Accurate characterization of rock anisotropy is crucial for underground engineering stability assessment. In this study, multi-directional drilling tests were performed on sandy mudstone and argillaceous sandstone, with real-time monitoring of feed force (F), torque (M), rotational speed (n), power (P), drilling velocity (v), and depth (h). Drilling-derived apparent friction angles (φ) in different directions were estimated using a force-equilibrium-based model. Based on drill bit geometry, an energy balance model for hollow drilling was developed, and a unit grinding energy (ηe) was derived. A preliminary drilling-derived anisotropy index (Bφ−ηe), based on the coefficient of variation (CV) and integrating φ and ηe, was proposed. Results show a strong linear correlation between thrust force and torque, both exhibiting a two-stage increase with drilling depth. For the tested drilling orientations, the anisotropy determined using the proposed method decreases in the following order: sandy mudstone 1, sandy mudstone 2, argillaceous sandstone 2, and argillaceous sandstone 1. The proposed index provides a preliminary basis for evaluating directional variations in rock anisotropy.
The tensile strength properties and the initiation and propagation of tensile cracks within micritic bioclastic limestone–which forms the surrounding rock mass at the tunnel exit of the Altash Water Conservancy Project–pose a threat to engineering stability. To investigate the differences in tensile strength and stress-induced crack evolution under natural and water-saturated conditions, a comparative experimental study was conducted using Brazilian splitting tests coupled with acoustic emission (AE) monitoring. The results demonstrate that, compared with natural specimens, saturated limestone exhibits a 16.54% reduction in tensile strength. The failure process can be categorized into three distinct stages: compaction, quasi-linear elasticity, and unstable crack propagation. Furthermore, AE analysis indicates that while overall AE activity decreases following water saturation, the proportion of tensile cracks increases from 92.30% to 95.14%. Conversely, under natural conditions, shear cracks are more active and initiate earlier. Microscopically, the high content of bioclasts and associated complex interconnected pores (e.g., body cavity and secondary dissolution pores) endow the rock with remarkable hydrophilic and water-retention characteristics. Coupled with the presence of the abundant hydrophilic mineral illite, these factors collectively exacerbate water-rock interactions, driving the significant degradation of the rock’s mechanical properties from both material and structural perspectives.
Zu-Guo Mo, Maojun Huang, Yong Wu et al.· Frontiers in Built Environme...· 0 citations
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