Stability Analysis And Reinforcement Measures For Highway Tunnels Crossing Coal Seam Goaf Groups

With the rapid development of Chinese economy,the traffic construction industry is advancing by leaps and bounds,and the highway network is becoming more and more denser.During the construction process,the highway tunnel will inevitably pass through one or more coal measures strata goaf.As a special adverse geological condition,coal goaf has weakness and spatial variability,which not only damages the natural environment and geological environment,but also brings great potential harm to tunnel construction and safe operation.Therefore,it is very necessary to carry out the stability analysis and reinforcement measures of highway tunnel crossing the coal seam goaf group to ensure that the highway tunnel passes through the goaf section safely,which has great engineering significance.This paper takes the Baizhao tunnel crossing multiple coal seam goafs as the engineering research background of the Zhao(Kua)Lu(Xi)highway project.Based on the geological survey report,the goaf geophysical exploration and the tunnel on-site monitoring,the mechanical theory analysis and numerical simulation combined research methods to systematically study the stability and reinforcement measures of tunnels passing through multiple coal seam mined-out areas.The main research contents and achievements are as follows:(1)The geophysical prospecting method(transient electromagnetic method)is used to conduct large-area exploration in ZK9+016?ZK9+100 and YK9+016?YK9+100 sections of tunnel site area.The geological radar method is used to make advance geological prediction for the surrounding rock in front of the tunnel face.The distribution of coal seam goaf in tunnel surrounding rock is comprehensively obtained,that is,the basic information of the spatial position,scale and damage scope of 1^#,2^#and 3^#goaf.(2)According to the"three zones"theory of goaf after coal seam mining,the failure characteristics and mechanical properties of surrounding rock are analyzed,and the failure height of goaf caving zone and water flowing fracture zone is calculated by using empirical formula.Combined with the theory of elastic-plastic mechanics and tunnel excavation mechanics,the mechanical characteristics of surrounding rock and stress field of tunnel in goaf are analyzed.Based on the ideal elastic-plastic model,the relationship between the range of plastic zone and excavation radius of tunnel surrounding rock is deduced,and the judgment basis and judgment method of stability of tunnel surrounding rock are summarized.(3)The uniaxial compression numerical tests of 50mm×100mm,100mm×200mm,250mm×500mm,500mm×1000mm and 1000mm×2000mm rock samples with height/diameter=2 were carried out by using RPFA^2Dsoftware respectively,and the triaxial loading and unloading numerical tests were carried out on 250mm×500mm samples with different confining pressures.The acoustic emission characteristics,strength variation characteristics and failure rules of rock materials under different sizes and different loading and unloading conditions were studied.The influence of size effect on the mechanical parameters and mechanical properties of the tunnel surrounding rock was analyzed.(4)Through MIDAS/GTS NX finite element simulation software was used to establish the three-dimensional solid model of the tunnel passing through the coal seam goaf stratum.Mohr-Coulomb failure criterion was selected to analyze the stability of the surrounding rock in the process of tunnel excavation under two working conditions of no reinforcement measures and reinforcement measures from the displacement,stress-strain,plastic zone range of the surrounding rock and the stress of the supporting structure.The study focuses on the improvement of tunnel stability by reinforcement measures of different types of mined-out areas.The simulation results show that the horizontal and longitudinal deformation of surrounding rock is greatly reduced,the phenomenon of stress concentration is obviously improved,and the plastic zone of surrounding rock is not connected with the plastic zone of goaf,or the connection degree is greatly weakened.The tunnel is in a safe and stable state.(5)This paper summarized the general disposal methods of the tunnel crossing the goaf of coal seam,and put forward different reinforcement measures and specific construction schemes for the Baizhao tunnel 1^#,2^#,3^#goaf.Combined with on-site monitoring and measurement,the disposal effect was preliminarily analyzed,and the results show that the overall deformation of surrounding rock is basically consistent with the numerical simulation results.