ABSTRACT:

The auxiliary tunnels and the underground testing sites #2 and #3 at the Jinping II hydropower station are buried in the entire Jinping mountain at a maximum depth of 2525 m. The in-situ stress level is very high and the self-weight stress at that depth reaches 66.5MPa. Many difficulties, such as the breakouts of the boreholes, core discing, and failing to be fractured, will be encountered in the stress measurements. The phenomena, including the brittle failures of the intact rock mass and core discing are directly related to high stress level. Hence, abundant information can be obtained by interpretation of these phenomena disclosed during the excavation of the auxiliary tunnels and the underground testing sites. Based on these data, the multi-methods integration analysis is carried out to estimating the regime, orientation and magnitude of the in-situ stress at this site. The analysis indicates that the accuracy of the results is proportional to the amount of available data. However, limited by the quantity of data at this site, it is difficult to obtain the horizontal components of shear stresses by applying this method. Nevertheless, the results can meet the requirements for the stability analysis of the surrounding rock mass of the headrace tunnels at this hydropower station.

1 INTRODUCTION

The in-situ stress provides the initial and boundary conditions for stability analysis and support design of the surrounding rock mass in underground engineering. The overcoring methods cannot be applied because of rock core fracturing or discing. And moreover, the hydraulic fracturing methods fail to fracture the rock at the borehole sidewall. These problems also occurred in the in-situ stress measurements at the URL 420 m level in Canada (Martin et al. 1996) and the Jinping II Hydropower Station auxiliary tunnels.

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