Hidden soil caves pose a serious threat to the stability and safety of subgrades. In this study, using the two-dimensional particle flow discrete element code, a total of eight subgrade models with circular soil caves of different dimensions, depths, and locations were established. Under self-weight and superimposed loading, the deformation characteristics of fill subgrade models, such as the evolution of displacement field and crack development process, were analyzed. The results show that under the self-weight, after the fill subgrade model of soil caves with diameters of 2 m, 4 m, 6 m, and 8 m is stable, the overlying soil layer of the soil cave corresponds to the transformation of slag falling, block falling, collapse and rapid collapse, respectively. The larger the dimension of the soil cave, the larger the number of cracks and damage areas, and the more prone the fill subgrade is to collapse. The superimposed load makes the fill subgrade compress from shallow to deep, significantly increasing the overall subgrade deformation, the number of cracks, and the development range. The evolution of the displacement field and crack propagation of the fill subgrade are also controlled by the buried depth and location of the soil cave. Whether the fill subgrade collapses is comprehensively controlled by the dimension and buried depth of the soil cave, the mechanical parameters of the soil layer, the load, and its scope of action. Thus, a comprehensive criterion of cylindrical collapse of the soil layer above the soil cave is constructed.

Despite the prevalence and validity of the universal distinct element code (UDEC) in simulations in geotechnics domain, water-weakening process of rock models remains elusive. Prior research has made positive contributions to a presupposed link between modelling parameters and saturation degree, Sr. Nevertheless, this effort presents inaccurate results and limited implications owing to the misleading interpretation, that is, devoid of the basic logic in UDEC that modelling parameters should be calibrated by tested macroscopic properties in contrast to a presupposed relation with Sr. To fill this gap, a new methodology is proposed by coupling a computationally efficient parametric study with the simulation of water-weakening mechanisms. More specifically, tested macroscopic properties with different Sr values are input into parametric relations to acquire initial modelling parameters that are sequentially calibrated and modulated until simulations are in line with geomechanical tests. Illustrative example reveals that numerical water-weakening effects on macroscopic properties, mechanical behaviours, and failure configurations are highly consistent with tested ones with noticeable computational expediency, implying the feasibility and simplicity of this methodology. Furthermore, with compatibility across various numerical models, the proposed methodology substantially extends the applicability of UDEC in simulating water-weakening geotechnical problems. (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/).

Wellbore breakout is one of the critical issues in drilling due to the fact that the related problems result in additional costs and impact the drilling scheme severely. However, the majority of such wellbore breakout analyses were based on continuum mechanics. In addition to failure in intact rocks, wellbore breakouts can also be initiated along natural discontinuities, e.g. weak planes and fractures. Furthermore, the conventional models in wellbore breakouts with uniform distribution fractures could not reflect the real drilling situation. This paper presents a fully coupled hydro-mechanical model of the SB-X well in the Tarim Basin, China for evaluating wellbore breakouts in heavily fractured rocks under anisotropic stress states using the distinct element method (DEM) and the discrete fracture network (DFN). The developed model was validated against caliper log measurement, and its stability study was carried out by stress and displacement analyses. A parametric study was performed to investigate the effects of the characteristics of fracture distribution (orientation and length) on borehole stability by sensitivity studies. Simulation results demonstrate that the increase of the standard deviation of orientation when the fracture direction aligns parallel or perpendicular to the principal stress direction aggravates borehole instability. Moreover, an elevation in the average fracture length causes the borehole failure to change from the direction of the minimum in-situ horizontal principal stress (i.e. the direction of wellbore breakouts) towards alternative directions, ultimately leading to the whole wellbore failure. These findings provide theoretical insights for predicting wellbore breakouts in heavily fractured rocks. (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 mechanical behavior of Methane Hydrate-Bearing Sediment (MHBS) is essential for the safe exploitation of Methane Hydrate (MH). In particular, the pore size and physicochemical characteristics of MHBS significantly influence its mechanical behavior, especially in clayey grain-cementing type MHBS. This study employs the Distinct Element Method (DEM) to investigate both the macroscopic and microscopic mechanical behavior of clayey grain-cementing type MHBS, focusing on variations in pore size and physicochemical characteristics. To accomplish this, we propose a Thermo-Hydro-Mechanical-Chemical-Soil Characteristics (THMCS) DEM contact model that incorporates the effects of pore size and physicochemical characteristics on the strength and modulus of MH. This THMCS model is validated using experimental data available in the literature. Using the proposed contact model, we conducted a series of investigations to explore the mechanical behavior of MHBS under conventional loading paths, including isotropic and drained triaxial tests using the DEM. The numerical results indicate that smaller pore sizes and lower water content-key physicochemical characteristics resulting from variations in electrochemical properties and the intensity of the electric field-can lead to reduced shear strength and stiffness due to the increased breakage of aggregates and weakened cementation. Additionally, heating was found to further accelerate the process of structural damage in MHBS.

The damage parameter is a variable used to describe the transition of geomaterials from a bonded state to an unbonded state. The correct expression of the damage evolution of structured soil is crucial in establishing constitutive models for structured soils. Currently, research on damage laws typically involves assuming expressions for damage parameters and then fitting these parameters using experimental results to establish the damage law. The rationality and applicability of these damage laws are yet to be validated. To derive a unified expression for the damage law of structured sands incorporating microscopic mechanisms, a prediction model based on symbolic regression is proposed. Firstly, using the definitions of damage parameters with microscopic physical significance, various damage databases are constructed using the distinct element method (DEM). Secondly, preliminary parameter screening is conducted on isotropic compression and constant p true triaxial compression stress paths using a method that combines input variables. p is the average effective stress. Combined with the genetic programming-based symbolic regression (GPSR), damage expressions with different complexities are derived. Finally, the best-performing expression is selected as the damage law for structured sand, namely the GPSR damage law, based on an analysis of prediction and generalization errors. The applicability of different expressions is compared using various DEM damage databases. The results show that the GPSR damage law represents damage parameters as functions of plastic deviatoric strain epsilon(s), normalized mean effective stress p/p(y) and coefficient of intermediate principal stress b. It effectively reflects the transition from structured soil to remolded soil. The outstanding prediction ability of the GPSR damage law on different damage databases further demonstrates its applicability to various geomaterials. The research findings are valuable to establish constitutive models for structured sands.

The tensile strength is an important parameter in engineering. Many engineering-related problems in buildings as well as the damage caused to them during natural disasters occur owing to a lack of tensile strength. The unconfined penetration (UP) test is an indirect method to measure the tensile strength of soil. Analyses of the mechanism of the UP test and simulations based on the discrete-element method have shown that the UP test is a complex process involving tensile and shear strengths. In this study, the authors use the modified Mohr-Coulomb model to establish a joint criterion for the failure of the tensile strength and the shear strength of loess, and derive expressions for the correlations between the relevant mechanical parameters. A combination of the results of the aforementioned model and laboratory tests showed the following: (1) the process of failure of loess samples during the UP test consisted of four stages: (I) the shaping of the wedge-shaped split body, (II) tension-induced fracture, (III) the yield stage, and (IV) damage to the sample; (2) the tensile strength of the loess decreased exponentially with its saturation; (3) the ratio of the unconfined compressive and cohesive strengths to the tensile strength of the remolded loess was 1.37 times that of the undisturbed loess, while the ratio of the unconfined compressive strength of remolded loess to its cohesion was similar to that of undisturbed loess; (4) the wedge-splitting angle ranged from 13 degrees to 23 degrees, and had a negative correlation with the internal angle of friction, a positive correlation with the water content, and decreased exponentially with the tensile strength.

Borehole instability in naturally fractured rocks poses significant challenges to drilling. Drilling mud invades the surrounding formations through natural fractures under the difference between the wellbore pressure (Pw) and pore pressure (Pp) during drilling, which may cause wellbore instability. However, the weakening of fracture strength due to mud intrusion is not considered in most existing borehole stability analyses, which may yield significant errors and misleading predictions. In addition, only limited factors were analyzed, and the fracture distribution was oversimplified. In this paper, the impacts of mud intrusion and associated fracture strength weakening on borehole stability in fractured rocks under both isotropic and anisotropic stress states are investigated using a coupled DEM (distinct element method) and DFN (discrete fracture network) method. It provides estimates of the effect of fracture strength weakening, wellbore pressure, in situ stresses, and sealing efficiency on borehole stability. The results show that mud intrusion and weakening of fracture strength can damage the borehole. This is demonstrated by the large displacement around the borehole, shear displacement on natural fractures, and the generation of fracture at shear limit. Mud intrusion reduces the shear strength of the fracture surface and leads to shear failure, which explains that the increase in mud weight may worsen borehole stability during overbalanced drilling in fractured formations. A higher in situ stress anisotropy exerts a significant influence on the mechanism of shear failure distribution around the wellbore. Moreover, the effect of sealing natural fractures on maintaining borehole stability is verified in this study, and the increase in sealing efficiency reduces the radial invasion distance of drilling mud. This study provides a directly quantitative prediction method of borehole instability in naturally fractured formations, which can consider the discrete fracture network, mud intrusion, and associated weakening of fracture strength. The information provided by the numerical approach (e.g. displacement around the borehole, shear displacement on fracture, and fracture at shear limit) is helpful for managing wellbore stability and designing wellbore-strengthening operations. (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 (http://creativecommons.org/ licenses/by-nc-nd/4.0/).

Landslide-generated debris flow is one of the severe outcomes of slope failure in hilly areas that have the potential to severely affect the physical and biological environment. The precise identification of the vulnerable areas depends on a broad understanding of the flow morphology and deposition process of the flowing debris. However, owing to varying compositions of the flowing debris coupled with inherent complex field topography, makes the flow prediction more complex. In this study, numerical analysis was performed using a distinct element-based numerical modelling technique. The numerical model was calibrated using a physical scale-down model of the residual soil slope. Calibration of the numerical model was performed under both dry and wet debris flow induced in the scale-down laboratory model. The final calibrated model was used to validate a case study. Following the successful validation of the developed numerical model, parametric analysis (various slope profiles, heights, inclinations, and particle size distributions of debris) was performed to study the rheology of the debris under both dry and wet conditions generated post-landslide event. This study gives a reliable idea about the possible flow behaviour in a simple residual soil slope and can be used as a guide to performing debris flow analysis for any natural slope. It also emphasises the need for the adoption of more robust debris runoff preventive measures in case of failures observed after precipitation owing to the very high energy and momentum of flowing wet debris in comparison with dry debris.