Experimental study on failure law of damaged surrounding rock and its anchorage mechanism

In response to the insufficient research on the damage weakening evolution law of surrounding rock during roadway (tunnel) excavation and retreat, as well as the inadequate understanding of the anchoring mechanism of rock bolts in such rock masses, a series of triaxial loading-unloading and uniaxial reloading experiments were conducted on sandstone specimens using the RMT-150 C rock mechanics testing system. Microstructural characteristics of the damaged rock mass were investigated through scanning electron microscope (SEM) analysis, revealing the damage weakening mechanism of the rock. Through laboratory experiments and theoretical analysis, in combination with the failure characteristics of rock-anchor composite bodies, the underlying causes of changes in failure modes were analyzed to explore the anchoring mechanism of bolts in damaged rock. The experimental results indicate that during the triaxial loading-unloading tests, the degree of rock damage varied depending on the axial unloading point, exhibiting a trend of first decreasing and then increasing. For damaged rock masses, the presence of bolts did not alter the failure modes, which manifested as splitting failure at low damage levels and shear slip failure at high damage levels. However, the bolts did influence the characteristics of the failure, with the failure features of damaged rock being more diverse under the influence of bolts. The failure mode of the rock is related to the degree of damage, which can be represented by the ratio η of wing crack length to main crack length. When η is large, splitting failure occurs, whereas smaller η leads to shear slip failure. Bolts can reduce the stress intensity factor at the crack tip to some extent, inhibiting the propagation of wing cracks. This study effectively explains, from a microscopic perspective, the relationship between the failure modes of rock-anchor composite bodies and the anchoring effect of bolts.