Study On Influences Of The Excavation Method On Surrounding Rock Deformation Of The Small Interval Loess Tunnel

With the vigorous development of Chinese infrastructure, highway tunnels and railway tunnels through the loess are becoming more and more. The small interval loess tunnels, between ordinary separated tunnel and multi-arch tunnel, have come into being, which make full use of the self-supporting and the self-stabilization ability. And it has been widely used because the interval is small and the line is flexible. It can replace the multi-arch tunnels, with high economic value and works value about route selection, land acquisition, environmental protection and other aspects. But because of special engineering geological conditions in the loess region, loess tunnel accidents often occur. The loess tunnel study has lagged behind the stone tunnel. The small interval loess tunnels research is relatively scarce especially, its surrounding rock deformation mechanism is not clear, and the construction method is also needed to be improved.For different effects of the small interval loess tunnel excavation on the surrounding rock deformation, the finite element method is adopted to perform numerical simulation on the tunnel excavation process where the distance of first hole advanced excavation is1.5B,2B,2.5B and3B (B means excavation span) respectively. On the basis of ensuring a stable tunnel structure, combined with the monitoring and measurement of specific projects, appropriate advanced excavation distance of the small interval loess tunnel is analyzed. The advice is provided to reasonable construction of the first and underwent hole of the small interval loess tunnel. Ring excavation reserving core soil method and unilateral pilot tunnel method are compared in the construction of the small interval loess tunnel. A reference for the selection of the small interval loess tunnel excavation is provided. In addition, different effects the tunnel deformation suffered by numerical simulation when the loess tunnel inverted arch takes different distances are compared. Combined with the monitoring and measurement of specific projects, a reasonable distance range of the application of tunnel inverted arch is analyzed. And the advice is provided to the construction of tunnel inverted arch to ensure the stability of the structure during tunnel excavation. The main research contents and conclusions are as follows:1. Analysis of distances of the tunnel first hole advanced excavation(1) The tunnel is excavated with ring excavation reserving core soil method when the distance between the two holes is1/3B、2/3B、B. And it is excavated with unilateral pilot tunnel method when the distance between the two holes is2/3B. The deformation and the stress state of surrounding rock of the small interval loess tunnel whose distance of the tunnel first hole advanced excavation is2.5B is significantly better than the ones whose distances are1.5B and2B. It is roughly equal with the one whose distance is3B. In order to ensure the security of the small interval loess tunnel excavation, it is recommended that distance of the tunnel first hole advanced excavation should be controlled more than2.5times of the excavation span;(2) The small interval loess tunnel is excavated with ring excavation reserving core soil method, and the deformation and the stress state of surrounding rock are not better or worse when the distance between the two holes is1/3B、2/3B、B;(3) When the distance between the two holes is2/3B, ring excavation reserving core soil method is better than unilateral pilot tunnel method for the small interval loess tunnel.2. Analysis of reasonable distances of the tunnel inverted arch(1) Surrounding rock deformation is significant when distance from the tunnel inverted arch to the tunnel face is3.75B;(2) When distance from the tunnel inverted arch to the tunnel face is2B, surrounding rock deformation is close to the one of2.5B. And they are both obvious. The surrounding rock deformation of2B is better than the one of2.5B;(3) The safe construction should narrow the distance between the inverted arch and the tunnel face, and when conditions permit, the distance should be controlled to be less than2B.Combined with the monitoring and measurement of specific projects, distances of the tunnel first hole advanced excavation and reasonable distances of the tunnel inverted arch of the small interval loess tunnel are explored and studied by numerical simulation of the finite element method in this paper. And deformation characteristic of the tunnel surrounding rock in the excavation process is analyzed. Both a reasonable range of distance is initially proposed. A reference for the selection and improvement of the small interval loess tunnel excavation methods is provided. It has the important significance for the small interval loess tunnel construction safety.