Study On The Technique Of Frame Foundation Applied For Railway Tunnel Subgrade In Karst Region

Taking typical karst treatment for tunnels in railway line from Chongqing to Huaihua as engineering background, this thesis aimed to make an insight into the treatment technique of frame structure foundation and soft sudgrade in filling karst cave in Ban tao tunnel, Wulong tunnel and Ganxigou tunnel which pass through the complicated karst section in karst area in this railway.Considering the concrete geological features of karst caves in these three tunnels, the finite element method is adopted to simulate the frame structure foundation for vacant karst cave and the results show that the traverse beam in the frame foundation is the key segment which bears larger internal forces and deformation. In-situ application has shown that frame structure foundation is one of the effective measures to deal with vacant karst cave in railway tunnel in karst area.In addition, this thesis performed the study and practice to cope with the soft subgrade in railway tunnel with application of new technique in composite foundation such as sprayed cement pile and steal pile. The utilization of this technique can not only abate the earthwork to fill and replace the soft subgrade of railway tunnel in karst cave, but enhance the bearing capacity of ground as well. Therefore, the sprayed cement pile and steal pile can be widely used.After the study of technique to deal with filling karst cave, this thesis derived the principle of "blocking up water first, taking combination of blockage and drainage, suiting measures to concrete geological conditions, and conducting comprehensive treatment". With regard to soft and loose subgrade, Such measures as replacing and filling method, steal pile and high pressure sprayed cement pile can also be used with the help of composite foundation so as to not only raise ground strength and its integrity, but prevent the ground from subsiding, muddiness. As for water drainage and leakage in large area or local section, for example, if the water quantity meets 2.0m~3/h≤Q<10m~3/h or water pressure satisfies 0.1MPa≤P<0.5MPa, radial and local grouting and supplementary grouting are adopted to block up ground water respectively.