In-situ stress characteristics and active tectonic response of Xianglushan tunnel of Middle Yunnan Water Diversion Project

Xianglushan tunnel of Middle Yunnan Water Diversion Project is a deep buried and long distance tunnel in the complex tectonic geological environment. In order to find out the distribution characteristics of in-situ stress, the results of measured stress by hydraulic fracturing method in 10 boreholes along the tunnel are statistically analyzed. The three principal stress relationships are mainly expressed as $S_{\text{H}}>S_{\text{V}}>S_{\text{h}}$, which accords with the strike-slip tectonic characteristics along the tunnel. The in-situ stress of shallow stratum in the project area is mainly affected by the lithology and faults. On the one hand, the horizontal principal stress of hard rock mass is obviously greater than that of soft rock mass. On the other hand, the stress level along the tunnel is relatively lower due to the development of faults. Meanwhile, the difference of mechanical properties in the local section of fault zones makes the horizontal principal stress in shallow stratum discrete. The maximum horizontal principal stress direction of Xianglushan tunnel is distributed in NNE ~ NEE direction. And it is almost parallel to the strike of a series of Holocene active faults along the tunnel and the direction of regional tectonic principal compressive stress, which is in response to the characteristics of focal mechanism solution and wedge block movement in the study area. Based on the measured stress data and the fault slip theory, the active faults along Xianglushan tunnel are in a relatively stable state at present. Among them, Heqing-Eryuan fault is closer to the critical condition of fault sliding. Therefore, with the accumulation of stress in fault zone, the stability of Heqing-Eryuan fault deserves further attention.