The influence of seismic history on the liquefaction resistance of saturated sand is a complex process that remains incompletely understood. Large earthquakes often consist of foreshocks, mainshocks, and aftershocks with varying magnitudes and irregular time intervals. In this context, sandy soils undergo two interdependent processes: (i) partial excess pore water pressure (EPWP) generation during foreshocks or moderate mainshocks, where seismic loadings elevate EPWP without causing full liquefaction and (ii) incomplete EPWP dissipation between seismic events due to restricted drainage. These processes leave behind persistent residual EPWP, reducing the liquefaction resistance during subsequent shaking. A series of cyclic triaxial tests simulating these mechanisms revealed that liquefaction resistance increases when the EPWP ratio r(u) < 0.6-0.8 (peaking at r(u) similar to 0.4) but decreases sharply at higher r(u). Crucially, EPWP generation during seismic loading plays a dominant role in resistance evolution compared to reconsolidation effects. Threshold lines (TLs) mapping r(u), the reconsolidation ratio (RR), and peak resistance interval (the range of r(u) where the peak liquefaction resistance is located) indicates that resistance decreases above TLs and increases below them, with higher cyclic stress ratios (CSR) weakening these effects. These findings provide a unified framework for assessing liquefaction risks under realistic multi-stage seismic scenarios.

Soybean urease-induced calcium carbonate precipitation (SICP) is an innovative and eco-friendly approach with demonstrated potential for mitigating soil liquefaction. However, the specific impacts of the concentrations of soybean urease and salt solutions require further elucidation. The research examines how the two compositions influence calcium carbonate formation. Dynamic characteristics of one-cycle SICP-treated clean and silty sand were analyzed based on cyclic triaxial tests. It was revealed that SICP-treated specimens of both liquefied sand and silty sand exhibit reduced accumulation of excess pore pressure and diminished strain growth under cyclic loading, thereby delaying liquefaction failure. Although higher concentrations of both soybean urease and salt solution can enhance liquefaction resistance, salt solution concentration has a more pronounced effect on improving liquefaction resistance due to the more production of calcium carbonate. Scanning electron microscopy observations confirmed the presence of calcium carbonate crystals at the interfaces between sand particles and between sand and fine particles. These crystals effectively bond the loose sand and fine particles into a cohesive matrix, reinforcing soil structure. A direct linear correlation was established between the liquefaction resistance improvement and precipitated calcium carbonate content. Notably, the one-cycle SICP treatment method adopted in this study demonstrates a better biocementation effect compared to cement mortar or multi-cycle MICP-treated sand under the same content of cementitious materials. These findings provide valuable insights for optimizing SICP treatments, aiming to reduce the risk of soil liquefaction in potential field applications.

This paper presents a comprehensive study on the evolution of the small-strain shear modulus (G) of granular materials during hydrostatic compression, conventional triaxial, reduced triaxial, and p-constant triaxial tests using 3D discrete element method. Results from the hydrostatic compression tests indicate that G can be precisely estimated using Hardin's equation and that a linear correlation exists between a stress-normalized G and a function of mechanical coordination number and void ratio. During the triaxial tests, the specimen fabric, which refers to the contact network within the particle assembly, remains almost unchanged within a threshold range of stress ratio (SR). The disparity between measured G and predicted G, as per empirical equations, is less than 10% within this range. However, once this threshold range is exceeded, G experiences a significant SR effect, primarily due to considerable adjustments in the specimen's fabric. The study concludes that fabric information becomes crucial for accurate G prediction when SR threshold is exceeded. A stiffness-stress-fabric relationship spanning a wide range of SR is put forward by incorporating the influences of redistribution of contact forces, effective connectivity of fabric, and fabric anisotropy into the empirical equation.

True triaxial tests were conducted on artificially frozen sand. The effects of the intermediate principal stress coefficient, temperature and confining pressure on the strength of frozen sand were studied. The stress-strain curves under different initial conditions indicated a strain hardening. In response to increases of either the intermediate principal stress coefficient or the confining pressure or to a decrease of temperature, the strength typically increased. Furthermore, a new strength criterion was proposed to describe the strength of artificially frozen sand under a constant b-value stress path, combining the strength function in the p-q and pi planes. Considering the low confining pressure, the strength criterion in the p-q plane fitted the linear relationship in the parabolic strength criterion well. The strength criterion in the pi plane was combined with stress invariants, and a new strength criterion was established. This criterion considers unequal tension and compression strength, and integrates temperature. Test results indicated its validity. All parameters of the strength criterion could be easily determined from the triaxial compression and triaxial tensile tests.

Gap-graded soil, characterized by the absence of certain particle sizes, is commonly used in infrastructure projects such as dams and roadbeds. A comprehensive understanding of both the macro- and micro-mechanical behaviors of discontinuously graded soils is essential for their effective use in engineering applications. In this study, drainage triaxial compression tests were conducted on four gap-graded soil samples with different fine-grain contents mainly using the DEM method, whereas the flexible boundary part was performed using the FDM-DEM method. The contacts were classified based on the magnitude of contact forces between coarse and fine particles, considering the coordination number of the particles involved and the normal angular distribution of these contacts. This classification enabled a detailed analysis of how fine particles contribute to stress transmission and structural evolution during shearing. The fabric tensor for these contact types provided further insights into the anisotropy of samples during shearing. On the microscopic scale, the evolution of contact numbers was found to closely align with the observed stress-strain behaviors. Increasing fine particle content significantly altered the role of fine particles in the stress transmission process. With low content of finer particles, initially, fine particles were situated within the voids formed by coarse particles, and the fine particles are gradually embedded into the coarse particles during the loading process. With the increase of fine particle content, fine particles constantly aggregate to block coarse particles and become the main medium of stress transmission.

This study presents the results of consolidated drained triaxial tests conducted to investigate the influence of various parameters on the volumetric change behavior of cemented sand reinforced with polyvinyl alcohol (PVA) fibers. The primary objective is to explore the interaction between fiber weight ratio, cement weight ratio, confining pressure, and relative density on the dilatation behavior of cemented sand reinforced with PVA fibers. PVA fibers were incorporated into dry sand-cement mixtures at weight ratios of 0.0%, 0.3%, and 0.6%. The specimens were prepared with cement content of 0%, 2%, and 4% by weight of dry sand and cured for 7 days. Two relative densities were used in specimen preparation, and triaxial compression tests were conducted under different confining pressures. The results reveal that decreasing relative density, increasing cement content, and adding fibers all contribute to a reduction in sample dilatation. Specifically, the peak dilation rate increases with higher relative density and cement content, while it decreases with higher fiber content and confining pressure. A notable aspect of this study is its investigation of how these parameters interact when combined, offering a deeper understanding of their collective effects on soil behavior.

Stress-strain behavior of two different soil specimens subjected to cyclic compressive loading are studied herein, the goal being to present a simple dynamic uniaxial mem-modeling approach that aids physical insight and enables system identification. In this paper, mem stands for memory, i.e., hysteresis. Mem-models are hysteresis models transferred from electrical engineering using physical analogies. Connected in series, a mem-dashpot and mem-spring are employed to model inter-cycle strain ratcheting and intra-cycle gradual densification of the two soil specimens. Measured time histories of stress and strain are first decomposed so that the two modeling components, mem-dashpot and mem-spring, can be identified separately. This paper focuses on the mem-dashpot, a nonlinear generalization of a linear viscous damper. A mem-spring model is also devised built on an extended Masing model. Nonlinear dynamic simulations (with inertia) employing the identified mem-dashpot and mem-spring demonstrate how well the identified mem-models reproduce the measured early-time data (first 200 cycles of loading). Choices of state variables inherited from bond graph theory, the root of mem-models, are introduced, while MATLAB time integrators (i.e., ode solvers) are used throughout this study to explore a range of contrasting damper and spring models. Stiff solver and the state event location algorithm are employed to solve the equations of motion involving piecewise-defined restoring forces (when applicable). Computational details and results are relegated to the appendices. This is the first study to use single-degree-of-freedom (SDOF) system dynamic simulations to explore the consistency of mem-models identified from real-world data.

This study reveals the mechanical behavior of silt in the Yellow River floodplain under 3D stress. A true triaxial apparatus was used to conduct consolidated drained shear tests under different intermediate principal stress coefficients (b) and consolidation confining pressures to investigate the influence of the intermediate principal stress on the deformation and shear strength of silt. The stress-strain curves exhibited strong strain-hardening characteristics during shearing. Due to enhanced particle interlocking and microstructural reorganization, the silt demonstrated complex b-dependent deformation and strength characteristics. The cohesion rose with increasing b, whereas the internal friction angle followed a non-monotonic pattern, increasing and decreasing slightly as b approached 1. The strength envelope of the silt fell between that predicted by the Lade-Duncan and the extended von Mises strength criteria., which is best predicted by the generalized nonlinear strength criterion when the soil parameter alpha was 0.533. The findings reveal the stress-path-dependent mechanisms of Yellow River floodplain silt and provide essential parameters for optimizing the design of underground engineering projects in this region.

Soil liquefaction caused by earthquakes is a devastating occurrence that can compromise the foundations of buildings and other structures, leading to considerable economic losses. Among the new remedies against liquefaction, Induced Partial Saturation (IPS) is regarded as one of the most promising technologies. In order to improve liquefaction resistance and the fluid phase's compressibility, gas or air bubbles are introduced into the pore water of sandy soils. This article deals with the general laboratory evaluation of a sand under partially saturated conditions and under cyclic loading to assess if this technology is applicable for a ground improvement of the examined soil. The use of the Axis Translation Technique for sample desaturation and diffusion-stable butyl membranes significantly influences the laboratory results. Additionally, it is found that the trapped air bubbles of the partially saturated samples act like a damping mechanism, which are reflected in the stress paths of the deviator stress q over the mean pressure p with an inclination of 1 : 3. Zum Verfl & uuml;ssigungsverhalten von teilges & auml;ttigtem SandDie durch Erdbeben verursachte Bodenverfl & uuml;ssigung ist ein verheerendes Ereignis, das die Fundamente von Geb & auml;uden und anderen Bauwerken gef & auml;hrden und zu erheblichen wirtschaftlichen Verlusten f & uuml;hren kann. Die induzierte partielle S & auml;ttigung (Induced Partial Saturation, IPS) gilt als eine der vielversprechendsten Technologien unter den neuartigen Baugrundverbesserungen gegen Verfl & uuml;ssigung. Um den Verfl & uuml;ssigungswiderstand und die Kompressibilit & auml;t der fl & uuml;ssigen Phase zu verbessern, werden dabei Gas- oder Luftblasen in das Porenwasser sandiger B & ouml;den eingebracht. Dieser Beitrag besch & auml;ftigt sich mit der generellen labortechnischen Evaluierung eines Sandes unter teilges & auml;ttigten Verh & auml;ltnissen und unter zyklischer Beanspruchung zur Beurteilung, inwiefern sich diese Baugrundverbesserung f & uuml;r den untersuchten Boden eignet. Die Verwendung der Axis Translation Technique zur Probenentw & auml;sserung und die Verwendung von diffusionsstabilen Butylmembranen haben einen erheblichen Einfluss auf die Laborergebnisse. Au ss erdem ist festzustellen, dass die eingeschlossenen Luftblasen der teilges & auml;ttigten Proben wie eine D & auml;mpfung wirken und sich in den Spannungspfaden der Deviatorspannung q & uuml;ber dem mittleren Druck p mit einer Neigung 1 : 3 widerspiegeln.

Discrete element simulation of triaxial tests is an important tool for exploring the deformation and failure mechanisms of geotechnical materials such as sands. A crucial aspect of this simulation is the accurate representation of lateral boundaries. Using coupled finite difference method (FDM)-discrete element method (DEM) approach, numerical simulations of consolidated-drained and consolidated-undrained triaxial tests were conducted under flexible lateral boundary conditions. These results were then compared with those of corresponding triaxial tests using rigid lateral boundaries. The results indicate that, compared to the rigid lateral boundary, the triaxial test using the FDM-DEM coupled flexible lateral boundary better captures both the macroscopic mechanical response and the microscopic particle kinematics of laboratory triaxial specimens. In the consolidated-drained triaxial tests, the strain softening and shear dilatancy of the specimen with the flexible lateral boundary are significantly weaker after reaching peak strength than those of the specimen with the rigid lateral boundary. In the consolidated-undrained triaxial tests, when the axial strain is large, the specimen with the flexible lateral boundary exhibits both a lower deviator stress and a smaller absolute value of negative excess pore pressure. Furthermore, in the consolidated-undrained triaxial tests, as the axial strain increases, the flexible lateral boundary provides weaker lateral constraint and support to the specimen compared to the rigid lateral boundary. Consequently, the stability of the force chains in the specimen with the flexible lateral boundary is lower, leading to more buckling events of force chains within the shear band. As a result, both the anisotropy and the deviator stress are reduced.