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.

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.

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/).