Contributions of In-Situ Stress Transient Redistribution to Blasting Excavation Damage Zone of Deep Tunnels

With the background of construction of the headrace tunnels with the deepest buried depth in China at present, by means of carefully acoustic velocity detection and analysis of Excavation Damage Zone (EDZ), the contributions to damage zones made by the effect of in situ stress transient redistribution are studied and compared with the extent of damage caused by the explosive load. Also, the numerical simulation was adopted to verify detecting the results. It turned out that the in situ stress transient redistribution during blasting has great influence on the development of EDZ of deep tunnels. The blasting excavation-induced damage zone of deep tunnels can be divided into the inner damage zone and the outer damage zone from the excavation surface into surrounding rocks. Although this damage zone dividing method is similar to the work of Martino and Chandler (2004), the consideration of developing a mechanism of the inner damage zone, especially the contribution of in situ stress transient redistribution, is totally different. The inner damage zone, which accounts for 29–57 % of the total damage zone, is mainly caused by explosive load and in situ stress transient adjustment, while the outer damage zone can be mostly attributed to the static redistribution of in situ stress. Field tests and numerical simulation indicate that the in situ stress transient redistribution effect during blasting contributes about 16–51 % to the inner damage zone in the 2# headrace tunnel of Jinping II Hydropower Station. For general cases, it can be concluded that the in situ stress transient redistribution is one of the main contributors of an excavation damage zone, and damage caused by in situ stress transient redistribution effect may exceed the damage caused by explosion load and become the main inducing factor for damage with the rise of in situ stress levels.