Hydraulic Fracturing Effect Of Water Inrush In Excavation Of Roadways Or Tunnels

Water inrush is a common geological disaster in underground engineering.When excavating mine tunnels,there is a common problem of water inrush process with high osmotic pressure and excavation disturbance,that is,the hydraulic fracturing effect during the water inrush process.In order to reveal the basic laws of the hydraulic fracturing effect of water inrush during excavation,this paper adopts physics experiments,theoretical analysis,numerical simulation to study this scientific problem,so as to provide theoretical basis and guidance for water inrush from underground projects.The water inrush test during the excavation process was carried out by using a4000 kN true triaxial underground water inrush experiment system.The experimental results show that the entire water inrush evolution process can be divided into the initial water seepage stage,the initiation and expansion of water pressure cracks,the formation of crack penetration and water inrush channels,the local water inrush and the overall water inrush.When the excavation surface is 3.5 cm away from the aquifer and the critical water pressure is 7 MPa,the test block begins to crack and expand at a distance of 9 mm from the middle of the distance-drilled excavation under the action of hydraulic fracturing and disturbance of the excavation.Eventually,water inrush occurred when it penetrated 5 mm from the edge of the excavation surface.The shape of the water inrush channel presents the characteristics of“lower on the middle high side”.The direction of the water inrush channel formed is perpendicular to the direction of the minimum principal stress,which is consistent with the expansion direction of the hydraulic crack after hydraulic fracturing.In addition,hydraulic fracturing water inrush has hysteresis.Based on the law of true triaxial hydraulic fracturing tests and the stress conditions of hydraulic cracks,a mechanical calculation model for water inrush from high pressure water during blasting excavation was established.Based on the dynamic and static superposition effect,the critical water pressure for rock initiation in intact rock masses was given.For fractured rock masses,two types of fracturing water inrush,which were water inrush caused by tension and shear and water inrush caused by compression and shear,were proposed.Two kinds of critical water pressures were calculated respectively.The relationship between critical water pressure,ground stress and blasting excavation perturbation load was given.The pore water pressure splitting factor was introduced in the calculation process.Using the calculated critical water pressure formula,water inrush in Jinping tunnel was verified.It is concluded that the type of fracturing water inrush is compression and shear.Theoretical results can guide the scene.The RFPA^2D-Flow numerical simulation software was used to analyze the evolution of the three fields,which include stress field,seepage field and fissure field,during water inrush.The simulation results show that the excavation changes the initial stress field of the tunnel.A stress disturbance zone is formed in front of the excavation surface,and stress concentration is formed.At the same time,the pressure water penetrates or flows into the surrounding rock,forming a hydraulic gradient and pore pressure gradient in the surrounding rock.As the excavation progresses,the distance between the excavation surface and the karst cavity decreases,and the stress gradient continuously increases,which aggravates the internal damage of the rock mass.The process of formation of water inrush channels is the result of the evolution of the fissure field.The excavation induced changes in the stress field and seepage field in the surrounding rock.Under the coupled action of stress-damage-infiltration,the rock cracks initiate,expande,and penetrate,and the crack propagation direction is vertical to the direction of the minimum principal stress,which is consistent with the direction of expansion of hydraulic fractures during hydraulic fracturing.After the water inrush channel is formed,the permeability and flow rate increase dramatically.The occurrence of cracks is the root cause of the significant change in permeability.