Face Stability Analysis Of EPB Shield-driving Tunnel In Sandy-clay Mixed Ground

Earth pressure balanced (EPB) shield machines are widely used in theconstruction of urban subway in recent years with the repaid development of urbanexpansion. Particular attention is paid to the face stability of EPB shield machine inthe sandy-clay mixed ground and it has become one of the key challenges in shieldtunneling.This thesis proposes to deal with this problem using three types ofapproaches, especially with the back ground of Beijing subway. Firstly, experimentsusing large-scale triaxial test to study the mechanical properties of mixed stratumunder varied stress level, petrofabric and composition of mixed grounds areperformed and the results are regarded as the strong basis for indoor model test andground conditioning. Secondly, an indoor model test with different cover depths(C/D=0.5,1,2) to simulate the construction of EPB driven-tunnel is applied to findout the law of support pressure on tunnel face, soil deformation and failuremechanism. In addition, the evolution of arching during face failure is investigated.The boundaries of the arch zones are also proposed. Then upper-bound approach inlimit analysis with rotating failure model is presented and the limit face supportingpressure is improved compared with the classical model of Mollon, Leca&Dormieux.Finally, a three-dimensional fast lagrangian finite difference program (FLAC3D),which can simulate large-strain deformation and progressive failure, is adopted tostudy the soil deformation and failure mechanism in EPB shield tunnelling on variousgrounds. Both of the local failure and global failure in collapse are studied whentaking into account the cover depth, soil character, ground spatial distribution and thecharacteristics of EPB shield tunnels in mixed ground.