Mechanics Characteristic Research On Shallow-Buried Excavation Method Proceed Of Large Cross-section Loess Tunnel Of Lanzhou Metro Tunnel

Due to the complexity of loess stratum character,different areas of the tunnels have different stratums,causes,types,water content, collapsibility, the related engineering mechanics index and engineering properties are different. The loess shallow tunnel excavation method, no matter from the aspects of design and construction are the lack of experience and theoretical guidance.Therefore selected Mechanics Characteristic Research on Shallow-Buried Excavation Tunnel with Large Cross-section in Loess Area of Lanzhou Metro Tunnel of this subject, and deeply analyzed initial supporting, the stress state of surrounding rock and the displacement of tunnel surrounding in the process of tunnel excavation under the different construction schemes. Expect to provide theoretical basis and guidance for the city subway tunnel engineering design and construction under complex conditions in the loess area.This article based on the first phase of the lanzhou urban rail transit line 1 as the background, for the above problems were studied and combined with field of shallow buried tunnel at home and abroad in recent years of research results. The three dimensional model is established by means of FLAC3 D, simulation of the lanzhou city subway excavation supporting process. To simulate the double wall pilot tunnel method of three kinds of different construction schemes,namely four steps excavation 5 meters in advance, four steps excavation 10 meters in advance, six steps excavation 5 meters in advance,in different advance heading length and different construction steps. Through the contrastive analysis of the three construction schemes, the following conclusions:(1)Four steps excavation 10 meters in advance scheme can be seen the length in advance effect stress state on the primary support. Increase the length of the pilot tunnel makes local tensile stress in the horizontal direction appear larger on Primary support,and the maximum tensile stress is 2.41 MPa. It more than the Code for design of metro, sprayed concrete tensile strength of 1.3 MPa.It is harmful to the structure safety. Therefore, in the big cross section in the process of the loess tunnel excavation supporting, suggested that control of the distance of the pilot tunnel.(2)Six steps excavation 5 meters in advance caused by the earth’s surface and vault subsidence is minimal, and the maximum subsidence caused by the construction steps and subsidence ratio is minimal. Therefore, in the process of construction in loess tunnel of large section,to increase the construction step and reduce the distance of the pilot tunnel are effective to control the surface settlement and arch sedimentation.(3)After primary support closed loop of the monitoring section,three schemes of every step surface settlement and arch sedimentation ratio decrease rapidly,and then gradually stabilized.Suggested that to closed primary support of excavation section in a timely manner is effective control the subsidence.(4)Six steps excavation 5 meters in advance in both headroom convergence monitoring section values to a minimum. Therefore, in the process of construction in loess tunnel of large section,to increase the construction step and reduce the distance of the pilot tunnel are effective to control the headroom convergence.(5)Six steps excavation 5 meters in advance caused by the amount of the maximum uplift at the bottom of the arch for three kinds of scheme is minimal. Therefore, in the process of construction in loess tunnel of large section,to increase the construction step and reduce the distance of the pilot tunnel reduce the amount of the maximum uplift at the bottom of the arch.