Research On Subsurface Excavation Construction Technology Of Shallow Buried Subway Tunnel In Karsts Area

The acceleration of urbanization has brought a huge challenge of urban traffic and more and more cities are joining in the construction of urban rail transit to ease traffic pressure. Subway tunnel is an important part of urban rail transit compared with other types of tunnel, which has many characteristics such as shadow buried depth and complex surroundings. Hence, it is very strict for the construction process especially for surrounding soil deformation and surface subsidence. Guizhou is one of the provinces of the district of karst in our country where geological conditions makes the construction of a shallow metro tunnel by excavation harder. Besides, Guiyang rail transit is the first attempt to build urban subway tunnel in Guizhou with the karst landform and there is a little experience of engineering for reference.In this paper, taking the shallow tunnel under the ground of Guiyang rail transit line 1 section 8 in engineering background and with investigation on the spot and data collection, the information about the geological conditions and the parameter of the mechanics obtain and establish the model of numerical analysis with FLAC3 D finite difference software through which systematically analyzes the construction methods,digging length, supporting structure and karst conditions about the influence of the stress and deformation of surrounding rock and verifies the reliability of the numerical model combined with the monitoring data and analyzes the reason for the difference between the simulated results and the measured data. The major achievements of this research are as follow:(1)Using CD method and bench method of which lead to oversize deformation in construction area would greatly cause dangers whereas using CRD method which has a small deformation would reduce risks.(2)The deformational degree of surrounding rock increases with the digging length and the surface subsidence at tunnel face is greatly influenced by digging length too.The crown settlement is obviously effected by the pressure of mountain. In strong bias area, the digging length should be narrow whereas the digging length can be increased to 1.5 m in weaker bias area.(3)The thickness of 28 cm initial support can not only satisfy the deformation requirement of the surrounding rock, but also provide some safety reserves. On the basis of the thickness of 28 cm initial support, a further increase of initial support stiffness and thickness leads to a little change of the surrounding rock deformation. The effect of the system bolted on stratum deformation is not obvious.(4)The influence of karst cave position and filling condition on the stress and displacement of surrounding rock is different. When the karst cave is located above the vault and filled nothing, the influence on tunnel vault and surface subsidence are the most and the minimal impact is located in the tunnel face.