Study On Supporting Sturctures Of Soft Rock Tunnel With Large Deformation And Small Distance

lanzhou to guangyuan section of the newly-built lanzhou-chongqing railway has manymountains, in this section, layer that most tunnels go through is soft rock under high stress.Joint fissure is developed, stability of the surrouding rock is poor, thickness of slabstone is small and the thin layer characteristics is obvious, thus large deformation occurred during theconstruction,what’s worse, the safety and rate is seriously affected. On the background ofLanzhou-chongqing railway xinchengzi tunnel, through on-site monitoring and numericalsimulation, This paper will focus on how can different supporting structures(such as longbolts,double early support and double secondary lining) control the large deformation of softrock and small distance tunnel. By using theoretical analysis, field test and numericalsimulation, studying the supporting structure of small distance tunnel under construction, thusproviding reliable theoretical and practical basis for subsequent research on smaller spacingsection.The research contents and methods：(1) By summarizing both the domestic and foreign research on supporting structure ofsoft rock tunnel with large deformation and small distance, the cause, sensiblefactors,mechanism and key technology for construction control of large deformation areanalyzed in this paper.(2) Based on engineering practice, by monitoring the surrounding rock deformation andsupporting structure(such as long bolts,double early support,double secondary lining)stress oftunnel under different supporting structure, the deformation and stress rule of surrounding andsupporting structure of xinchengzi tunnel are analyzed, and the effect of different supportingstructures is also comparatively analyzed.(3) By using MIDAS∕GTS software, the numerical simulation is applied in tunnelconstruction under different supporting structures, the curve lines that key parts of rock andsupporting structure stress change with time are drawn, thus concluding its change rule overtime and space. Considering different factors, recommended supporting structure iscontrastively analyzed.(4) By comparing field measured and numerical simulated data of displacement insurrounding rock and the supporting structure stress, synthetically select deformation controleffect to optimize the supporting structure.Research results:(1) The main reason that causes large deformation of Xin Chengzi Tunnel is that the geological condition of where the tunnel locates has seriously developed joint and fracture inbroken and extremely broken zone, the ground stress is relative high, rheological effect ofsurrounding rock is obvious; the horizontal shear failure is the main kind of failure,surrounding rock represents characteristics that horizontal convergence is bigger than verticalsettlement, cumulative total deformation is large and the cycle is long.(2) Deformation rule vs. time of large deformation tunnel of soft rock excavated bythree-step method obtained through field measurement analysis is: rapid changing time ofvault displacement is20~25days; rapid changing time of upper step is15~20days; rapidchanging time of lower step is10~15days. The excavation deformation of upper stepaccounts for60%~70%of total amount of deformation, middle step accounts for10%~15%of total amount of deformation, and the lower step accounts for about5%of total amount ofdeformation.(3) Aaccording to the numerical simulation of excavation of soft rock tunnel with smalldistance under different supporting structures, the change rule of stress-strain of tunnel underdifferent supporting structures is in line with the actual situation, big displacement of tunnelappears in the side wall and arch foot, which is relatively easy to fail, so it should bestrengthened; excavation of second tunnel has certain influence on the displacement and forcebearing of the supporting structures of the first tunnel, and the influence will become moreand more serious with the decreasing of space.(4) By analyzing the field monitoring and numerical simulation, concluding that theeffect of double layer initial support and secondary support is good, but the stress of first layerof double layer secondary support is too large, which has security risk, so double layer initialsupport with greater stiffness is recommended, and the surrounding rock in double layerinitial support can be further reinforced through a delay of the construction of second layer ofinitial support and the application of bolt grouting before construction, which can also givefull play to its self bearing capacity, and reduce the stress of secondary lining.As for soft rocktunnel, it shall be noted that sufficient pressure should be retained in primary support,deformation energy of surrounding rock should be reasonably released, self-stability ofsurrounding rock should be given full play, the second reinforcement of rigidity supportingprinciple is scientific and rational, namely the "stiff after soft, resist after release, leaveenough deformation, restrict each other, fully improve the stability of surrounding rock" asbasic principle.