This paper presents pore unit assembly-discrete element model (PUA-DEM), a pore-scale hydromechanical framework that resolves interactions between mobile granular particles and multiphase fluids in unsaturated granular media. The framework uniquely integrates DEM with pore-scale hydrodynamic models to capture unsaturated flow dynamics, while leveraging a two-way coupling mechanism to ensure bidirectional fluid-grain feedback through stabilized domain partitioning. Further innovations include a dynamic pore-merging and retriangulation algorithm that enhances computational efficiency for large-scale systems. Validated against experimental data for glass beads and Ottawa sand, PUA-DEM accurately reproduces critical hydromechanical phenomena-including capillary/viscous fingering, wetting-induced granular swelling/collapse, and quasi-static deformation-under diverse saturation and loading regimes. Numerical case studies reveal how capillary forces and wetting fluid saturation collectively govern granular response, from pore-scale meniscus evolution to macroscale flow instabilities. By bridging pore-and particle-scale physics, PUA-DEM advances predictive modeling of partially saturated granular systems, offering transformative insights for geohazard mitigation, sustainable agriculture, pharmaceutical manufacturing, and energy-related engineering applications.

In subsurface projects where the host rock is of low permeability, fractures play an important role in fluid circulation. Both the geometrical and mechanical properties of the fracture are relevant to the permeability of the fracture. To evaluate this relationship, we numerically generated self-affine fractures reproducing the scaling relationship of the power spectral density (PSD) of the measured fracture surfaces. The fractures were then subjected to a uniform and stepwise increase in normal stress. A fast Fourier transform (FFT)-based elastic contact model was used to simulate the fracture closure. The evolution of fracture contact area, fracture closure, and fracture normal stiffness were determined throughout the whole process. In addition, the fracture permeability at each step was calculated by the local cubic law (LCL). The influences of roughness exponent and correlation length on the fracture hydraulic and mechanical behaviors were investigated. Based on the power law of normal stiffness versus normal stress, the corrected cubic law and the linear relationship between fracture closure and mechanical aperture were obtained from numerical modeling of a set of fractures. Then, we derived a fracture normal stiffness-permeability equation which incorporates fracture geometric parameters such as the root-mean-square (RMS), roughness exponent, and correlation length, which can describe the fracture flow under an effective medium regime and a percolation regime. Finally, we interpreted the flow transition behavior from the effective medium regime to the percolation regime during fracture closure with the established stiffness-permeability function. (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/).

One effective technique for mitigating the earthquake-induced liquefaction potential is the installation of stone columns. The permeability coefficients of stone columns are high enough to cause a high seepage velocity or expedited drainage. Under such conditions, the fluid flow law in porous media is not linear. Nevertheless, this nonlinear behavior in stone columns has not been evaluated in dynamic numerical analyses. This study proposes a dynamic finite element method that integrates nonlinear fluid flow law to evaluate the response of liquefiable ground improved by stone columns during seismic events. The impact of non-Darcy flow on the excess pore pressure and stress path compared to conventional Darcy law has been investigated numerically in stone columns. Furthermore, the effects of different permeability coefficients and stone column depths have been studied under near and far field strong ground motions. The results indicate that the non-Darcy flow increases the excess pore water pressure as high as 100% in comparison to the Darcy flow.

Application of biopolymers to improve the mechanical properties of soils has been extensively reported. However, a comprehensive understanding of various engineering applications is necessary to enhance their effectiveness. While numerous experimental studies have investigated the use of biopolymers as injection materials, a detailed understanding of their injection behavior in soil through numerical analyses is lacking. This study aimed to address this gap by employing pore network modeling techniques to analyze the injection characteristics of biopolymer solutions in soil. A pore network was constructed from computed tomography images of Ottawa 20-30 sand. Fluid flow simulations incorporated power-law parameters and governing equations to account for the viscosity characteristics of biopolymers. Agar gum was selected as the biopolymer for analysis, and its injection characteristics were evaluated in terms of concentration and pore-size distribution. Results indicate that the viscosity properties of biopolymer solutions significantly influence the injection characteristics, particularly concerning concentration and injection pressure. Furthermore, notable trends in injection characteristics were observed based on pore size and distribution. Importantly, in contrast to previous studies, meaningful correlations were established between the viscosity of the injected fluid, injection pressure, and injection distance. Thus, this study introduces a novel methodology for integrating pore network construction and fluid flow characteristics into biopolymer injections, with potential applications in optimizing field injections such as permeation grouting.

Experiments on fluid flow in porous media, using fluids loaded with solids of various grain sizes, have been conducted in a modified Hele-Shaw setup. This setup utilised weakly cemented porous media with specific hydraulic and mechanical properties. Fluid injection in coarse granular media with clean or low-concentration fine particles, results in infiltration only, with pressure close to the material tensile strength, while injection in finer granular material causes damage alongside infiltration, with the fluid pressure still close to the material tensile strength. When larger particle sizes or higher particle concentrations are used in the mixture, the fluid travels further within the porous medium, primarily influenced by the grain size of the granular medium. In the latter case, the Darcy flow equation with an effective permeability term can be employed to determine the pressure differential. For the largest particle sizes included in the fluid, the equation is still applicable, but the effective permeability requires adjustment for particle size within the fluid rather than the granular medium. This is crucial when the injection point is locally clogged. The experiments show that fracturing conditions are controlled by different mechanisms. Dimensional and statistical analysis was used to classify the injection pressures to regimes predicted by fracturing theory or by Darcy law with modified effective permeabilities. The findings show that both the material properties and fluid composition are important designing parameters.

Fractures with fluid flow can lead to the damage of rock carving relics. During the detection of fractures, millimeter-scale fractures are usually difficult to determine due to their small apertures. Considering the rapid variation of water content in the fracture seepage zone can lead to anisotropy, this article proposes a new methodology to detect these millimeter-scale fractures with fluid flow using a time-lapse full-polarimetric ground penetrating radar (FP-GPR) scheme and an anisotropy analysis method. The time-lapse FP-GPR detection can monitor the water flow in the fracture and the infiltration in the rock, and the Freeman decomposition, H-Alpha decomposition, and a polarimetric phase (PP) feature are adopted to quantify and analyze the anisotropic effects over time. In the numerical test, we adopt hydrological modeling to build realistic dielectric models for time-lapse FP-GPR simulations. The results indicate that the variations of water contents and several polarimetric features, i.e., the surface-like scattering power, the double-bounce scattering power, and the averaged scattering angle, are consistent and are essentially related to the anisotropy of the seepage zone. Finally, we introduce the field tests performed at the experimental station of the Dazu Rock Carvings in Chongqing, China, which contain two cases I and II. Case I is an experiment on a surface fracture of a cliff, whereas case II is a detection test of a buried fracture. The results verify the effectiveness of the proposed methodology.

Dynamically loaded soils can exhibit large-deformation flow liquefaction or limited-deformation cyclic mobility mechanisms, depending on the initial state of the soil. Undrained cyclic triaxial tests were performed on saturated calcareous and silica sand specimens prepared with different relative densities and subjected to various effective confining pressures and cyclic stress ratios to study the flowability of viscous liquefied sand. The cyclic shear stress-strain rate relationship for calcareous and silica sands transitioned from an elliptical shape to an asymmetric Lame curve shape as excess pore pressures accumulated under cyclic loading. The asymmetric Lame curve-shaped relationship demonstrates that the saturated sand exhibited low shearing resistance and high fluidity under elevated excess pore pressures for the conditions evaluated. The average flow coefficient, kappa over bar , defined as the maximum shear strain rate triggered by the unit average cyclic shear stress, and the flow curve defining the variation in kappa over bar with the number of loading cycles, describes the flowability of the saturated sand and is used to quantify the cyclic failure potential of the saturated sand under a proposed viscous fluid flow failure criterion. The effect of relative density, effective confining pressure, and cyclic stress ratio on the flow curves and the number of cycles to failure under the proposed viscous fluid flow failure criterion is discussed and compared with the cyclic resistance determined from widely used excess pore pressure- and strain-based cyclic failure criteria. The viscous fluid flow cyclic failure criterion is more stringent than these alternative criteria, and the corresponding axial strains are consistent with those associated with liquefaction triggering under cyclic strain approach.