Study On Stability And Support Structure Optimization Of Soft Fractured Slate Tunnel

With the continuous development of traffic construction in Western Sichuan,more and more tunnels passed through the soft fractured slate.During the construction process,the tunnel frequently encounters problems such as large deformation of the surrounding rock,damage of the primary support structure,which seriously threatens the safety of construction and operation.Therefore,studying the properties of the slate,analyzing the stability of the tunnel,and proposing an optimized type of support structure are of great significance to the design and construction of the subsequent slate tunnel.Relying on Haizishan tunnel engineering,rock testing,field monitoring and numerical simulation are comprehensively adopted in this paper.The characteristics and deterioration mechanism of soft fractured slate are obtained.The tunnel stability evaluation standard and the deformation control criteria are proposed.Meanwhile,two optimization methods of the reinforcement support stiffness and the yielding support are compared.The main research contents and results are as follows:(1)Compared with the natural sample,the elastic modulus,cohesion and friction Angle of the saturated sample decreased by 48.3%,31.4%and 23.0%respectively,and the Poisson’s ratio increased.The deformation modulus,cohesion and friction angle of the slate rock mass are 13.7%,9.4%and 49.0%of the sample parameters.The proportion of clay minerals in slate samples is 64%.Under the synergistic action of excavation disturbance and water,the slate deteriorates seriously when it encounters water.(2)The safety factor of 1.31～1.40 is put forward as the evaluation standard of tunnel stability.Under the original design support,when the safety factor is less than 1.31,the failure probability is close to 80%,and the tunnel surrounding rock is extremely easy to lose stability.When the safety factor is between 1.31 and 1.40,the failure probability is 25%,and the tunnel surrounding rock is relatively easy to lose stability.When the safety factor is greater than 1.40,the surrounding rock of the tunnel is relatively stable.The calculation method for the safety factor of surrounding rock with overburden depth greater than 50.0m is proposed.According to the correction factor λ and the change value of the safety factor per unit height of each area ?Ki,the formula K=K2+λ?K is used to obtain the stability safety factors of surrounding rock under different buried depths and different stratum conditions.(3)In combination with the typical monitoring section deformation and code,the deformation control standard of vault subsidence(20,56 and 126mm)and horizontal convergence(24,67 and 151mm)in water-rich fractured slate under three burial depth ranges is obtained.(4)Surrounding rock deformation and initial support stress under two kinds of optimal support methods are analyzed.When the stiffness of the primary support is increased,the relationship between the maximum compressive stress and the thickness of the primary support is σ=365.6d-0.776.When the initial support stress decreases,it will bear greater pressure of the surrounding rock.This method has high cost and poor effect.When the support resistant limiting damper is used for yielding support,the inward convergence between the left and right arch feet of the arch becomes larger,and the minimum principal stress at the same position is reduced.The minimum principal stress in the high value range(between the right and left waist)is reduced to the stress near the arch foot.The relationship between the maximum compressive stress and the thickness of the primary support isσ=497.4d-0.920.The surrounding rock pressure is properly released,and the initial support stress is smaller under the same stiffness,which indicates that the optimization method of yielding support is more effective.