Study On The New Tectonic Movement Phenomena And Geological Stability Of Nibashan Tunnel Site

The Nibashan tunnel is located in the intersection part of the three fault zones, and is the transitional zone of the western Sichuan plateau and Sichuan basin. There's less special study of this area in the past, and the depth of the basis geological research is also shallow. In the field investigation of the Nibashan tunnel, it is found that the alluvial gravel-boulder bed of the quaternary period of the Caodaping fault is covered by cataclastic dolomite. It is proved that the Caodaping fault moved since the late Pleistocene by dating experiment of fault, and the relationship between it and Xianshuihe fault is not clear. On the plane, the strike direction of the Caodaping fault is consistent with that of Xianshuihe fault. And they are approximately in just one line. There for, it is necessary to research the relationship between the Caodaping fault and Xianshuihe fault further and evaluate the stability of the tunnel site. The main contents and conclusions achieved in this thesis are as follows:(1) The results of electron microscope scanning of quartz show that the lightly erosive quartz is developed, which indicates that the main active ages of Caodaping fault is from early Pleistocene to middle Pleistocene, and it also moved since late Pleistocene.(2) The regional geological background (regional landform, stratum, tectonic evolution, geophysical characteristic, new tectonic movement and seismic activity), regional fault (Xianshuihe fault, Longmenshan fault, Anninghe Fault, Yingjing-Mabian fault) and the geometry and activity characteristic of the major fault of the tunnel site are comprehensively analysed in the thesis. Based on the results of fault dating, regional tectonic environment and the positional relation of faults on the plane, we have reasons to doubt that the tunnel site maybe the locked segment between the Xianshuihe fault and Mabian-Yanjin fault.(3) The methods such as focal mechanism solution, crustal deformation survey and in situ stress measurement are used to analysis the regional crustal stress field in this thesis. The results show that regional recent tectonic stress field is obviously affected by geotectonic and the maximum principal stress is nearly horizontal whose direction is approximately EW-NW. The method of 3d numerical simulation is used to simulate regional crustal stress field and analyze the spatial variation law of the crustal stress field. According to the results, regional maximum shear stress shows obviously division. Under the action of northwestward stress, the image of the maximum shear stress presents the form of "X". On the basis of tectonic characteristic of region and tunnel site and characteristic of the regional stress field, it is considered that Caodaping fault of tunnel site maybe the southeast end of Xianshuihe fault, and may move again in future.(4) The method of true 3d is used to build model and simulate the crustal stress field of tunnel site. The results show that the distributing of the maximum principal stress and maximum shear stress are mainly under the control of the landform and fault. Collectively speaking, there isn't obvious stress concentration. The tectonic stress field is took as the main thread in the thesis and the regional geological characteristics and the new tectonic movement phenomena found in tunnel site are analyzed comprehensively to evaluate the stability of tunnel site.