In the design and construction of underground engineering (tunnels, caverns, shafts, hydro-electric stations...), investigation the mechanical behavior of underground excavations, that include the influence of the rock mass characteristics on the Excavation Damaged Zone (EDZ), is necessary. More ever, the presence of the excavation damaged zone (EDZ) around an excavation boundary can significantly affect the overall performance of the excavation and the general safety of people and equipment. Hence, it has been an important subject of research on various rock mass excavation projects. For Houziyan Hydropower station in Sichuan Province of P.R. China, the understanding of the EDZ is essential for optimal design and construction of rock support. In this paper, the investigation of the main objective EDZ was conducted at the Hydropower station using geophysical test methods (acoustic waves and digital borehole camera techniques). The results of the experiments indicated that tensile failure was the microscopic failure mechanism of rock in the excavation damaged zone. The damage of caverns surrounding rock masses was usually resulted from the redistribution of stress undisturbed rock mass in caverns excavation process. The results are consistent with our predictions of forming progression and failure mechanism of rock on construction.

1. Introduction

In recent years, more and more vast and large-scale Hydropower stations have been designed and constructed in China. These constructions all involved in large-scale excavations, which provoked a zone, excavation disturbed or damaged zone (EDZ) beyond the excavation boundary, where the rock was disturbed or damaged and its physical, mechanical properties were greatly changed (Malmgren, Saiang, Töyrä, & Bodare, 2007). EDZ is the zone around an excavation where in situ rock mass properties and conditions have been altered due to stress redistribution, fracturing, blasting damage have taken place (Martino & Chandler, 2004), (S. Wang, Lee, Ranjith, & Tang, 2009). In general, a redistribution of stresses and rearrangement of rock structures will occur in this zone and result in drastic changes of stress distribution, mainly through the fractures and cracks induced by excavation. Therefore the investigation of the EDZ around an underground excavation is especially important for underground construction and supports that require a long term stability, and it is always one of the major research topics for the underground research facilities and stability for many countries including China (Sheng, Yue, Lee, Tham, & Zhou, 2002), (Zou & Xiao, 2010), (Feng, Rutqvist, & Pan, 2013). There are some researches that have studied the EDZ around underground opening, for example: using a micromechanics model based on the images of the distribution given by the digital borehole camera (Li et al., 2011); using Acoustic wave method for assessment EDZ (Xu et al., 2014), (Kwon, Lee, Cho, Jeon, & Cho, 2009). Another researcher adopted the acoustic emission and microseismic events (Cai & Kaiser, 2005); for instance quantification of the damage degree and accumulative opening displacement of cracks (Wu, Liu, Liu, Zhuang, & Yan, 2009); the realistic modeling and fracture process (S. Wang et al., 2009).

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