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.

Waves can cause significant accumulation of pore water pressure and liquefaction in seabed soils, leading to instability of foundations of marine hydrokinetic devices (MHKs). Geostatic shear stresses (existing around foundations, within slopes, etc.) can substantially alter the rate of pore pressure buildup, further complicating the liquefaction susceptibility assessments. In this study, the development of wave-induced residual pore water pressure and liquefaction within sandy seabed slopes supporting MHK structures is evaluated. Unlike most earlier studies that excluded the impact of shear stress ratios (SSR) on the residual pore pressure response of sloping seabeds, asymmetrical cyclic loadings are considered herein for a range of SSRs. To obtain wave-induced loading in the seabed (and cyclic shear stress ratios, CSRs), the poroelasticity equations governing the seabed response, coupled with those for fluid and structure domains, are solved simultaneously. Utilizing an experimental model based on anisotropic cyclic triaxial test data that includes CSR and SSR impacts, an equation for the rate of pore pressure buildup is developed and added as a source term to the 2D consolidation equation. Numerical investigations were performed by developing finite element models in time domain. The models were calibrated using particle swarm optimization method and validated against wave flume experimental data. The results indicate that the consideration of static shear stresses has led to sudden rise in residual pore pressures followed by fast dissipations at early and late time steps, respectively, beneath the structure. The exclusion of SSR is shown to cause significant overestimation of pore pressure accumulations at late cycles, potentially causing significant overdesign of MHK foundations. The impact of proximity to the free drainage boundary, CSR amplitude, and loading frequency on the accumulation of residual pore pressure is illustrated. The residual liquefaction susceptibility of the seabed is shown to decline by increase of the seabed slope angle.

In eutrophic shallow lakes, cyanobacterial blooms will occur frequently and then settle into sediment, leading the formation of fluid sediment. Several factors including temperature can influence surface sediment properties. In this study, the influence of temperatures on surface sediment properties was determined in microcosm experiments through monitoring sediment physicochemical and rheological properties. During one-month incubation, it was found that surface sediment density and water content varied exponentially with increase in temperatures from 10 to 35 degrees C. The results of particle size distribution indicated that cyanobacterial blooms biomass (CBB) degradation in sediment led to sediment flocculation and agglomeration. In the meantime, there were high ratios polysaccharide/protein in extracellular polymeric substances (EPSs), which enhanced the sediment particle agglomeration. Further, the yield stress in rheological test for sediment with ( R2 = 0.97) and without ( R2 = 0.85) CBB presented an exponential decay with increase in temperatures. And a threshold value at 20 degrees C for sediment critical shear stress ( tcr ) indicated that sediment could be resuspended easier when temperature was more than 20 degrees C. Altogether, this study showed that the increase in temperatures with a threshold at 20 degrees C, can cause sediment particle flocculation, resulting in a loose and fragile structure. And the results would be helpful to sediment management considering environmental effects of sediment suspension for eutrophication shallow lakes. (c) 2024 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V.

Several studies focus on enhancing soil strength through the incorporation of natural or synthetic fibers. However, there is limited published data on the effectiveness of rice husk in soil reinforcement. The use of rice husk as a reinforcing material is supported by the fact that rice is one of the most produced and consumed cereals globally. In this article, we analyze the behavior of a clayey soil from southern Brazil with the addition of 0.5, 0.75, and 1% rice husk (RH), comparing it to coconut coir (CC) and curau & aacute; fibers (CU). In unconfined compressive strength tests (UCS), increases in soil strength of 20, 40, and 140% were observed for RH, CC, and CU, respectively, compared to pure soil. From consolidated undrained triaxial compression tests, both unreinforced soil and soil reinforced with 1% RH, CC, and CU were examined. The triaxial tests revealed an increase in the internal friction angle of 72 and 98%, alongside a decrease in cohesion of 57 and 94% due to the addition of CC and CU, respectively, in terms of effective stress. In contrast, RH did not significantly enhance the soil's behavior, likely due to its shorter fiber length.

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.

The differential settlement of warm permafrost foundations significantly impacts the safe operation of highway and railway embankments. The use of geotextile encased lime energy columns (GELECs) has been proven to be effective in pre-thawing shallow layers of warm permafrost as a novel method to reduce the post-construction settlement of embankments. Understanding the interaction between the GELECs and the soil is crucial in illustrating the load transfer mechanism. This study conducted a series of large-scale direct shear tests on GELEC-soil in degraded permafrost environments using an improved temperature-controlled direct shear test apparatus with assembled large shear boxes. The effects of different shear rates, water contents, and types of geotextiles on the mechanical behavior of the interface were analyzed. The strength development of the interface under various curing times was studied in detail. The experimental results indicate that the interface strength increases significantly during the initial stage of curing while the rate of strength increase diminishes over time. The improvement in peak shear strength is primarily attributed to the increase in interfacial cohesion, and the increasing trend of the cohesion follows an exponential decay function. And the microscopic strengthening mechanism of the interface was analyzed through SEM tests. Finally, a nonlinear elastic model incorporating a parameter to represent the variation of cohesion was developed to describe the shear stress-strain relationship at the GELEC-soil interface under different curing times.

Offshore wind turbines are subjected to more significant wave and wind environmental loads at extreme weather conditions, making subsoil experience various loading stages with different amplitudes. To investigate the coupling effect of both cyclic shear stress ratio (CSR) and stage amplitude ratio (Ar) between normal and extreme weather conditions, a series of bi-directional simple shear tests with five different Ar and three CSR values were conducted on marine sand using the variable-direction dynamic cyclic simple shear (VDDCSS) apparatus. In the tests, soil samples were compacted under vertical stress and then sheared in undrained conditions by applying two shear stresses acting in different horizontal directions. Test results indicated that the cyclic strain, pore water pressure ratio, and cyclic strength were significantly determined by the value of stage amplitude ratios and the CSRs: at the same CSR, cyclic strains, and pore water pressure increased while cyclic strength decreased with the Ar. Comparing the test data between various cyclic stress ratios found that the CSRs can accelerate shear strains, pore pressure accumulation, and cyclic strength attenuation.

Numerical simulation of the stress-strain behavior of materials under various loading conditions requires an appropriate constitutive model, with the yield surface being one of its key components. Extensive studies have been conducted to identify the yield surface of soil materials (sand and clay) through laboratory methods. However, the identification of the yield surface of waste materials has received less attention to date. Waste materials are artificial soil-like substances produced during the crushing and concentration processes in mineral processing plants. In this paper, a laboratory program was carried out using an advanced triaxial stress-path and stress-control apparatus on reconstructed saturated samples from the Sungun mine, located in the northwest of Iran. These samples were reconstituted using the wet tamping method. Through analysis and interpretation of the results, the yield surfaces of these lightly over-consolidated materials, with both isotropic and anisotropic initial consolidation conditions, were determined. The dependency or non-dependency of the obtained yield surfaces was evaluated, and the effect of consolidation stress and the angle of the applied stress path on the variation trends of the secant shear modulus and secant bulk modulus-indicating structural anisotropy of the waste materials-were assessed. Finally, the structural anisotropy of the samples was examined using SEM images and statistical processing of particle orientation and a mathematical model for the obtained yield surfaces was proposed.

Energy piles are highly favored for their excellent, low energy consumption in providing heating for public residences. The temperature field changes the activity of the diffuse double electric layer (DEL) on the particle surface, thereby altering the distribution of the stress field in the soil and ultimately affecting the mechanical properties of the interface between the energy pile and the soil. Therefore, studying the influence of water content on the mechanical behavior of the soil-structure interface in the temperature field is crucial for energy pile safety. This study used a modified temperature-controlled direct shear apparatus to obtain the influence of water content and temperature on the shear behavior of the soil-structure interface. Then, the test results were analyzed and discussed. Finally, three results were obtained: (1) The water content of bentonite (wbent) had a significant impact on the shear stress-shear displacement curve of the soil-structure interface; when the wbent was less than the wp of the bentonite, the tau-l curve exhibited a softening response, then displayed a hardening response. (2) The shear strength of the soil-structure interface gradually decreased with the increase of wbent. (3) The shear strength of the soil-structure interface increased with increasing temperature under various wbent and vertical loads.

Soil instability and potential failure under principal stress rotation require greater attention than ever before due to increased operation of heavier and longer high-speed trains. This study focuses on the interplay between cyclic vertical stress and torsional shear stress on the failure condition of a low-plasticity subgrade soil, facilitated by a hollow cylinder apparatus. Combined vertical and torsional loading significantly influences strain response, with increasing torsional stress leading to higher strain accumulation. Moreover, the data indicate that an increase in torsional shear stress is generally accompanied by a swift rise in the EPWP and a corresponding decrease in the soil stiffness. In view of this, a novel parameter, the overall stiffness degradation index (delta o) that simultaneously captures both the vertical and torsional shear effects under principal stress rotation is proposed as an early indicator of instability. In addition, a normalised torsional stress ratio (NTSR), which is the ratio of the amplitude of torsional shear stress to the confining pressure, is introduced to assess the impact of torsional shear stress. Whereby, higher NTSR values correlate with premature inception of failure. These experimental results provide new insights for a better understanding of soil instability under simulated railway loading.