Study On Construction Technique For Two Line Large Span Railway Tunnel With Shallow Depth And Its Deformation Control

Taking Jia Xianggou tunnel and Bao Shiyan tunnel in the railway line which runs from Daxian to Chengdu for two-propose of passenger and cargo transport at the speed of 200km/h as engineering background, this thesis has elaborated work on rock deformation in grade III, IV and V surrounding rock., the axial forces in rock blots and working statues of tunnel lining of the two line large span tunnel with shallow depth. The deformation criteria for each grade surrounding rock, working states of bolt and support structure have been obtained. The key achievement goes to solving the construction technique for large span tunnel buried shallowly in horizontal strata. This has not only ensured the safety and stability of surrounding rock and tunnel support, but also accelerated construction procedure and shortened construction period. Expected economic effect has been acquired in the light of this study.The prerequisite is to solve rock pressure applied to tunnel lining under specific geological condition before determining the structure design and construction technique. Both Jia Xianggou tunnel and Bao Shiyan tunnel pass through horizontal strata with clay and sandy mudstone, their overburden ranges from 8m～20m. Through the study of this subject, several conclusions have been drawn as follows.With respect to horizontal strata with clay and sandy mudstone, 60% of the horizontal convergence and roof subsidence for surrounding rock occurs chiefly during construction of the upper drift, 40% of the subsidence and displacement convergence takes place during the construction of the bottom drift. The roof subsidence and horizontal displacement vary radically after the excavation of upper drift. The second lining can be erected after 30~40days when bottom drift has been excavated. The second lining in runnel crown bears much less surrounding rock pressure, but the side wall bears greater pressure. During the tunnel construction, primary support undertakes main rock pressure; second lining is used to preserve safety storage and strength. The seconding lining is safe.After in-situ monitoring, the maximum depth of plastic zone both in the tunnel roof and left and right sidewall reaches 1.5m～3.3m, this shows that the rock bolts (whose length is 3.0m～3.5m) installed in gradeⅣand gradeⅤare feasible and safe.Moreover, as far as construction technique is concerned, the obtained results show that bench method is suitable for shallow depth tunnel constructed by drilling and blasting method, but the distance between upper drift and bottom drift should probably be controlled within 40m～60m.