Research On Safety Control Technology Of Metro Shield Tunnel Construction In Karst Area

During constructing the subway in karst area,many problems spring out.If the karst is not handled properly,it is easy to cause the head felling of shield,water and mud inrush in tunnel,land subsidence and other accidents,affecting the normal construction of the tunnel,and even leading to casualty accidents.The success or failure of the shield tunnel construction depends largely on the geological conditions and the shield equipment,which both complement and restrict each other.Especially,the construction of the shield tunnel in the karst area is more stringent for the two factors.While karst brings troubles to subway tunnel construction,in turn,tunnel construction will also have an impact on the surrounding environment,leading to new karst problems.Therefore,based on the construction of the karst section of the metro line 2 in Nanning,this paper carries out the safety control technology of metro shield tunnel construction in karst area through theoretical analysis,numerical simulation,field test and other research methods.The main research contents and results are as follows:(1)Though change the relative positions between the karst caves and the tunnels,and the size of karst cave,the numerical simulation method is used to analyze the stress and strain of the surrounding rock and segments,the law of settlement during the construction of shield tunnels under three conditions(The three conditions refer to the cave is above,below and on the side of the tunnel).It is concluded that the scope of karst treatment around the tunnel under three conditions should not be less than 0.5 times the tunnel diameter of the shield tunnel.(2)According to the construction experience and numerical simulation results of the karst section of the metro line 2 in Nanning,this paper summarizes the treatment principles of treating karst:1)The pretreatment on the ground and by the shield machine is main,the supplemental measures in the cave as a supplement;2)Karst treatment after ground treatment.The scope of treatment is as follows:When the bottom stratum of the tunnel is limestone,the caves within 2m of the tunnel structure and 5m below the tunnel structure should be treated.When it is clay or silty clay,the caves within 3m of the tunnel structure and 5m below the tunnel structure should be treated.Meanwhile,the scope of treatment should not be less than 0.5 times the diameter of the shield tunnel.(3)Different treatment modalities were proposed for different types of karst caves.The treatment effect is tested by the field test of the compressive strength of the core sample,the average number of strokes in the grouting section,and the permeability coefficient.The test results show that the rock and soil after treatment can meet the design requirements.(4)Geological conditions are the basis for shield construction and shield equipment is the key.In order to ensure that the tunnel boring mashine can smoothly pass through the complex geology of the karst area,an earth pressure balance tunnel boring machine is selected and the key parts are optimized.The optimization results are as follows:1)The basic structure of the cutterhead is a spoke panel type,with 4 main beams and 4 sub beams,and the opening ratio is 40%;2)For the limestone stratum,the strength and wear resistance of the cutterhead should be strengthened,and the matching drive power should be provided;3)Designing 6 improver injection port in the cutter head,configuring enhanced spatula with large the spacing,seting up the soil tank mixing system and other measures to prevent the mud cake;4)In view of the spewing problem of the complex stratum,the design of the screw conveyor must be in the form of anti-sloshing,and a small 9.5° angle conveyor should be used to enhance the ability to transport slag;5)Two grouting openings are added at the top of shield tail to optimize the grouting design;6)Advance the geological reinforcement in front of the excavation surface of the shield machine at the unstable face and gushing stratum.(5)Based on the surface settlement data caused by the shield construction in the karst area,the grey correlation analysis method was used to analyze the sensitivity of the shield parameters and rank the impact degree.According to the analysis,the sensitivity is ranked as follows:cutterhead torque,shield total thrust,grouting pressure,driving speed,grouting amount,and earth pressure.This provides a theoretical basis for the rational setting of shield parameters in such areas and the development of surface settlement control measures for shield tunnels.(6)Combining with the actual site conditions during the construction of the shield tunnel in the karst area,through the analysis of the boring parameters and monitoring datas,comprehensively consider the economic,technical and social benefits,the corresponding shield parameter control standards and measures for clay(silty clay)layer,strong weathering siliceous rock layer and gravel soil layer,and the weathering limestone layer in karst area are proposed respectively,which will provide reference for similar projects in the future.