| The gushing water at the fracture zone and the wide fracture zone are characterized by high speed and large water volume,which seriously threaten the safety of tunnel construction.The common tunnel water inrush prediction method is mainly based on Darcy’s law and ignores the local loss at the intersection of cracks.There are problems such as the high-speed motion law of the water flow and the excessive estimation of the absolute head pressure in the fracture-type water channel.The result is a huge difference between the predicted value of the water inrush and the measured value.Aiming at the problem of distortion of high-pressure water surge prediction results in fractured tunnels,based on the natural fund project(41602241),this paper firstly constructed a physical model of single-fracture and cross-fracture,studied the law of water flow in cracks and the expression of the local head loss at the intersection of the crack.On this basis,the physical model of the fracture network is further constructed to study the expression and distribution of the flow resistance of the fracture network.Based on the above research results,the non-Darcy flow water prediction model of sparsely fractured rock mass tunnel considering local flow resistance is established,which lays a theoretical foundation for accurately predicting the water inrush of fractured tunnel.A natural marble block with a length of 70cm and a thickness of 1.8cm was used to construct a single-fracture seepage test model with a gap width of 1.17mm.The test results show that the water flow in the fracture is non-Darcy flow state(Re=19856.45~35633.80);Compared with the Darcy linear equation,the Fochheimer equation can better describe the motion law of water flow in the fracture due to both the viscous term and the inertial term.A natural marble block with a length of 70 cm and a thickness of 1.8 cm was used to construct cross-fracture seepage test model of 30°,60°and 90°.Based on the research results of the single-fracture seepage law and the head loss between the pressure-measuring points and flow test values,the hydrological parameters of the entire fracture segment are obtained,then calculate the local head loss at the intersection of the cracks based on the relationship between the total head loss and the head loss and local head loss along the path.The results show that with the increase of the intersection angle,the local flow resistance at the intersection of the fracture increases;The local loss coefficient at the intersection of the fracture can be expressed asζ=0.3392(sinθ)+1.0638;The local flow resistance accounts for about 4.8%to 11.5%of the total flow resistance,If it is ignored,the error caused by it is obvious.A fracture network test model consisting of two inlets,two outlets,eight sections of straight cracks,and four intersections(all 90°intersections)was constructed using natural marble blocks of length 80cm,width 70cm and thickness 1.8cm.The test results show that when the fracture network is subjected to the same pressure difference,the flow rates of 1#and 2#,3#and 4#are the largest,and the flow of 1#and 3#is the smallest;the network flow resistance is related to the flow.It can be expressed as R_w=f(Q,a,b,θ).The local flow resistance accounts for about 6.1%to 23.2%of the total network flow resistance,and the ratio will increase greatly with the increase of the number of exports,and its impact cannot be ignored.Based on the above research results,the tunnel non-Darcy flow water prediction model considering local flow resistance is established when different fracture distributions are established:Q=f(H,a,b,l,θ),which lays a theoretical foundation for accurately predicting tunnel water inrush..The key to applying this model is to obtain the fracture distribution and the hydrological parameters of the fracture outcrop. |
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