Application And Mechanism Of Initial Support And Surrounding Rock In Weak Rock Tunnel

With rapid development of tunnel construction in China, the contradiction between scale of tunnel construction and theory of tunnel design is becoming increasingly conspicuous. Action effect of anchor to surrounding rock, the mechanical characteristics and applicability to surrounding rock of section steel and grid steel, and characteristics of shotcrete hardening which have influence on deformation of the surrounding rock and initial support have no further study. The paper based on data and literatures. Damage feature, mechanics characteristics, ultimate bearing capacity, applicability, and ability to control the deformation of surrounding rock of grid steel and section steel have been studied. Based on the early strength of the shotcrete laboratory test, analysed the interaction and coupling effect of the initial support and the surrounding rock considering the shotcrete hardening characteristics. Main research work and results were as follows:(1) The mapping function of successive approximation method from underground cave outer domain to unit circle outer domain has been put forward. The stress and displacement solution of without support tunnel and arbitrary cross-section under gravity stress have been derived using conformal transformation.(2) First, mechanical characteristics, failure feature and ultimate bearing capacity of grid steel and section steel were analyzed by lab test and finite element method. Then further analyses of the mechanical characteristics and control effect for the surrounding rock of grid steel and section steel by similar model test. Finally, some suggestions on the applicability of the grid steel and section steel were given by field test and Verification.The stiffness of the section steel was bigger than the stiffness of the the grid steel. The section steel could immediately bear load of the surrounding rock, and the ability of resisting deformation of the surrounding rock be greater and provide greater supporting force than the grid steel. So the section steel should priority be used in soft surrounding rock or large deformation of surrounding rock.The cohesiveness between the shotcrete concrete and grid steel was better than the section steel. The grid steel belonged to flexible support structure, and could better cooperative to the shotcrete deformation. Beause the stiffness was moderate and uniformly forced of the grid steel in the forth grade surrounding rock. So the grid steel was especially suitable for broken and weak surrounding rock. (3) Based on laboratory test of the shotcrete early strength, anayling harden feature of the shotcrete for deformation and mechanical characteristics of the surrounding rock by the finite element secondary development. Harden feature of the shotcrete helped to play self-supporting capacity of the surrounding rock in better quality rock and intact rock mass, which could reduce the stiffness of the initial support cost of the project in design. But the feature of the shotcrete was disadvantageous to controlling deformation of the surrounding rock. Early strength of concrete should be used to enhance the early rigidity of the initial to supply greater support force in soft surrounding rock or large deformation of surrounding rock.(4) Based on early strength of the shotcrete with time variation, the Calculation Method of support characteristic curve considering the shotcrete hardening characteristics was given. Based on convergence confinement method, evaluation method of supporting opportunity and the safety assessment method of the initial support in soft rock tunnel were determined.(5) Based on the safety assessment method of the initial support considering shotcrete hardening characteristic, change law of the safety factor of the initial support under construction were analyzed.The safety factor of the initial support was dynamic due to hardening characteristics of shotcrete. The safety factor of the initial support decreased at first, then increased, finally stabilized.(6) Deformation characteristics, failure feature and interaction between surrounding rock and grid steel and section steel were analyzed by lab test. Time change law and Distributed law of contact pressure between surrounding rock and grid steel and section steel by field test.