Deformation and damage mechanisms of Y-intersecting jointed rocks under uniaxial compression

Abstract

This study systematically analyzes the influence of different combined joint dip angles on rock mass failure modes and damage mechanisms through uniaxial compression tests on granite specimens with prefabricated Y-shaped discontinuities, combined with digital speckle and acoustic emission (AE) monitoring. The results show that as the dip angle of the primary joint increases, the failure mode transitions from overall failure to wedge block ejection and shear failure. A failure mode identification model was established based on main crack dip angle thresholds (40°, 45°), uniaxial compressive strength thresholds (40, 90 MPa), and energy core zone proportion thresholds (20%, 10%), achieving an accuracy of 93.3%. In the overall failure and wedge block ejection modes, a sharp increase in shear crack ratio and a sudden drop in the acoustic emission b-value occur in the high-stress phase (>0.6σc), while in the shear failure mode, significant fluctuations are observed due to the shear-tension alternation, making it difficult to identify a single critical point. Additionally, joint slip in the overall failure and wedge block ejection modes primarily occurs during the failure instability phase (>0.8σc). These findings provide theoretical support for stability evaluation of complex fractured rock masses and practical guidance for engineering safety construction.

1. Introduction

The presence of discontinuities such as joints, fractures and faults in natural rock masses significantly affects their stability [1]. Particularly in underground engineering, joint and fracture zones in rock masses often lead to tunnel instability problems, including collapses, roof falls and rock bursts (Fig. 1) [2–5]. The deformation and instability of jointed rock masses pose significant challenges to tunnel stability management. In practical engineering, joints often intersect, and their geometric configurations directly control the hazard level of instability modes by regulating crack propagation paths and energy release chronology. Therefore, the study of the mechanical behaviour and failure mechanisms of intersecting joints not only provides a theoretical basis for optimising differentiated support designs, but also serves as a critical prerequisite for hazard classification and early warning, thereby ensuring the safety of deep underground engineering projects.

In recent years, scholars both domestically and internationally have conducted extensive research on the failure modes of different types of jointed rock masses, with a focus on single-joint, double-joint, and multi-joint rock masses [2,6–10]. For single-joint rock masses, studies have shown that failure modes include tensile failure, sliding along the primary joint, and mixed failure [11], with the geometric dip angle and morphology of the joints being key factors in determining the failure mode. As the number and spatial complexity of joints increase, the failure behavior of rock masses becomes more complicated, influenced by multiple parameters and displaying multi-modal characteristics [12,13]. Among these factors, the geometric angle of the joints remains a core controlling variable. For example, Lin et al. [8], through experiments on layered rock masses, found that when the joint angle is less than 30°, crack propagation dominates the failure mode; when it exceeds 60°, high-strength rock layers inhibit crack propagation, and the low-strength layers become the point of failure. Numerical simulations by Chen et al. [14] further indicate that the peak strength and elastic modulus of rock masses containing X-shaped fractures decrease as the fracture angle β (β<90°) increases, but increase as the direction angle α (the angle between the bisector of β and the loading axis) increases. These changes are highly coupled with the stress threshold for crack initiation.