Mechanical properties of NPR anchorage in layered composite rock mass under the influence of different dip angles

Understanding the mechanical behavior of anchored composite rocks is essential for revealing the deformation and failure mechanisms of bolted layered rock structures. The study examines the mechanical characteristics of sandstone-mudstone composite specimens under different bedding dip angles and cable support types, using acoustic emission (AE) and digital image correlation (DIC) techniques for monitoring. Experimental results indicate that specimens anchored with PR cables exhibit more extensive damage and a higher crack density than those reinforced with NPR cables. Both types of anchored composite specimens display a clear bedding angle effect (15°–45°) on their compressive strength and surface displacement evolution. Specifically, compressive strength decreases progressively with increasing dip angle, and high-displacement zones on the specimen surface expand accordingly. Meanwhile, the failure mode shifts from localized small-scale cracking at 15° to interface slip failure at 45°. Notably, the bedding angle affects the AE characteristics of PR-anchored and NPR-anchored specimens in different ways. In PR-anchored samples, the proportion of shear cracks and high-frequency AE signals increases with dip angle, reflecting a clear sensitivity to bedding orientation. In contrast, NPR-anchored specimens exhibit relatively stable tensile-shear crack proportions and AE frequency distributions regardless of dip angle. These findings provide a theoretical basis for addressing stability challenges in roadways with layered sandstone–mudstone roofs and contribute to the advancement of bolting support technologies for composite rock masses with inclined bedding structures.