Most underground constructions are located in a jointed rock mass. The excavated openings will disturb the initial stress state of the rock mass, resulting in compressive and tensile stress concentration around the opening and joints. The redistributed stress will cause crack initiation, propagation and coalescence, which greatly reduces the strength of rock mass and threatens the stability of the underground structures. Under complex geological conditions, the failure of surrounding rock mass is complex due to the diversity of joint occurrence. Therefore, the interaction mechanism between the opening and its surrounding joints should be further studied.

In this research, prismatic sandstone specimens were pre-flawed with a circular opening and two joints (called H-J specimen for short). The different relative positions between the opening and joints are specially researched by varying the ligament angle of the rock bridge. The H-J sandstone specimens were subjected to an uniaxial compression tests, assisted by a camera and acoustic emission (AE) system. Also, particle flow code (PFC) were used to further reveal the stress distribution around the opening.

The experimental results show that different relative positions between the opening and joints greatly influence the strength and deformation characteristics. The minimum strength occurs when ligament angle is 45°, while the cracking process and energy release are more intense and sudden when ligament angle is 0° ~30° and 90°. The numerical results reveal that the stress distribution around the opening is closely related to the coalescence failure patterns and the maximum principal stress on the opening sides and shoulders may reflect the strength characteristics of underground structures.

1 Introduction

With the growing demand for energy and space in human society, the underground engineering has developed rapidly in the recent decades. However, complex geological rock mass poses great challenges to underground constructions for its discontinuity, inhomogeneity and anisotropy, mostly caused by the existence of flaws, joints, cracks, and faults, etc. (Hoek and Brown 1997). Generally, the stress disturbance caused by the underground excavation in jointed rock mass will cause stress concentration around the opening and flaws, finally resulting in crack initiation, propagation and coalescence failure of surrounding rock mass. Therefore, the cracking behavior and stress interaction between the opening and its adjacent joints should be further understood. Most joints or flaws are nonpersistently existed in the rock mass (Lajtai et al. 1994). The strength and stiffness of rock mass are greatly influenced by the geometric and mechanical properties of non-persistent joints. Therefore, the strength, deformation and cracking process of pre-cracked rock or rock-like specimens have been widely investigated based on the dip angle of single joint (Wong and Einstein 2009), parallelism (Yin et al. 2014) or coplanarity (Yang 2011) of two joints, and the dip, space, continuity of joint sets (Chen et al. 2018), etc. It can be concluded that the wing cracks, anti-wing cracks and secondary cracks usually initiate from the joint tips. The coalescence modes (tension, shear, or mixed tension and shear) are influenced by different relative positions of joints, indicating different interaction mechanisms. However, it is more complex when the opening is excavated in the jointed rock mass for the interaction between the opening and its surrounding joints, which may result in more unconventional and complex failure around the opening. In general, the failure of opening mainly located on the top and sides, such as the roof fall and rib spalling of tunnel or roadway (Lajtai et al. 1991; Martin et al. 1997). However, when a fault or weak plane is encountered, the deformation and plastic region size of opening tend to become larger, for the activation or shear deformation of fault or weak plane (Jeon et al. 2004; Hao and Azzam 2005; Yan et al. 2012). In order to further explore the failure characteristics and mechanisms of the excavated opening in jointed rock mass, many simplified small-scale physical tests were conducted (Sagong et al. 2011; Yang et al. 2017; Yang et al. 2019). Their results proved that the strength and deformation characteristics, crack coalescence patterns, stress distribution around the opening are all highly dependent to the joint angle.

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