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/).

Drilling pressure relief is one of the methods to reduce the risk of coal bursts in deep mines. However, the effect of the drill hole orientations has not been studied well enough to understand their impact on the burst failure mechanism. In this study, we investigated two designs of drill hole orientations. The first design includes drill holes located on the upper free face of the rectangular samples and labelled as upper hole (UH) and centre hole (CH) - the long axes of the drill holes are aligned with minor principal stress, sigma(3), direction. The second design includes drill holes at the top (TH) and the side (SH) of the rectangular samples in which the long axes of the drill holes are aligned with the maximum, sigma(1), and intermediate principal stress, sigma(2), directions, respectively. The coal samples with the proposed drill hole orientations were subjected to the true-triaxial unloading coal burst tests. The results show that the drill holes reduce the risk of coal bursts. However, we found that the intensity of coal burst was significantly reduced with the SH-type, followed by the CH-types. We also observed that the coal burst intensity is reduced better for the CH, UH, TH, and SH-type drilling patterns. However, it was found that the orientations of drill holes have little influence on the failure mode (splitting). The acoustic emission (AE) activities for coal with drill holes noticeably decreased, especially for the UH and CH layouts. The drill holes reduced the upper limit of the AE entropy (chaos of microcracks generation). However, regarding reducing the coal burst risk, the TH and SH are less effective than UH and CH. (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/).

For the soft-plastic loess tunnel engineering, collapse and damage of the surrounding rock during excavation are often driven by the combined action of the seepage water and the unloading effect. Under water pressure and unloading, the soil suffers complex stress-seepage coupling action causing the inevitable change of permeability and mechanical properties. In this paper, seepage control devices were added to the GDS test device, and a new triaxial permeability measurement system was developed. Triaxial unloading-seepage tests were conducted on soft-plastic loess under the effect of hydraulic coupling. The variation of permeability characteristics of Q(2) type soft-plastic loess under lateral unloading and the soil mechanical characteristics under different seepage pressure were analyzed. Meanwhile, microstructure characteristics of soft-plastic loess during the triaxial test were obtained by scanning electron microscope to clarify the deformation and seepage mechanism. The results show that the strength of soft-plastic loess decreases significantly with the increase of osmotic pressure. Under the condition of 50 and 100 kPa osmotic pressure, the cohesive force of soft-plastic loess decreases by 15.5% and 39.0% and the friction angle decreases by 9.4% and 22.6%, respectively. The permeability coefficient of loess increases slowly at first and then increases rapidly during the unloading process. The main reason for the significant increase of permeability coefficient is the penetration of soil fissures and the formation of shear bands after entering the plastic deformation stage.