# Scale-Model Experiment on the Delayed Failure Mechanism of Deep Tunnels in Brittle Rock
## 1 Introduction  
Changes in local stress state due to deep tunnelling perturb the stability of the rock mass surrounding excavations. The subsequent readjustment of the rock towards a new equilibrium does not occur instantaneously but as a gradual process over time. This process can include two types of inelastic deformation, namely creep-like movements and violent failures, or rockbursts [1]. As the creep rate of the brittle rock is low, the rockbursts in deep tunnels tend to occur in a sudden way, with little warning of the deformation [2]. With rockbursts being found frequently in deep tunnels with brittle rock surrounding, causing men injury and destroying equipment [3], researchers conducted a theoretical and experiential studies on the delay mechanism of the rockburst [2, 3, 4, 5, 6, 7, 8]. Early research recognized that the delay time of the rockburst phenomenon determined the relaxation time. As the specific potential energy of weak rocks at a given time will be less than that of hard rock, the probability of the occurrence of brittle fractures will be greater in hard rock [4]. The relaxation time used in this study was measured by the ultrasonic method on rock cores; the engineering level evaluation method was not implemented [4]. Subsequent research on rockbursts focused on some engineering cases and rockburst prevention and control. [PERSON] and [PERSON] investigated the unique problemsassociated with the design of rockburst support for shallow-dipping tabular excavations [1]. [PERSON] et al. studied rockbursts in the mine, his research shows that rockbursts are caused by a combination of movement, special breaking laws of thick conglomerate, and special breakage-induced stress fields [1; 9]. [PERSON] and [PERSON] reviewed, selected, and published rockbursts' case data and identified the contributing factors, including unfavourable stress states, excavation geometry (size and orientation in relation to the principal stresses), and rate and direction of advance [3]. [PERSON] et al. conducted rockburst tests on rectangular prismatic specimens of six types of intact hard brittle rocks, i.e., granodiorite, granite, marble, basalt, sandstone, and limestone, under one free-face true triaxial loading conditions. The results show that the inherent microstructures and fracture evolution under loading are the primary factors accounting for different rockbursts in various rock types [2]. [PERSON] et al. monitored microseismicity during the excavation of deep tunnels, the difference in the mechanism of strain rockburst and strain structure slip rockburst was found [10]. The delayed failure mechanism has not reached a consensus yet. To figure out the delayed failure mechanism, the focus of recent research has begun to shift towards the study of time-to-failure of rocks. Time-to-failure is the key indicator of delayed rockburst occurrence. [PERSON] and [PERSON] reviewed the theory of the time-to-failure and proposed a simulation scheme to reproduce daily failure processes in macroscopic scale [11]. [PERSON] et al. researched the time-dependent failure mechanism of rocks and associated the application in slope engineering with numerical simulation [12]. [PERSON] and [PERSON] proposed a time-to-failure creep model to simulate the lifetime of intact brittle rock under uniaxial and triaxial loading conditions [13]. [PERSON] et al. predicted the time-to-failure of red sandstone by a temporal precursor of acoustic emission signals [14]. [PERSON] proposed a unified approach to evaluate stresses in the limit state zones and the time-to-failure in rock mass with regard to internal friction [15]. [PERSON] et al. established a time-lag numerical model using particle flow code 2D to investigate the failure characteristics and the time-lag of rock masses with structural planes under unloading conditions typical of hard brittle rock [16]. The time-to-failure, as the key indicator of delayed rockburst, was the main concern in recent studies.