Influence Of Concealed Karst Cave On Tunnel Surrounding Rock Stability And Key Support Technology

Tunnel projects often have long,large,and deep engineering characteristics,while soluble rock formations exist in nearly one-third of our country.In the karst development area,different distribution characteristics of karst caves always affect the safety of tunnel engineering and restrict the safety development of deep engineering.To explore the influence of the distribution characteristics of karst caves on the stability of tunnels and optimization of support parameters is a key scientific issue to ensure the safety and stability of tunnel surrounding rocks in karst areas.This paper takes the zhaiguan tunnel as the research background.Theoretical analysis,monitoring statistics,and numerical simulation methods are used to systematically study the stress and deformation state of the surrounding rock of the tunnel in different hidden karst cave locations.The impact analysis of the tunnel surrounding rock deformation and the supporting structure's stress are analyzed in the following aspects:(1)Collect engineering data of karst phenomena development section,count the height,high collapse ratio,relative position and distance of karst caves,and obtain and analyze the distribution characteristics of hidden karst caves.Among them,concealed caves are located from the crown of the tunnel to the waist of the tunnel.The shape of the cave has a high collapse ratio in the range of 0.5-2,a high collapse ratio of about 1.0 accounts for 75%,and a height of about 3-5m.The relative distance between the cave and the tunnel is about 5-10 m.The theoretical analysis of the stability of the surrounding rock of the tunnel with concealed karst caves is organized and analyzed.The internal rock factors affecting the stress distribution are mainly the lateral pressure coefficient,,value,residual value.In addition,because of the location,size,number,shape,etc.of karst caves,the traditional theoretical methods can not be well described for the stability analysis of tunnels with concealed caves by summarizing the theoretical analysis.(2)Using numerical simulation technology,through the analysis of the numerical results of tunnel stability in the state of concealed caves and non-cavity caves at different positions,the law of the influence of the concealed caves at different positions on the lateral and longitudinal deformation of the tunnel surrounding rock is obtained.The concealed karst cave at the vault increases the settlement deformation from the vault to the waist of the arch,reduces the horizontal deformation at the location of the abutment,and has the greatest influence on the stress values in the z direction and the x direction of the vault and abutment.The stress values in the x and z directions show a significant and increasing trend.The concealed karst cave at the abutment is more obvious in the z-direction settlement and deformation of each part of the tunnel,which aggravates the settlement from the crown to the position of the abutment and foot The stresses in the x and z directions of the spandrel range are significantly increased,and the impact is greater.The concealed karst cave at the arch waist increases the settlement deformation in the area from the arch top to the arch foot,and is concentrated at the arch waist.Obviously increased the deformation value in the x direction at the vault position.The stress in the z direction at the position of the sill is significantly increased.Hidden karst caves at different locations mainly affect the x and z directions of tunnels in adjacent locations.(3)Three different bolt lengths and three different bolt spacings are set for tunnels with concealed karst caves at different positions,and numerical simulation tests are carried out on the two working conditions of lengthening without encryption and encryption without lengthening.After the anchor rod is applied,the concentrated area of settlement deformation increases from the vault to the vault to the arch waist,and the convergence deformation of the tunnel changes from the original arch shoulder to the arch foot to the arch waist to the arch foot area,reducing the deformation.Concentration range.When the length of the anchor rod is 4m,it is most beneficial to the deformation constraint of the surrounding rock of the tunnel.Excessively increasing the length of the anchor rod cannot improve the settlement and convergence deformation of the surrounding rock of the tunnel.When the concealed karst cave is located from the crown to the shoulder,the tunnel settlement and convergence deformation can be improved to a greater extent as the distance between the anchor rods decreases.The optimal anchor rod spacing is 0.8m.When the concealed karst cave is located at the arch waist,when the anchor spacing is reduced from 0.9 to 0.8m,there is no significant improvement in tunnel settlement and convergence deformation.The optimal anchor spacing is 0.9m.(4)The statistical analysis of the monitoring data of the settlement and convergence deformation of the dome of a typical concealed cave tunnel is carried out,and compared with the influence law revealed by the numerical test results.Finally,it is found that the cave at the abutment has the greatest influence on the tunnel deformation And,the hidden karst cave mainly affects the deformation of the surrounding rock on the side of the adjacent tunnel,which is consistent with the numerical test results.