Research On Passive Failure Of Shield Tunnel At Tunneling Face And Cross Section

Earth pressure balanced or slurry shield tunneling method is commonly used to construct tunnels in soft ground to improve stability and safety.When the pressure at the tunnel face is excessive,the accident of blow out may take place.In addition,a large grouting pressure of grouting around the tunnel segments may also lead to passive failure,especially for the shallow covered tunnel.Furthermore,the passive failure of the advancing tunnel could influence the existing tunnel in the congested urban area.Consequently,this study focuses on the passive failure of tunnel.The limit analysis method was mainly used to investigate the mechanism of passive failure of tunnel.The software Optum3 D and Optum2 D incorporated upper bound solution was performed to model and analyze different cases.The critical pressure of passive failure and the slip surface were obtained and analyzed.The most important factors on the failure mechanism of the tunnel includes the ratio of tunnel cover depth to diameter(C/D ratio),the friction angle and the cohesion the surrounding soil.The parametric studies were carried out.It is found that as the critical passive failure pressure on tunnel face increases with the increasing of C/D ratio and friction angle and cohesion.Considering the higher efficiency of 2D analysis and its wide use in practice,this study was aimed to find a relationship between the 2D and 3D results.The 3D effect of passive failure at tunnel face was studied and a design chart was obtained for different strengths of soil and C/D ratios of tunnel.A modification of 2D analysis can been easily conducted using the design chart.The section shape effect of tunnel was researched.The passive face failure of circular,rectangular and hexagonal tunnel were modeled.Due to the similar the overall failure surfaces,there was no significant shape effect on the critical passive failure when the area of different sections was the same.For the passive failure on the side of tunnel liners,the passive failure pressure at the crown,spring line and invert of the tunnel section were compared in cases of different soil strength and C/D ratio.Compared to other position,the crown of tunnel has the lowest critical failure pressure in the same situations because of its shortest slip surface extended to the ground surface.For the impact of the advancing tunnel on the existing tunnel,the dangerous distance was the emphasis.The results show that the most dangerous distance of the advancing tunnel to the existing tunnel is about 2D.A closer distance than 2D was not the most dangerous because the slip surface in this situation has surpass the existing tunnel.