To investigate the mechanical characteristics of frozen silty clay under complex stress paths, using the true triaxial instrument for permafrost, tests were carried out under triaxial compressive and plane strain stress states using the true triaxial instrument for permafrost to analyze deformation characteristics and strength evolution law under different stress paths and minor principal stresses (sigma(3)) and establish strength criterion under plane strain conditions. PFC3D numerical simulation results were compared to test results and meso-crack evolution law was discussed. The results showed that stress-strain curves were characterized by strain hardening. Destructive strength showed a gradual increase with the increase of sigma(3) and the values obtained from plane strain tests were higher than those of triaxial compression tests. Volume strains basically showed shear shrinkage characteristics and all sigma(3) directions were expansion deformation. Strength at damage under plane strain state was approximated based on generalized Mises and Lade-Duncan plane strain strength criterion using generalized plane strain strength criterion. Stress-strain curves obtained from numerical simulation tests in PFC3D basically agreed well with those obtained from indoor test results. The number of tensile and shear cracks in the developed numerical model under various stress paths were increased with generalized shear strain.

Preexisting cracks inside tight sandstones are one of the most important properties for controlling the mechanical and seepage behaviors. During the cyclic loading process, the rock generally exhibits obvious memorability and irreversible plastic deformation, even in the linear elastic stage. The assessment of the evolution of preexisting cracks under hydrostatic pressure loading and unloading processes is helpful in understanding the mechanism of plastic deformation. In this study, ultrasonic measurements were conducted on two tight sandstone specimens with different bedding orientations subjected to hydrostatic loading and unloading processes. The P-wave velocity was characterized by a similar response with the volumetric strain to the hydrostatic pressure and showed different strain sensitivities at different loading and unloading stages. A numerical model based on the discrete element method (DEM) was proposed to quantitatively clarify the evolution of the crack distribution under different hydrostatic pressures. The numerical model was verified by comparing the evolution of the measured P-wave velocities on two anisotropic specimens. The irreversible plastic deformation that occurred during the hydrostatic unloading stage was mainly due to the permanent closure of plastic-controlled cracks. The closure and reopening of cracks with a small aspect ratio account for the major microstructure evolution during the hydrostatic loading and unloading processes. Such evolution of microcracks is highly dependent on the stress path. The anisotropy of the crack distribution plays an important role in the magnitude and strain sensitivity of the P-wave velocity under stress conditions. The study can provide insight into the microstructure evolution during cyclic loading and unloading processes. (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/).

To study the crack evolution patterns in expansive soils under wetting-drying cycles, a series tests were conducted on the expansive soil from a canal side slope in the South-to-North Water Diversion Middle Route Project. Six indoor wet-dry cycle tests were performed on the samples with compaction degrees of 97%, 88%, and 79%. The crack image processing system by using Python was developed for quantitative analysis of crack ratios the expansive soil samples. Furthermore, PIV (particle image velocimetry) technology was also utilized to monitor the entire process of crack development. Results show that the evolution of crack ratios over time in the expansive soil samples can be divided into four stages, crack formation, crack development, crack closing, and crack stabilization stages. The higher the compaction degree of an expansive soil sample, the shorter its duration of the crack formation stage, and the shorter the time required for the crack ratio to reach its peak. The stress and displacement field nephograms of the samples can effectively reflect the crack evolution process on their surfaces. In addition, closing ratio was proposed to studied the crack closing capacity in expansive soil samples. The crack closing ratio decrease with the increase of the number of wet-dry cycles, as well as the compaction degree decreases. The primary cause of crack closing in compacted expansive soil is uneven shrinkage in the vertical direction, which arises from differing evaporation rates between the upper and lower parts of the sample.

The size of mineral grain has a significant impact on the initiation and propagation of microcracks within rocks. In this study, fine-, medium-, and coarse-grained granites were used to investigate microcrack evolution and characteristic stress under uniaxial compression using the acoustic emission (AE), digital image correlation (DIC), and nuclear magnetic resonance (NMR) measurements. The experimental results show that the characteristic stress of each granite decreased considerably with increasing grain sizes. The inflection points of the b-value occurred earlier with an increase in grain sizes, indicating that the larger grains promote the generation and propagation of microcracks. The distribution characteristics of the average frequency (AF) and the ratio of rise time to amplitude (RA) indicate that the proportion of shear microcracks increases with increasing grain size. The NMR results indicate that the porosity and the proportion of large pores increased with increasing grain size, which may intensify the microcrack evolution. Moreover, analysis of the DIC and AE event rates suggests that the high-displacement regions could serve as a criterion for the degree of microcrack propagation. The study found that granites with larger grains had a higher proportion of high-displacement regions, which can lead to larger-scale cracking or even spalling. These findings are not only beneficial to understand the pattern of micro- crack evolution with different grain sizes, but also provide guidance for rock monitoring and instability assessment. (c) 2024 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license.

To investigate the macroscopic mechanical properties and failure evolution mechanism of desulfurization gypsum-fly ash fluid lightweight soil, a microscale numerical model using PFC2D (Particle Flow Code) was constructed. Uniaxial compression tests were conducted to determine the microscopic parameters of the model, extracting information on the discrete fracture network type, quantity, age, and particle displacement trend. The crack morphology and propagation evolution of desulfurization gypsum-fly ash fluid lightweight soil were explored, and the destructive properties of desulfurization gypsum-fly ash fluid lightweight soil material were evaluated through energy indicators. The research findings suggest that the discrete element numerical model effectively simulates the stress-strain curve and failure characteristics of materials. Under uniaxial compression conditions, microcracks dominated by shear failure occur in the initial loading stage of desulfurization gypsum-fly ash fluid lightweight soil, with a through crack dominated by tensile failure appearing once the load exceeds the peak stress. The dissipated energy evolution in the flow state of desulfurization gypsum-fly ash fluid lightweight soil is relatively gentle, leading to delayed cracking after surpassing the peak stress point.