Research Of Rational Application Of Reserving Core Soil With Annular Excavation Method Used In Tunnel

During mountain tunnel constriction, the surrounding rock turns to be unstable easily in the soft strata, so it is very important to select constriction method correctly. It is fast to progress and stable for the working face to use revering core soil with annular excavation method, so this method has been used in many tunnels, however, the study to it lacks much.Aiming at the surrounding rocks ranked V and VI, this paper developed numerical models, and calculated the engineering situation while the buried depth was 20m, then analyzed the influence to surrounding rock for the change of the length of core soil. Besides, with the analysis to the stabilization of surrounding rock of kinds of buried depths, this paper reached the applicable range of this constriction method. By changing surface slope, this paper also researched the rule of tunnel stabilization. Conclusions were as follows:(1)When the surrounding rock was ranked V and the buried depth was 20m, rational length of core soil was 2.5-3.0m. In this condition, with the increase of the length of core soil, the plastic area around annular working face decreased and tended to invariant when the length was longer than 3.5m; the maximum longitudinal displacement of working face didn't change basically, but longitudinal displacement of many places in annular working face increased slightly; the vault settlement changed slightly too. When the surrounding rock was ranked VI and the buried depth was 20m, rational length of core soil was 2.5-3.5m. In this condition, with the increase of the length of core soil, the plastic area around annular working face decreased and tend to invariant; the maximum longitudinal displacement of working face decreased firstly and then increased; the vault settlement decreased constantly and tended to invariant when the length was longer than 3.5m.(2)This constriction method was suitable to the tunnel buried no longer than 40m while the rock was ranked V. In this condition, with the increase of the buried depth, the plastic area around annular working face increased, and became coherent while the buried depth reached 80m; the longitudinal displacement of working face increased and added fast while buried depth was between 80m to 160m; the vault settlement increased and the maximum added fast. This constriction method was suitable to the tunnel buried no longer than 20m while the rock was ranked VI. In this condition, with the increase of the buried depth, the plastic area around annular working face increased, and became coherent while the buried depth reached 40m; the longitudinal displacement of working face increased and added fast while buried depth was between 80 to 160m; the vault settlement increased and the maximum added fast while buried depth was deeper than 20m.(3) In the rock ranked V, surface slope effected the longitudinal displacement of working face much, but little to the vault settlement, and the working face was not stable enough while the surface slope was between 20°to 40°. In the rock ranked VI, surface slope effected the longitudinal displacement of working face very much, and much to the vault settlement, besides, the working face was not stable enough while the surface slope was between 20°to 30°.