Study On Construction Methods Of Large Cross Section Tunnel In Soft Rocks

Along with the large-scale construction of the railway in our country, the design and construction technology of the large span tunnel has made breakthrough and development. In railway tunnel, three-line tunnel and four-line tunnel are usually large-span tunnels. With the "The Belt and Road Initiative" and the rapid development of high-speed railway, highway, railway tunnel construction will be more and more. Due to the influence of the selection criteria, topography and other factors, it is bound to have a lot of large span tunnel, and it will certainly travel through many complex strata, especially the soft rockss, such as fault fracture zone. But there is no specific design description in the existing technical specifications, so we need to do the corresponding calculation and analysis of the specific engineering, to study the construction method of large span tunnel in the soft rocks.Based on the construction of the Shi Ziyuan tunnel, this paper studies two kinds of construction methods used in large cross section tunnel in soft rocks. The calculation results of the analysis to the roundabout heading are compared with the monitoring results, determine the reasonable parameters of rocks, for large section tunnel calculation. The three-dimensional numerical simulation of double side wall method and double-side-wall heading method was simulated. Optimization of construction scheme is determined by analysis of two construction methods in excavation sequence, how partial excavation, demolition of the timing of the temporary support, the tunnel face distance. Analysis of stress characteristics of the two kinds of method is made. After analysis and comparison, the following conclusions are drawn:(1) The numerical simulation of circuitous heading extraction is made. Thtough the comparation of the calculating displacement and stress results and monitoring results, the parameters used in the calculation of the rocks are determined.(2) Double side wall method is optimized. The reasonable excavation sequence and excavation height were determined. When the excavation face distance is 3.6m, the plastic zone is no longer in the tunnel. When the distance between the support and the last side of the excavation face (the middle of the arch face) is 9.6m, the displacement is convergent, that is, the influence of the front construction is small.(3) Double-side-wall heading method is optimized. In the case of a certain deformation of the rocks of the arch permitted, the soil method of the two excavation arch is better. When the distance between the support and the 4 excavation face is 13.2m, the displacement is convergent, that is, the influence of the front construction is small.(4) Vault settlement results show that when the double side wall heading construction method is used, attention should be paid to the arch of soil excavation of crown settlement and the excavation of soil arch should be divided with benching tunnelling method. If double-side-wall heading method is used, should pay attention to vault settlement of the demolition of arch support.Before the demolition of arch support, it is recommended to strengthen the initial support strength of arch part.(5) Stress results show that maximum shear stress of the side wall and inverted arch of double side wall method is largest; The maximum principal stress of vault and arch foot of double-side-wall heading method is the largest. The central part of the invert, the maximum shear stress of bilateral wall is 3.95 MPa; the maximum shear stress of the bilateral wall heading method is 1.16 MPa. Double-side-wall heading method in high side wall can well bear arch load transfer and of tunnel sidewall and invert arch part to the very good protective effect. For complex geological conditions and tectonic regions with greater force, especially the sidewall and invert parts stress larger tunnel and double-side-wall heading method can better improve the structure of the force.