Geostress Field Analysis of Futang Tunnel, before and after Wenchuan Ms8.0 Earthquake

After an earthquake, such as the "5.12 Wenchuan earthquake", some of the high stresses in the hanging wall of seismic faults should be released. However, the rock mass surrounding the Futang Tunnel, is still under high stress, even after the earthquake, as evidenced by rockburst and core cracking. To investigate this issue, in-situ stress tests and back analysis of the tunnel's regional stress field, before and after the earthquake, were conducted to deduce the formation mechanism of this high stress. Research shows that the high stresses are produced by the integration of the continuous effects of fault thrusts, epigenetic and superficial reformation, medium differences, the effect of distance to the fault, and so on. The stress on the Yingxiu side of the Futang Tunnel decreased significantly after the earthquake, but on the Wenchuan side, there was little decline, and so the stress is still high. The maximum stress in Section K18+850-K21+450 is between 20-25 MPa. This unique stress distribution means that both sides of the tunnel require different engineering features, and have important engineering consequences for the design of support parameters, the construction method and the construction cost in different tunnel sections.