A series of true triaxial unloading tests are conducted on sandstone specimens with a single structural plane to investigate their mechanical behaviors and failure characteristics under different in situ stress states. The experimental results indicate that the dip angle of structural plane (B) and the intermediate principal stress (o2) have an important influence on the peak strength, cracking mode, and rockburst severity. The peak strength exhibits a first increase and then decrease as a function of o2 for a constant B. However, when o2 is constant, the maximum peak strength is obtained at B of 90 degrees, and the minimum peak strength is obtained at B of 30 degrees or 45 degrees. For the case of an inclined structural plane, the crack type at the tips of structural plane transforms from a mix of wing and anti-wing cracks to wing cracks with an increase in o2, while the crack type around the tips of structural plane is always anti-wing cracks for the vertical structural plane, accompanied by a series of tensile cracks besides. The specimens with structural plane do not undergo slabbing failure regardless of B, and always exhibit composite tensile-shear failure whatever the o2 value is. With an increase in o2 and B, the intensity of the rockburst is consistent with the tendency of the peak strength. By analyzing the relationship between the cohesion (c), internal friction angle (4), and B in sandstone specimens, we incorporate B into the true triaxial unloading strength criterion, and propose a modified linear Mogi-Coulomb criterion. Moreover, the crack propagation mechanism at the tips of structural plane, and closure degree of the structural plane under true triaxial unloading conditions are also discussed and summarized. This study provides theoretical guidance for stability assessment of surrounding rocks containing geological structures in deep complex stress environments. (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/).

Large-scale and heavily jointed rocks have inherent planes of anisotropy and secondary structural planes, such as dominant joint sets and random fractures, which result in significant differences in their failure mechanism and deformation behavior compared to other rock types. To address this issue, inherent anisotropic rocks with large-scale and dense joints are considered to be composed of the rock matrix, inherent planes of anisotropy, and secondary structural planes. Then a new implicit continuum model called LayerDFN is developed based on the crack tensor and damage tensor theories to characterize the mechanical properties of inherent anisotropic rocks. Furthermore, the LayerDFN model is implemented in the FLAC3D software, and a series of numerical results for typical example problems is compared with those obtained from the 3DEC, the analytical solutions, similar classical models, laboratory uniaxial compression tests, and field rigid bearing plate tests. The results demonstrate that the LayerDFN model can effectively capture the anisotropic mechanical properties of inherent anisotropic rocks, and can quantitatively characterize the damaging effect of the secondary structural planes. Overall, the numerical method based on the LayerDFN model provides a comprehensive and reliable approach for describing and analyzing the behavior of inherent anisotropic rocks, which will provide valuable insights for engineering design and decision-making 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/).

The role of structural planes in controlling mudstone landslides is a key issue in the study of geo-disasters in the Loess Plateau of China. In this study, the effects of sliding-control structures on the mechanisms of mudstone landslides are investigated via three model experiments with different slope structures. The results show that the hydrological response and failure mode of the experimental slope vary with the structural conditions. The vertical joints serve as preferential seepage paths, which accelerate rainfall infiltration, resulting in earlier responses of volumetric water content and pore water pressure. With the incorporation of vertical joints, the slope failure mode tends to transform from shallow failure to deep-seated failure. The presence of a weak interlayer leads to significant increases in the velocity and runout of the sliding mass. The variation in the slope failure extent and deformation characteristics with varying sliding-control structures further changes the temporal and spatial distributions of volumetric water content and pore water pressure. The different slope failure modes correspond to different sliding-control mechanisms, which are dominated by the types of structural planes and their interactions with hydrological responses. In the action of these mechanisms, pore water pressure and seepage force play significant roles in the reduction of effective stress and shear strength.

Geological disasters occur frequently in the Loess Plateau due to the joint fissures in the strata and human engineering activities. Against this background, the deformation and failure mode of the loess slope with the structural plane under excavation and the extension mechanism of the structural plane are analyzed and summarized. The results showed that: (1) Through the physical model test, the deformation failure mode of the slope is summarized as the tension-splitting, pressure-sliding shallow failure. The collapse failure process is defined as four stages: Compression deformation, creep deformation, slip deformation and slip failure. (2) Slope displacement is concentrated beneath the pressure plate, increasing linearly under load conditions but becoming nonlinear after excavation conditions. As the excavation angle rises, the displacement range along the structural plane gradually extends toward the slope toe. The displacement time-history curve shows three stages: The lifting load stage, the cumulating deformation stage, and the sliding failure stage. (3) The stress redistribution caused by excavation, prompting deformation and potential failure. As internal stress nears the soil strength limit, human-induced disturbances exacerbate stress redistribution, leading to accumulated stress. Finally released through deformation and cracking. Each excavation condition modifies the original loading transfer path, driving stress redistribution at the slope surface and at the structural plane's tip. (4) The sudden drop in stress level and sudden rise of accumulated settlement are the characteristics of slope sliding failure. The position of the structural plane determines the position of the slope sliding surface. (5) According to the external characterization of the structural plane, the extension process of the structural plane can be defined as four stages: Initiation of crack extension, classification deformation, sub extension and compression sealing. According to the extension of the structural plane, the spreading cracks of the slope's internal structural plane are defined as two types: Fractured cracks and shear cracks.

The loess structural planes of different formation, scales, origin, and types are widely developed in loess slopes, which can significantly control the structure, hydro-mechanical properties, damage regulations and deformation failure pattern of the slope. A series of major engineering projects have been implemented on the Loess Plateau of China. These projects have formed many loess slopes, which are prone to failure induced by loading. However, the failure mechanism of heap-loading loess slope, especially the influence of structural plane on slope failure, is not clear. Therefore, based on the investigation and analysis of the characteristics of loess structural planes, a large-scale model experiment was carried out, and the deformation process and failure mechanism of loess landslide induced by loading were systematically investigated. The soil pressure distribution, plastic state, and deformation characteristics of the slope were analyzed to reveal the influence of the structural plane on slope failure. The results show that the existence of the structural plane changed the stress field of the loess slope, forming a preferential yielding region around the structural plane, making the structural plane more likely to become a potential sliding surface. Different increases of earth pressure in the x- and z-direction is the main reason for the change in the extension angle of the structural plane. The propagation of the shear zone presents a typical double slip surface structure. The failure process of the loess slope induced by loading could be generalized into structural plane extension, shear band initiation, shear band penetration, and sliding failure stages.

In geotechnical engineering, it is of great importance to study the mechanical properties of the interface between geomaterials and structure. In order to understand the shear behavior of the interface between soil-rock mixture and structure, the discrete element method is used to simulate the interface shear test of soil-rock mixture and rough structural plane. Two confining pressures and three roughness are considered. In terms of mechanical properties, macro-mechanical behavior and micro-mechanical behavior are analyzed. In terms of deformation, in order to identify the local area that plays a critical role in the interface, three methods are used to quantitatively describe the shear band thickness, including localized deformation, new contact, and particle displacement. Particle displacement is a physical quantity that can be directly observed in laboratory tests, while localized deformation must be calculated and analyzed. And new contact is a quantity that can be captured by discrete element simulation. Different analysis methods of shear band thickness are suitable for analyzing different problems by different test methods. The results show that the thickness of shear band increases with the increase of structural plane roughness. The mechanism of this phenomenon is explained by the microscopic particle inlay and the active and passive earth pressure on both sides of the structural plane unit.

Structural planes play an important role in controlling the stability of rock engineering, and the influence of structural planes should be considered in the design and construction process of rock engineering. In this paper, mechanical properties, constitutive theory, and numerical application of structural plane are studied by a combination method of laboratory tests, theoretical derivation, and program development. The test results reveal the change laws of various mechanical parameters under different roughness and normal stress. At the pre-peak stage, a non-stationary model of shear stiffness is established, and threedimensional empirical prediction models for initial shear stiffness and residual stage roughness are proposed. The nonlinear constitutive models are established based on elasto-plastic mechanics, and the algorithms of the models are developed based on the return mapping algorithm. According to a large number of statistical analysis results, empirical prediction models are proposed for model parameters expressed by structural plane characteristic parameters. Finally, the discrete element method (DEM) is chosen to embed the constitutive models for practical application. The running programs of the constitutive models have been compiled into the discrete element model library. The comparison results between the proposed model and the Mohr-Coulomb slip model show that the proposed model can better describe nonlinear changes at different stages, and the predicted shear strength, peak strain and shear stiffness are closer to the test results. The research results of the paper are conducive to the accurate evaluation of structural plane in rock engineering. (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/).