Recently, the biostimulation has received attention due to its sustained mineralization, environmental adaptability and lower cost. In the current study, a series of isotropic consolidated undrained triaxial shear (CU) tests were performed on biocemented soil treated through biostimulation approach to examine the effect of cementation levels on the undrained shear behaviors. The test results demonstrate that the biocementation generated by the biostimulation approach can improve the shear behaviors remarkably, with the observed changes in stress-strain relationship, pore water pressure, stress path, stiffness development, and strength parameters. The variations of the strength parameters, i.e., effective cohesion and effective critical state friction angle, with increasing cementation treatment cycles can be well fitted by an exponential function and a linear function, respectively, while the variation of the effective peak-state friction angle is relatively small. The increased shear strength, stiffness, effective cohesion, and strain softening phenomenon of biocemented soils are related to the densification, increased particle surface roughness, and raised interparticle bonding caused by biostimulation approach. The liquefaction index decreases with the increase in cementation treatment cycles, especially at lower initial mean effective stress (100 and 200 kPa), indicating that the biostimulation approach may be a viable method for anti-liquefaction of soil.

The water-holding and strength characteristics of unsaturated expansive soil and modified soil in a high-fill canal embankment along the central line of the South-to-North Water Diversion Project were investigated using a pressure plate apparatus and a GDS unsaturated triaxial test system. The soil-water characteristic curves (SWCCs) of expansive soil and modified soil were obtained by curve-fitting the results of water-holding characteristic tests, thereby revealing the distinctions in water-holding characteristics of the two soil types. The laws governing the effects of matrix suction on the stress-strain relationships and shear strength of the two soil types were explored through unsaturated triaxial drainage shear tests. According to the test results: (1) The moisture content and void ratio of each soil type decreased gradually with the increase in matrix suction, although the void ratio of modified soil decreased at a slower rate than that of expansive soil. (2) Matrix suction induced a transition from strain hardening to strain softening; (3) The shear strength of both soils increases with the matrix suction and confining pressure, with the increment of expansive soil greater than that of modified soil. Notably, the influence of confining pressure became progressively more significant with increasing matrix suction for both soils; (4) The cohesion and internal friction angle of expansive soil and modified soil increases with the matrix suction, with 200 kPa as the critical point of increasing rate; (5) The expansive soil differs from modified soil in cohesion and internal friction angle under different matrix suctions, with matrix suction of 400 kPa as the critical point. (6) The matrix suction thresholds of 200 kPa and 400 kPa can serve as references for engineering design and construction, as well as seepage prevention and slope reinforcement. This study provides technical parameters and theoretical support for the design, construction, and long-term stability of embankments on the expansive soil in the South-to-North Water Transfer Project site.

Landslides developing in bedding-plane sediments are predominantly controlled by basal shear zones, where clay-rich materials localize deformation along bedrock surfaces. The mechanical behavior of shear-zone soil is further influenced by the characteristics of soil-rock interface. This paper investigates the residual strength of the soil-rock interface samples through ring shear and large-scale direct shear tests under varying stress and rate conditions. Shear zone materials from two landslides sites are paired with manufactured base and natural rock to compose the interface samples. Experimental results find that the residual strengths of shear zone materials are altered by different interfaces. At a low normal stress level, the mechanical behaviors of soils show strong dependence on surface asperities. As driven by increasing shear stress, the smooth interface sample exhibits accelerated failure progression with significant loss of resistance. The surface morphology and rheological behavior explain that the basal shearing easily occur along a relatively smooth interface, resulting weakening at high velocity and stress states.

Sandy hydrate reservoirs are considered an ideal target for the extraction of marine natural gas hydrates (NGH). However, engineering geological risks, including reservoir sand production and seabed subsidence during the extraction process, present a significant challenge. In 2019, China discovered a high-concentration sandy NGH reservoir with favorable commercial development potential in the Qiongdongnan Basin of the South China Sea, establishing the region as a key focus for future exploration and development efforts. A thorough comprehension of macro-meso mechanical properties of this specific sandy NGH reservoir is essential for the safe and efficient extraction of hydrates. In this study, a novel method is proposed to calculate hydrate saturation of hydrate-bearing sandy sediments (HBSS) with hexagonal close-packed state. A series of undrained biaxial compression with flexible boundary show that hydrate cementation enhances the strength of the sample. However, an excessively high hydrate saturation is likely to induce strain softening, whereas an increase in confining pressure helps to mitigate strain softening. Hydrate cementation promotes the formation of abundant force chains. The inhomogeneous displacement, sliding, and relative rotation of the particles are the primary factors contributing to the formation of X-shaped shear bands, which is related to cemented bond breakage. The primary cause of hydrate cementation failure is tensile stress failure. External loading induces force chains to undergo buckling, fracturing, and restructuring, which governs fabric development. The research outcomes offer novel insights into the inhomogeneous deformation and macro-meso mechanical properties of HBSS at the particle-scale.

Due to the widespread prevalence of respiratory diseases such as COVID-19 and H1N1, the use of disposable masks has increased significantly. Consequently, the environmental issues arising from their accumulation have become increasingly severe. This study, therefore, aims to investigate the potential of using masks as soil reinforcement materials. This study conducted triaxial and seepage tests on mask-calcareous sand mixtures with varying ratios to examine the effects of mask content on the strength, modulus, particle fragmentation, and permeability coefficient of calcareous sand, as well as the influence of different mask sizes on shear strength and shear dilation. The results demonstrate that with an increase in mask content, the peak stress ratio of the mask-calcareous sand mixture increases by 4% per level, and the internal friction angle rises by approximately 1.6% per level. Conversely, water permeability and shear swelling are reduced, and particle loss decreases by over 70%. The reinforcing effect of the mask is attributed to the high friction between the mask and the calcareous sand at the contact interface, which restricts the movement of soil particles during deformation, thereby enhancing the overall strength of the mixture. Among the three mask sizes, the smallest mask-calcareous sand mixture exhibited the greatest improvement in shear strength, and the shear shrinkage effect was more pronounced. This indicates that particle size also significantly influences the mechanical properties of the mixtures. The reinforcing effect of the mask on the soil results from the high friction at the interface between the mask and the calcareous sand. When the soil deforms, the mask enhances the overall strength of the mixture by restricting the movement of soil particles. Considering the impact of masks on the performance of calcareous sand, it can be concluded that the optimal mass content of masks is 0.3%. This study offers a new perspective on the reuse of discarded masks in civil engineering applications.

In the waterway construction projects of the upper reaches of the Yangtze River, crushed mudstone particles are widely used to backfill the foundations of rock-socketed concrete-filled steel tube (RSCFST) piles, a structure widely adopted in port constructions. In these projects, the steel-mudstone interfaces experience complex loading conditions, and the surface profile tends to vary within certain ranges during construction and operation. The changes in boundary conditions and material profile significantly impact the bearing performance of these piles when subjected to cyclic loads, such as ship impacts, water level fluctuations, and wave-induced loads. Therefore, it is necessary to investigate the shear characteristics of the RSCFST pile-soil interface under cyclic vertical loading, particularly in relation to varying deformation levels in the steel casing's outer profile. In this study, a series of cyclic direct shear tests are carried out to investigate the influential mechanisms of roughness on the cyclic behavior of RSCFST pile-soil interfaces. The impacts of roughness on shear stress, shear stiffness, damping ratio, normal stress, and particle breakage ratio are discussed separately and can be summarized as follows: (1) During the initial phase of cyclic shearing, increased roughness correlates with higher interfacial shear strength and anisotropy, but also exacerbates interfacial particle breakage. Consequently, the sample undergoes more significant shear contraction, leading to reduced interfacial shear strength and anisotropy in the later stages. (2) The damping ratio of the rough interface exhibits an initial increase followed by a decrease, while the smooth interface demonstrates the exact opposite trend. The variation in damping ratio characteristics corresponds to the transition from soil-structure to soil-soil interfacial shearing. (3) Shear contraction is more pronounced in rough interface samples compared to the smooth interface, indicating that particle breakage has a greater impact on soil shear contraction compared to densification.

This study presents a comprehensive investigation into the cyclic shear behavior of sand-fly ash mixtures through experimental and data-driven modeling approaches. Cyclic direct shear tests were conducted on mixtures containing fly ash at 0%, 2.5%, 5%, 10%, 15%, and 20% by weight to examine the influence of fly ash content on the shear behavior under cyclic loading conditions. The tests were carried out under a constant stress of 100 kPa to simulate field-relevant stress conditions. Results revealed that the fly ash content initially reduces shear strength at lower additive contents, but shear strength increases and reaches a maximum at 20% fly ash content. The findings highlight the trade-offs in mechanical behavior associated with varying fly ash proportions. To enhance the understanding of cyclic shear behavior, a Nonlinear Autoregressive Model with External Input (NARX) model was employed. Using data from the loading cycles as input, the NARX model was trained to predict the final shear response under cyclic conditions. The model demonstrated exceptional predictive performance, achieving a coefficient of determination (R2) of 0.99, showcasing its robustness in forecasting the cyclic shear performance based on the composition of the mixtures. The insights derived from this research underscore the potential of incorporating fly ash in sand mixtures for soil stabilization in geotechnical engineering. Furthermore, the integration of advanced machine learning techniques such as NARX models offers a powerful tool for predicting the behavior of soil mixtures, facilitating more effective and data-driven decision-making in geotechnical applications. Evidently, this study not only advances the understanding of cyclic shear behavior in fly ash-sand mixtures but also provides a framework for employing data-driven methodologies to address complex geotechnical challenges.

Evaluation of hydromechanical shear behavior of unsaturated soils is still a challenging issue. The time and cost needed for conducting precise experimental investigation on shear behavior of unsaturated soils have encouraged several investigators to develop analytical, empirical, or semi-empirical models for predicting the shear behavior of unsaturated soils. However, most of the previously proposed models are for specimens subjected to the isotropic state of stress, without considering the effect of initial shear stress. In this study, a hydromechanical constitutive model is proposed for unsaturated collapsible soils during shearing, with consideration of the effect of the initial shear stress. The model implements an effective stress-based disturbed state concept (DSC) to predict the stress-strain behavior of the soil. Accordingly, material/state variables were defined for both the start of the shearing stage and the critical state of the soil. A series of laboratory tests was performed using a fully automated unsaturated triaxial device to verify the proposed model. The experimental program included 23 suction-controlled unsaturated triaxial shear tests on reconstituted specimens of Gorgan clayey loess wetted to different levels of suctions under both isotropic and anisotropic stress states. The results show excellent agreement between the prediction by the proposed model and the experimental results. (c) 2025 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/).

The effects of cyclic heat treatments on the fracture shear behaviors are rarely reported. To enhance our understanding, granite fractures having almost the same roughness were first exposed to cyclic heating at 400 degrees C and air-cooling treatments, and then direct shear tests were performed under four levels of normal loading. The influences of thermal cycles on roughness degradation and shear properties are analyzed. The roughness degradation in the joint roughness coefficient and the three-dimensional (3D) roughness metric exhibit linear increasing tendency with increasing thermal cycles. Typical fracture shear properties, including cohesion and friction angle, peak and residual shear strength, peak and residual shear displacement, and initial and secant shear stiffness, fluctuate generally within the first 10 thermal cycles, followed by gradual decreasing tendencies. The thermal effect on the shear properties become weaker as the number of heat treatments increases from 10 to 80. Nonuniform expansion and shrinkage of mineral grains after thermal treatments produce micro-cracks within the rock matrix and on the rock surface, suggesting that asperities are easier to be sheared-off. Thermal alteration in fracture peak-shear strength could be attributed to the deterioration in rock strengths and the mismatch in opposing fracture walls. The observations would provide better insights into rock friction after high temperatures in geothermal energy exploitation. (c) 2024 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/).

To understand the shear characteristics of particles more comprehensively, the shear behavior of rigid particles, deformable particles, and breakable particles is investigated in this work. The rigid particles are modeled by the spheropolygon-based DEM. The deformable spheropolygon-based discrete element method is employed to study the shear behavior of deformable and breakable particles. Firstly, the influence of different circularization radii on rigid particles is studied. It is found that with a larger circularization radius, the edges and corners of the particles become less pronounced, and the particle shape approaches a circle, resulting in a smaller shear force. Secondly, the shear characteristics of breakable particles are examined. The experimental results indicate that particle fragmentation primarily occurs during the early stages of the shear process. Additionally, under high tensile strength, the impact of particle fragmentation on the mechanical properties of granular materials can be disregarded. Lastly, a comparison of shear forces is conducted among rigid, deformable, and brittle particles. The results show that particles assumed to be rigid generate the highest shear forces. On the contrary, deformable particles undergo deformation during shear, while brittle particles experience breakage, leading to a relatively loose packing and consequently less shear force.