 50 to 300 kPa, comprising three stages(C-W-D): consolidation, pre -humidification, and cyclic loading. The deformation behavior under humidification and cyclic loading was analyzed. The microstructure evolution of loess during three stages was examined using scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP). Results indicated that: (1) Cyclic deformation increased with the rise in vertical stress sigma v , humidification parameters S w , and dynamic shear stress amplitude gamma d . The sensitivity of cyclic deformation to sigma v and gamma d gradually decreased as S w increases. (2) The total deformation in the three stages correlated positively with S w , sigma v , and gamma d . The proportion of humidification deformation and cyclic deformation in the total deformation was largely unaffected by sigma v , with cyclic deformation gradually dominating as gamma d increases. (3) The prehumidification stage promoted aggregates and the formation of numerous intergranular pores. Cyclic loading mainly leads to the change of pore structures, forming obvious seismic damage area. Based on the relationship between humidification deformation and cyclic deformation, a loess deformation prediction model was proposed, which can comprehensively consider S w , sigma v , and gamma d . This can provide a theoretical reference for earthquake disaster prediction in collapsible loess areas.