Study On Structure Selection And Optimization Technology For Tunnel Crossing Large Karst Cave

The Lu Jiazhai tunnel is located in the territory of Ping Dingying Town and Yin-Zhan Township,Weng an,Fuquan City,Guizhou,with a center mileage of DK52+665,a full length of 4430 m and a maximum buried depth of about 125 m.According to the site geological survey data,the hidden river channel is developed at the line DK50+790 to DK50+816.9,the line crosses the dark river channel and the tunnel elevation is located within the elevation range of the river.It is speculated that the tunnel is located in the horizontal circulation zone or the seasonal variation belt of the underground river channel,and there are some hidden dangers in the tunnel construction.The tunnel through a large cavity is the process of tunnel excavation from a stress field to the stress field within the influence of a large cavity,while the large dissolution cavity in the karst area has a special stress field distribution in the underground,and the stress field,the plastic zone and the stress concentration around the cavity are different.Engineering geology and hydrogeology are complex and changeable.It is very difficult to ascertain the stress around the cavity.This paper mainly takes the surrounding rock itself as the research object,using the theory of elastoplastic mechanics and the method of numerical simulation,the mechanical response of the karst cavity and the tunnel excavation near the large cavity.On the basis of this,the influence range of the cavity to the surrounding rock mass is given and the construction is instruct.The main contents and conclusions are as follows:(1)In a certain rock mass parameter range,the stress characteristics and plastic zone distribution of deep buried spherical caverns are analyzed by the Elastoplastic Solution of deep buried spherical caverns under the existing equal stress ratio.The stress characteristics and plastic distribution of deep buried spherical caverns under the unequal stress ratio are analyzed by FLAC 3D,and the numerical simulation junctions are analyzed.By fitting the relationship between the long axis of the plastic zone and the analytical solution of elastoplastic,the improved formula for estimating the long axis of the ellipsoidal plastic zone under the unequal stress ratio under a certain parameter condition is obtained.Meanwhile,according to the stress characteristics of the cavern,the approximate calculation method for the long axis of the plastic zone of the minimum section encircling circle of the square cavern is put forward.(2)The three-dimensional numerical model of tunnel crossing large cavity is established by FLAC 3D.In the process of tunnel approaching and entering the cavity,the deformation characteristics of the surrounding rock are studied.The law of the change of the time and space effect curve of the surrounding rock deformation with the distance of the excavation section and the cavity is summarized.Curve.Combining the distribution of the stress field and plastic zone around the tunnel and the cavity,the plastic zone between the plastic zone and the cavity caused by the longitudinal excavation section of the tunnel is determined.It is the position of large deformation.After excavation,the tunnel will enter the stress relaxation area through the stress concentration area of the cavity.The deformation of the surrounding rock will gradually decrease,and the surrounding stress concentration area will shift outward.(3)Based on the theory of the new Austrian excavation method,combined with the variation of the ground stress area in the tunnel excavation in the influence range of the cavity,the structure of the tunnel crossing the large cavity area is optimized.The method of the pre reinforcement for the maintenance of the rock mass during the construction process is put forward,and the reinforcement measures are combined with the numerical simulation.The effect is analyzed,and its mechanism is revealed.(4)In recent years,large dissolving cavities encountered by tunnels have been collected and summarized.Combined with the development mechanism and characteristics of karst,the factors affecting the selection of crossing structures are expounded.Based on the research results of this paper,the typical project of tunnel crossing large karst is deeply analyzed,and the scope of influence,the way of influence and the treatment method are determined.