Study On Safe Distance And Water Lnrush Risk Assessment Between Water Conveyance Tunnel And Hidden Cave

With the development of national economic construction,infrastructure construction has gradually advanced to the southwest.A number of water conservancy and hydropower projects have been built on the Jinsha River,Yalong River,Dadu River,Nujiang River,Hongshui River and the upper reaches of the Yellow River,etc.Abundant water resources have brought opportunities for the development of hydropower in the Southwest as well as challenges for project construction.In southwest China,carbonate strata are widely distributed,water systems are staggered,underground structures are complex,and karst caves are often developed.When the tunnel is excavated through the surrounding karst caves,it is often endangered by water and mud outburst,which seriously threatens the safety of the project.Therefore,it is of great practical significance to deeply study the safe distance between tunnel and karst cave and establish an accurate prediction model of the safe distance between tunnel and karst cave and an evaluation model of water inrush risk grade.In this paper,with Xianglushan Tunnel as the engineering background of Central Yunnan water diversion project,the stress of surrounding rock of underground cavern is studied through theoretical analysis.The influence factors are qualitatively analyzed by using numerical simulation method,and then the orthogonal test is designed to quantitatively analyze the situation of karst cave above,below and on the side of the tunnel.Multiple regression method was used to establish the safe distance prediction model of tunnel and karst cave respectively under three conditions and through engineering verification.Finally,the risk grade evaluation method of water conveyance tunnel and surrounding rock of concealed karst cave based on TOPSIS method was proposed.The main research contents and conclusions are as follows:(1)The stress of surrounding rock in circular cavern is analyzed based on elastoplastic mechanics;The rock layer between the tunnel and the underlying karst cave is simplified as a fixed beam model,and the stress analysis is carried out by using the strength theory,and the critical distance of tunnel instability is deduced when the karst cave is located below the tunnel.The mechanism of rock instability between tunnel and cave and the criterion of rock instability are summarized,and the criterion of safety distance between tunnel and cave is put forward.(2)The simulation methods and key points of tunnel construction under the influence of concealed cave are studied,and the important factors affecting the safe distance between tunnel and cave are selected.A two-dimensional numerical model of the cave under the tunnel is established by using ABAQUS software,and the influence factors of surrounding rock level,cave span,tunnel buried depth,cave water pressure and cave location are qualitatively analyzed.(3)The range of influencing factors was determined according to the engineering geological data,and the orthogonal test combination was designed for numerical simulation calculation.The critical safety distance between the tunnel and the karst cave was obtained under 75 working conditions in which the karst cave was located at the upper,lower and side of the tunnel,and its sensitivity to different influencing factors was analyzed.The prediction models of the safe distance between tunnel and cave are established by multiple regression analysis.The results show that the influence degree of each factor on the safety distance between the water conveyance tunnel and the karst cave is slightly different.In order from high to low,it is as follows: the level of surrounding rock,the span or water pressure of the karst cave,the depth of the tunnel and the water pressure of the tunnel.Under the three working conditions,the tunnel water pressure range is not more than 10% of the horizontal range of surrounding rock,indicating that the surrounding rock level has a great effect on the safe distance,and the tunnel water pressure has a weak effect on the safe distance.It is found that the predicted safe distance between tunnel and karst cave is basically consistent with the designed distance,which indicates that the prediction model is reliable in engineering application.(4)Taking full account of the numerical simulation results and the actual situation of the project,the factors such as water pressure,safety factor and location of the karst cave were introduced to enrich the evaluation indexes.Based on TOPSIS method,the karst tunnel water gushing risk grade evaluation model was established and successfully applied to Xianglushan Tunnel.The results show that the model has good accuracy and practicability in predicting the risk of water inrush in many tunnel segments,and can provide a reasonable reference for the design and construction of water conveyance tunnels in karst areas.