###### Abstract  
The instability of jointed rock masses has been a persistent concern in China's underground geotechnical engineering, particularly regarding rock mass instability triggered by structural activation, such as faulting. This form of instability constitutes a significant type of dynamic geological hazard in the field of geotechnical engineering. Research on the mechanism of jointed rock mass instability typically concentrates on various characteristics associated with structural activation but frequently neglects the interplay between coalescence instability within the jointed zones and the intact zones, as well as the development and evolution of abrupt water channels. To delve into the coalescence instability characteristics between jointed and intact zones, this study conducted uniaxial compression tests on macro-scale pre-fabricated jointed sandstone. The research results show that the failure process of the specimen consists of a strong deviation linear stage, a sub-critical stage, and an unstable stage. The main failure process occurs during the sub-critical stage. jointed rock masses; coalescent failure; energy evolution; identification information +
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In the research concerning rock joint dip angles and connectivity, it has been observed that the pre-peak energy self-inhibition effect in jointed rock increases with an increasing joint dip angle, initially strengthening and subsequently weakening. This effect exhibits a positive correlation with joint length [6]. The sequence of failure in jointed rock undergoes changes as the joint dip angle increases, transitioning from filled-type weak joints to the rock matrix [7]. Joint dip angles of 45\({}^{\circ}\) and 90\({}^{\circ}\) exert the most significant influence on the mechanical properties of jointed rock, while the impact is minimal at a 15\({}^{\circ}\) dip angle [8]. Non-connected joints noticeably degrade the rock mass, with the strength of jointed rock positively associated with the joint dip angle. With increasing joint connectivity, the fracture toughness of jointed rock follows an approximately hyperbolic trend, decreasing [9]. An equivalent elastic model for rock masses featuring intersecting connected and non-connected joints has been developed using the equivalent elastic modeling approach [10]. In recent years, many coal mines in eastern China have reached depths exceeding one kilometer, resulting in extensive bidirectional jointing within the surrounding rock of tunnels, especially under high stress and intense mining conditions. This has contributed to increased complexity in the discontinuity and deformation damage of the surrounding rock. The process of crack propagation in rock specimens with two joints differs from that in specimens with a single joint, and the development of cracks is influenced by factors such as joint spacing, dip angle, and location. Consequently, numerous scholars have delved into the mechanisms of crack propagation through the interaction of two joints. In the case of layered jointed rock masses, as the joint angle increases, the impact of joint surfaces on failure characteristics decreases [11]. In non-connected jointed rock masses during unloading, the maximum shear stress occurs on both sides of the joint, followed by the center of the rock bridge, while the joint surface experiences the minimum shear stress [12]. The numerical values of elastic strain energy and dissipation energy for double-jointed rock specimens exhibit significantly smaller abrupt increases or decreases compared to single-jointed specimens [13]. Under dynamic loading, specimens with double cracks at different angles display an X-shaped distribution of the primary fracture [14]. In jointed rock masses, coplanar joint tips predominantly undergo pure shear failure, while adjacent joint tips experience both tension and shear failures. Non-coplanar joints primarily exhibit a combination of shear and tension failures [15].