A Study On The Ground Water Mechanism In The Fault Zone Of Nibashan Tunnel

The ground water is an unfavorable conditions for tunneling. In the section where the rock is broken or the joint and fissures are well developed with some opening, water inflow easily takes place in the situation where the aquifuge strength is low and the ground water pressure is high. At the same time, collapse or other grological calamities could be induced. Once this happens, it will take a lot of efforts and resources to deal with the accident, and has a bad effect on the construction progress.In this paper, the basic engineering geological condition of the tunnel area was summerized firstly. And then, by the simulation of the seepage field of the tunnel area, the following conclusions can be obtained:(1) At the shallow buried section of the tunnel, the headwater of undergroud water is low. Though the headwater is high at the deep buried section of the tunnel, as the increase of buried depth, the loss of groundewater head will increase rapidly at the nearby of the tunnel, and the possibility of the occurrence of water inrush in the tunnel under the effect of high pressure is low.(2) The demanded safe thickness of the aquiclude at the front of each fault is0.31～0.91m. So in the construction of some needed water isolation measures such as the construction of catheter circumferential grouting, a certain thickness must be reserved to ensure the efficiency of the aquiclude.(3) The critical headwater pressure for the expansion of the mode Ⅰ cracks in tunnel surrounding rock is23.5Mpa. And that for the expansion of the mode Ⅱ cracks is15.8Mpa. In actual, the headwater of each fault is lower than the critical water pressure, so the underground water near the tunnel have no split effect to tunnel surrounding rock.According to the simulation of the excavation, such conclusions have been draw as following:(1) the hydrostatic pressure from water head has little impact on tunnel’s stain. Ground water changes the nature of tunnel surrounding rocks mainly by soften and erosion.(2) Digging with CD method, the deformation of the surrounding rock can be controlled effectively and it’s good for dealing with the bad condition that might happened in left part of the tunnel. Deserved to be mentioned, the primary support is very important.