Research On Safety Thickness Of Waterproof-Resistent Slab Of Karst Tunnels

With the rapid development of transportation in Western China,the number of karst tunnels with large buried depth and high hydraulic pressure increase dramatically.Due to the complex geological conditions,there are usually water bursting and mud gushing during constructing.These disasters are seriously threatening the safety of workers and tunnel construction.Reserving appropriate thickness of waterproof-resistant slab is an effective method to ensure the safety and economical efficiency of tunnel engineering.In this paper,water blasting of tunnels are simulated by the finite-element program considering fluid-solid coupling.The calculation model of minimum safety thickness of waterproof-resistant slab is established by multi-linear fitting and theoretical analysis.Meanwhile the processes of water inrush and the forms of water inrush channels under different conditions are studied which aiming to provide reference for the design and construction of karst tunnels.The major work is as follows:(1)When the caves are located around tunnels or in front of the tunnel face,the influence of the buried depth of tunnels,the diameters of tunnels and cavities,modulus,cohesion and water pressure on the minimum safety thickness of waterproof-resistant slab under different relative positions is studied by orthogonal test.The results show that modulus has no effect on the minimum safety thickness of waterproof-resistant slab.With the increase of buried depth,the minimum safety thickness of waterproof-resistant slab around tunnels increases first and then decreases,while that in front of the tunnel face shows a linear increasing trend.The influence trend of other factors on minimum safety thickness of waterproof-resistant slab is independent of the position of the cavity,showing an approximate linear relationship.(2)According to the numerical simulation results of cavity around the tunnel,the process of water the process of water inrush and the water inrush channel are analyzed.The water inrush processes are different due to the diverse thickness of waterproof-resistant slab.Correspondingly,there are three forms of water inrush channel:1)the water inrush channel is formed along the shortest path between the tunnel and the cavity;2)the water inrush channel is formed when hydraulic fractures extend to the shoulder and foot of the tunnel on the same side;3)the water inrush channel is formed when hydraulic fractures extend to the shoulder and foot of the tunnel on another side.(3)According to the numerical simulation results of cavity in front of the tunnel face,the process of water the process of water inrush and the water inrush channel are analyzed.Owing to the diverse buried depth of the tunnel and the diameters of the cavity,the water inrush processes are different.Correspondingly,there are two forms of water inrush channel:1)the water inrush channel is formed when the hydraulic cracks extend to the top or bottom of the tunnel face;2)the water inrush channel is formed along the shortest path between the tunnel and the cavity.(4)Based on the results of numerical sirmulation,the empirical formula of minimum safety thickness of waterproof-resistant slab is established by multi-linear fitting analysis.Meanwhile,the theoretical formulas of the minimum safety thickness of waterproof-resistant slab are derived based on the bending strength theory when the waterproof-resistant slab is simplified as a fixed supported beam or a circular plate with fixed edge.What's more,assuming the waterproof-resistant slab encounter general share failure,the expressions of the minimum safety thickness of waterproof-resistant slab around the tunnel of in front of the tunnel face are derived by the equilibrium of the waterproof-resistant slab.The applicability of the formulas of the minimum safety thickness of waterproof-resistant slab are verified by case studies.(5)The influence of the buried depth of tunnels,the diameters of tunnels and cavities,modulus,cohesion and water pressure on the failure form and the minimum safety thickness of waterproof-resistant slab are qualitatively analyzed by the control variable method,which supplements the quantitative analysis mentioned above.What's more,the failure mechanism of the waterproof-resistant slab is also researched.