Numerical study on hydraulic failure characteristics of deep-buried solution-fissured limestone as an inrush prevention layer

ZHOU Fubiao; LIU Lei; MU Meng; LAO Jiarong; CHENG Xiaojie; MENG Yan

doi:10.11932/karst2025y018

Abstract：In tunnel engineering in karst areas,the tunnel mud and water inrush is one of the most common geological disasters.These events significantly impact tunnel construction,safe operation,and the safety of personnel and equipment,particularly in tunnels with large burial depths located in water-rich karst areas.Numerous factors affect tunnel mud and water inrush,including tunnel burial depth,groundwater pressure,surrounding rock stress,and the characteristics of the water inrush prevention layer-such as its thickness,karstification rate,fissure density,and mechanical strength.The water inrush prevention layer refers to the rock mass situated between the tunnel and groundwater,serving as the critical barrier that prevents groundwater from rushing toward the tunnel free face.The strength of the rock mass in this inrush prevention layer determines its capacity to resist groundwater pressure.Key factors affecting its overall strength include the thickness,karstification rate,fissure density,and mechanical strength of the prevention layer.The development of solution-fissures within this rock mass reduces its effective thickness and structural integrity,thereby diminishing its ability to withstand groundwater inrush toward the tunnel free face. To investigate the impact of the development of solution fissures in the limestone inrush prevention layer on its failure characteristics under hydraulic action in deep tunnels,limestone samples were collected,and tests were conducted to determine the physical and mechanical parameters of rock.The true density,tensile strength,elastic modulus,Poisson's ratio,cohesion,and internal friction angle of the limestone material were obtained.Four different models for water inrush prevention were designed with the use of finite difference numerical simulation software.These models are square-rectangular in shape,with cylindrical holes cut opposite each other in the middle of the upper and lower surfaces,but not completely through.The remaining middle layer forms a disc-shaped batholite that serves as the water inrush prevention layer.Shallow circular holes and strip-shaped grooves are cut on the upper surface of this disc-shaped water inrush prevention layer,representing the solution fissures developed within the water inrush prevention layer.The circular holes and strip-shaped grooves differ among the four models,representing varying karstification rates.Based on this,the models were assigned the measured physical and mechanical parameters.Numerical simulations of the model for inrush prevention batholitel were conducted under fixed geostress at a depth of 1,000 m,with gradually increasing water pressure at an interval of 50 m after constraining the bottom and surrounding areas of the model.The maximum shear stress and maximum shear strain increment of the model's force response are used as the criteria to analyze the characteristics of each stage of the model's force deformation and failure,as well as the stress-strain distribution contour maps.The similarities and differences among the four models were compared.The research findings show that: (1)The deep-buried solution-fissured limestone used as inrush prevention batholitel undergoes four stages of deformation and failure under high geostress and gradually increasing water pressure:elastic deformation,plastic deformation,residual strength,and failure. (2)The higher the karstification rate of the deep-buried limestone used as an inrush prevention batholite,the lower the critical water pressure required to cause hydraulic failure of this batholite.A relationship curve between the critical water pressure for failure of the inrush prevention batholite and the karstification rate has been established. (3)Under the effect of water pressure,the inrush prevention batholite without solution fissures is more susceptible to tensile failure near the upper surface,mixed tensile-shear failure around the lower surface,and tensile failure in the central area of the lower surface. (4)The development of solution fissures affects the distribution of stress and strain within the inrush-prevention batholite.Under the effect of water pressure,the inrush-prevention batholite with solution fissures is more prone to mixed tensile and shear failure at the edges and tips of the solution fissure zones,compressive shear failure at the periphery of the lower surface of the batholite,and tensile failure in the central area of the lower surface. (5)Tensile and shear deformations at multiple adjacent solution fissure tips tend to interconnect,forming a continuous tensile and shear fracture zone.Under water pressure,the inrush prevention batholite may experience tensile failure,tensile-shear failure,compressive-shear failure,or a combination of these failure modes,depending on the development of the solution fissures. The research findings offer valuable insights into the deformation and failure characteristics of hydraulic fracturing in deeply buried rock formations,aiding in the prevention of water inrush.