# Formation Mechanism and Control Technology of an Excavation Damage Zone in Tunnel-Surrounding Rock
## 1 Introduction  
The drilling and blasting methods are the most commonly used in the construction of rock tunnels. The protected surrounding rock is damaged, and its stress is redistributed after the rock within a tunnel outline is thrown out by the blasting load during excavation. The strength of the protected surrounding rock is reduced, and the original cracks of it are further widened to form a circle of loosened rock around the tunnel. This has a significant influence on the supporting parameters, durability, and structural safety of the tunnel. The range, fragmentation degree, and permeability of loosened rock circles around the tunnel were extensively studied in terms of hydropower station chamber excavation, tunnel engineering, and waste storage cavern engineering [1]. Failure zone, fracture zone, and disturbance zone are formed in the surrounding rock after tunnel excavation. A large number of rocks are separated from the surrounding rock and become unstable bodies in the failure zone, and the surrounding rock generates microcracks or fissures in a fracturedzone. In the disturbance area, the stress of the surrounding rock and groundwater pressure has changed, yet other physical and mechanical properties of the surrounding rock have changed little [2]. At present, the range of loosened rock circles around the tunnel formed and their physical and mechanical properties after tunnel excavation have been studied by many scholars through theoretical analysis, field tests, and numerical simulation. A Russian scholar named [PERSON] proposed the natural equilibrium arch theory and initiated a theoretical study of the loosened rock circle around a tunnel in the early 20 th century. Subsequently, scholars established wedge theory, elastic medium theory, flexible medium theory, loose fracture theory, elastic-plastic medium theory, fracture zone diagram theory, and discontinuous zone theory [3]. [PERSON] et al. [4] and [PERSON] et al. [5] derived an analytical solution of the radius and stress of the plastic zone of a circular roadway based on elastic-plastic theory, the Drucker-Prager criterion, and the double-shear unified strength criterion. [PERSON] et al. [6] and [PERSON] et al. [7] derived an approximate solution to the boundary equation of the plastic zone of the surrounding rock of an unequal-pressure circular roadway based on the Hoek-Brown strength criterion and the Mohr-Coulomb strength criterion. [PERSON] et al. [8] and [PERSON] et al. [9; 10] verified the elastic-plastic damage mechanical model of the surrounding rock and computed the plastic zone range. Based on elastic-plastic theory, many scholars have used different strength criteria and approved analytical methods to calculate the loosened rock circles around standard circular tunnels. Nevertheless, these computational results are problematic when meeting engineering needs. In order to obtain more practical theoretical analysis results, [PERSON] et al. [11] deduced an analytical solution in which the plastic zone of a tunnel is controlled by the yield criterion during the failure process, and stress field changes were obtained in the plastic zone of the tunnel. At the same time, the development of science and technology has brought a variety of advanced monitoring instruments, which have made outstanding contributions to the monitoring test of surrounding rock loose circles. Many scholars adopted multipoint displacement measurements, the seismic wave method, the geological radar method, and the borehole imaging method to study loose circles in tunnels. At present, the single-hole acoustic monitoring method [12] and the borehole acoustic monitoring method [13; 14] are the most widely used methods to study the range of tunnels surrounding rock loose circles. With improvements in computing ability and numerical algorithms, researchers have begun to use numerical simulations to calculate the range of loose circles.