Study On The Combined Effect Of Excavation Method And Advanced Support Of Large-Section Sandy Loess Tunnel

With the implementation of "One Belt,One Road" initiative and the vigorous development of transportation infrastructure in western China,a large number of tunnel projects have been built in loess areas.The sandy loess has high coarse particle content,loose structure and low strength,which makes the tunnel excavation prone to disasters such as palm face runoff and landslide,and the stability of palm face is a key problem in construction.In this paper,the longest loess tunnel in Pan Xian Mountain on Zhongwei-Lanzhou Passenger Dedicated Line is taken as the engineering support.Through theoretical analysis,numerical simulation and field tests,the research work on the mechanism of over-supporting action of large section sandy loess tunnel,the stability of tunnel surrounding rock under different combinations of excavation method and advanced support measures,and the force characteristics of supporting structure is carried out,and the following conclusions are obtained.(1)Introduced the basic contents of ADECO-RS,and the reinforcement mechanism of several commonly used tunnel face reinforcement measures is analyzed.Based on the limit equilibrium theory and elastic foundation beam theory,the formula for calculating the stability coefficient of palm face under the action of palm face anchor and advanced support is derived,and the formula is verified by the finite element strength reduction method(SRM).The numerical analysis of tunnel face stability under different reinforcement methods is carried out for the dependent project cases,and the stability coefficient of tunnel face is obtained as follows: horizontal rotary jet grouting pile > shed pipe > tunnel face bolt.Considering the construction characteristics of loess tunnel,the reinforcement method of overrunning horizontal rotary pile and shed pipe is recommended.(2)According to the general situation of the relying project,Midas GTS NX software was used to conduct 3D construction numerical simulation analysis of nine combinations of three excavation methods(three bench excavation,three bench temporary inverted arch,three-bench core soil method)and three advanced support measures(leading small conduit,shed pipe,horizontal rotary pile),and studied the displacement field of tunnel surrounding rock,plastic zone and palm face under different combinations of working conditions.The results show that the combination method of three bench temporary inverted arch + horizontal rotary jet grouting pile has the best control on the surrounding rock deformation and can be the preferred method for large section sandy loess tunnel.In the premise of determining the optimal method,the study of different rotary pile application length(6m,10 m,14m)on the tunnel surrounding rock deformation,the initial support structure force,the results show that when the step excavation length is certain,the optimal length of rotary pile application for10 m.(3)Selected a typical section of the relying project,carried out a long time,multi-frequency field monitoring work,study the deformation of the tunnel surrounding rock,the initial support and secondary lining force characteristics.The results show that: vault settlement and horizontal convergence are influenced by construction disturbance,the data has a certain lag,the actual measured data are smaller than the numerical simulation data,and both show S-shaped change pattern.The curves of rock pressure,steel frame stress,contact pressure between initial support and secondary lining,secondary steel axial force and concrete stress change with time roughly show three intervals: construction disturbance section(violent fluctuation section),slow change section and stable section,and the maximum value appears in the construction disturbance section,and the elevation arch fluctuates more than the curve above the side wall,and the time to reach the stable section is longer.In the sandy loess stratum,the second lining bears part of the surrounding rock pressure,and the contact pressure on the elevation arch is larger,which is different from the role of safety reserve in the NATM.Comprehensive site monitoring information,the support structure is in a safe state,which verifies the effectiveness of the combination method of three bench temporary inverted arch + horizontal rotary jet grouting pile.