Characteristics of the Excavation Damaged Zone (EDZ) have important significance on surrounding rock stability assessment and support design. Through detecting and analysis of the EDZ of Jin-ping Access Tunnel with a general depth 1500m~2000m, the influence of blasting load, in-situ stress transient unloading and static stress redistribution on EDZ were studied and compared. The damage mechanism of in-situ stress transient unloading to rock mass was also studied adopting numerical simulation technique. The result indicates that the extent of EDZ is significantly influenced by the state of in-situ stress, and the EDZ of Jin-ping Assistant Tunnel could be divided into two parts: Inner Damage Zone (IDZ) and Outer Damage Zone (ODZ). The IDZ is attributed to blasting load and transient unloading of in-situ stress, and is characterized by a more rapid reduction of acoustic velocity of rock mass, while the ODZ is caused by redistribution of in-situ stress and characterized by a gradual reduction of acoustic velocity. In addition, the extent of IDZ is significantly larger than that of ODZ, and its distribution on tunnel section is strongly controlled by the redistribution stress field after excavation, which indicates that the effect of in-situ stress transient unloading during blasting is one of the causes of IDZ, and it is confirmed by both detecting results and theoretical analysis in this paper.
In the process of implementing the Western Development Strategy of China, the construction of large-scale hydropower projects involves the work of large-scale rock excavation under middle-to-high in-situ stress and complex geologic condition, and has to face the problems in controlling the excavation-induced damage and displacement for massive rock structures. Because of the promotion of building underground nuclear waste repositories, a large number of studies have been carried out since the end of 1970's with regard to the excavation-induced response of rock mass surrounding underground openings. Some important progresses have been made in the mechanism of the Excavation Damaged Zone (EDZ), rock properties of EDZ, the inspection and diagnose techniques of EDZ, etc[1-4]. Experience at the URL has shown that when developing tunnels in crystalline rock, damage occurs from energy imparted to the rock from the excavation method, redistribution of in situ stresses around the opening and subsequent factors that influence the growth of damage around the opening such as subsequent excavation and thermal loading [1]. In reality, when a tunnel is excavated by long round drill and blast method, some of the strain energy in the system is converted into kinetic energy that needs to be dissipated. Theoretical analysis and field photographic data obtained using high speed camera both indicated that the unloading process of in-situ stress during blasting excavation is a transient (or dynamic) process because the whole time during which the exploded rocks were thrown away from mother rock is in magnitude of some milliseconds. Early in 1960s, Cook found that tensile stress could be occurred by the over loosening induced by the instant unloading of in-situ stress during rock excavation [6].
