Study On The Deformation Regularity And Supporting Structure Stress Of Large Section Tunnels In Xigeda Formation Strata

Xigeda formation strata is a kind of fluvio-lacustrine deposits’half-diagenesis in southwest of China, which is also a kind of soft rocks with poor engineering properties. Its water stability is bad, so it is easy to mud and disintegrated with the strength greatly reducing in the presence of wate. In the process of tunnel excavation, some emergencies is easy to produce, such as large deformation of surrounding rock, the initial support structure cracking supporting, sloughing and collapsing etc. We will be facing with a series of problems, when we build large secion tunnels in the strata. Such as the deformation regularity of tunnels in the Xigeda formation strata; distribution laws of tunnel surrounding rock pressure; the shared ratio of secondary lining; the security of supporting structures’strength; the effect of construction project and surrounding rock with different poor strengths on the large section tunnels’ deformation and supporting structure stress; deformation controlling measures of surrounding rock, etc. Therfore, the paper is based on the Tong Zilin tunnel located in MPZQ-2 Section of the reconstructed railways from Chengdu to Kunming, using the methods of literature research, theoretical analysis, experimental study on mechanical characteristics of of Xigeda formation, field test and numerical simulation to study on the above questions. Meanwhile the following main research results are obtained.(1) Based on the experimental research on the samples of Xigeda formation, we select 5 monitoring section in the entrance of Tong Zhilin tunnel within the depth of 45m, which is used to measurement the deformation and stress of tunnel supporting structures. With the numerical simulation method, we study on the deformation regularity and supporting structure stress of large section tunnels in Xigeda formation strata. The results showed that:the surrounding rock deformation is given to vertical settlements, the influence scope of advanced vault settlement is 1.3 times of tunnel diameter, the influence scope of advanced horizontal convergence is 1.7 times of tunnel diameter; the prediction formula of the tunnel crown settlement with the change of time is U=102.15·exp(-5.33/X), the prediction formula of the tunnel arch foot horizontal convergence with the change of time is L1=19.55·exp(-7.49/X), the prediction formula of the tunnel wall waist horizontal convergence with the change of time is L2=17.86·exp(-23.26/X); the tunnel deformation behind the working face is mainly affected by the excavations of system step; we recommend the standard to calculate the surrounding rock pressure of large section deep-buried tunnels in Xigeda formation strata; the shared ratio of the steel-truss structures’section is range from 37.7% to 54.3%, the shared ratio of the shotcrete structures’section is range from 37.7% to 54.3%; the force in shotcrete structures is about 1/10 of the force in steel-truss structures.(2) Through numerical simulation, we analysis the effect of construction project on the large section tunnels’deformation and supporting structure stress. The results showed that: Different construction methods make a biggest effect on crown settlement in Xigeda formation strata; when the rational deformation of tunnels is range from 10cm to 15cm, three-step temporary inverted arch method is recommended to choose; when the rational deformation of tunnels is less than 10cm, the CRD method is recommended to choose.(3) In view of the Xigeda formation strata’s engineering properties what is easy to slime and collapse if the strata is infiltrated by water, using numerical simulation makes a research on the effect of the large section tunnels’deformation and supporting structure stress with different poor strengths. The results showed that:The strength of surrounding rock outside the tunnel excavation contour line within the 0.25 times hole diameter, is very important to control the deformation of surrounding rock in Xigeda formation strata. The surrounding rock ma is possible to loss of stability, when the coefficient of surrounding rock strength degradation is over 2.0; the strength of the surrounding rock within tunnel excavation contour line are important to control extrusion deformation of working face; in order to ensure the stability of working face, and control effectively, we suggest controlling the coefficient of surrounding rock strength degradation less than 1.5.(4) From three aspects including of predeformation, extrusion deformation and tunnel deformation, using numerical simulation method analysis some measures’results on controlling the deformation of surrounding rock in Xigeda formation strata, such as pre-Lining, strengthening core soil in advance, spraying concrete on working face, improving performance of the sprayed concrete, optimizing the space between steel frame; combining with the engineering actual situation, summarizes some measures used on controlling disaster-induced of tunnels in Xigeda formation strata.