Cut-and-cover And Structural Response Analyses Of A Tunnel With Load-reduction Backfilling

In this thesis,a railway tunnel constructed by cut-and-cover method is taken as the research object,which collapsed due to the subsurface excavation method.The reasons for the collapse of the tunnel and the failure of the subsurface excavation measures are analyzed.The main reason is that the shallow depth of the tunnel,continuous rainfall,surface water seepage and karst water enrichment led to a decrease in the strength of the tunnel rock mass.Moreover,after the first collapse,only the retaining structure strengthening measures were undertaken,and effective measures to improve the strength of the rock and soil were not employed,which is the cause of the second collapse.After that,the MIDAS GTS NX software are used to analyze the deformation and stability of the foundation pit,and the force and displacement of the cut-and-cover tunnel during the excavation and backfilling process are analyzed.The results show that the slope of the foundation pit presents a tendency of uplift,and the steep terrain around the foundation pit shows a trend of settlement.Moreover,the support system can effectively improve the safety of the tunnel,and the pre-tension axial force of the steel support can effectively control the horizontal deformation of the foundation pit.The response analysis of the cut-and-cover tunnel in the foundation pit is carried out.Different from the structural weak area in the straight wall of the conventional cut-and-cover tunnel,the cut-and-cover method with a pit structure forms a “safe” zone in the straight wall.The structural safety factor of the straight wall is the highest.In the comparative analysis with the conventional design method,it is found that there is a difference between the force of the cut-and-cover pit with the vertical retaining structure on both sides and the force of the cut-and-cover backfill.The retaining structure and the supporting system reduce the horizontal earth pressure at straight wall,which increases the safety of the cut-and-cover pit structure.In the end,the two-dimensional plane strain model of the load-reduction backfilling of the open excavation deep foundation pit is established,and the specific load-reduction effect of different EPS materials is analyzed.The results show that after the EPS board is used,the displacement of the crown,the soil pressure on the crown and the internal force of the open pit are significantly reduced.And with the same thickness,the load-reduction effect of EPS20 board is more obvious than that of EPS25 board.For the same kind of EPS board,the load-reduction effect increases with the increase of board thickness.