Engineering Geology Research On Northern Margin Fault Zone Of Western Qinling

The northern margin fault zone of western Qinling is an important NWW regional active fault zone and a dividing line of geological structure units in China. It is situated at the northeastern Tibetan plateau, relatively wide to stretch over the area of Gansu and Shaanxi, cross tunnel in Yintao project. It is the largest regional active fault zone across tunnel in Yintao project, the wall rocks of tunnel through the fault zone have a poor stability and the construction will be difficult and encounter with various geological problems.Relying on the Yintao Project, taking the the northern margin fault zone of western Qinling as the major study object, based on the field investigation and laboratory test for Zhangxian fault, reviewing literature about the northern margin fault zone of western Qinling, study on the spatial extensions, geological structure, material Character and activities of the fault zone, analyze the changes of engineering geological conditions of tunnel in Yintao project.(1) The northern margin fault zone of western Qinling is rupture segmentation, can be divided into3in firstgrade and6in second grade. It is composed by several approximately parallel faults in vertical and flower structures in the profile.(2) The northern margin fault zone of western Qinling consists of a main fault and several subsidiary faults, which are commonly arranged as en echelon (or parallel) pattern. Fracture influence zone consists of series of asymmetric compact syncline, subsidiary faults (anti-dip or cocurrent), and the derivative little faults.(3) The material of the northern margin fault zone of western Qinling consists of fault gouge and fault breccia. The fault rock which is mainly maroon, black and khak has a higher content of coarse grain than that of clay. The middle part of the fault zone with equal contents of coarse grain and silt has a lower content of coarse grain than that of the eastern part. The microstructure types of the fault gouge are mortar texture, aphanitic texture, dispergated texture and reticular texture, and the mineral composition of the fault gouge are maily quartzs, calcites and kaolinites. It has a high compressibility and a poor shear strength.(4) The northern margin fault zone of western Qinling formed mainly in early aleozoic. Based on its active characteristics, its activity is recognized as three stages, it experienced intense thrusting since late paleozoic; The transformation from thrust to left slip of northern margin fault zone of western Qinling finished in the period of the mesozoic; From quaternary on, its amplitude of the vertical movement become smaller. According to Historical Seismic Data and displacement of faulting data, since late pleistocene, fault activities continuously become weaker. And in contrast to the two ends, middle part of fault activities has a higher intensity. According to size composition, microstructure and scratch trace of fault gouge show that, it is multistage fracturing tectonism. The results suggest that it is characterized essentially by sticky slip, with some steady creeping slip.