| When tunnel construction or geological defects cause voids or tunnels above the top arch to pass through the goaf and karst area,the overhead rock mass above the top arch brings great hidden danger to the overall safety and stability of the tunnel,in order to maintain the smoothness of the tunnel.Normal operation,to ensure the safety of life and property of vehicles and pedestrians,the impact of overhead rock mass on the structural stability of the tunnel should be correctly evaluated.Therefore,this paper analyzes the development characteristics and potential failure failure modes of tunnel arch roof rock mass with specific engineering cases,discusses the main factors affecting the stability of the rock mass and the instability induced factors,and then makes the stability of the aerial rock mass Evaluation.Through the numerical simulation method,the hazard of the collapse impact of the rock mass on the lining structure is evaluated,and the deformation response characteristics of the lining structure under different prevention measures of the rock mass are compared.Then the lightweight concrete seal is proposed according to the indoor concrete ratio test and numerical simulation results.The control measures for filling in holes provide reference and reference for tunnels facing similar problems.Through the above research,the main results of this paper are as follows:(1)Through detailed field investigation and combined with 3D laser scanning,the engineering geological features such as topography and stratum lithology of the study area are mastered,combined with the development of joint fissures in the rock mass and mutual cross-cutting relationship,etc.,and the upper part of the cavity is obtained(overhead rock)The dangerous rock mass developed on the outer side of the body may fall down and collapse under the action of long-term gravity.After the collapsed damage of the hollow roof in the vertical and near horizontal fractures,the suspended rock mass may gradually collapse from the bottom to the top.(2)Through freeze-thaw strain test in fractured rock mass,the law of rock fracture propagation under water-ice phase transformation is analyzed.The attenuation law of rock mechanical properties after different freeze-thaw cycles is analyzed by freeze-thaw cycle test and uniaxial compression test.The test results show that the residual micro-strain of the fractured water-bearing specimen is positive under different freeze-thaw cycles,that is,the expansion deformation of the specimen crack under freeze-thaw cycles,and the strength of the rock sample decreases gradually with the increase of freeze-thaw cycles.(3)Considering the influence of connectivity of main control structural plane on the stability of large dangerous rock blocks above the cavity entrance,the stability of dangerous rock blocks is evaluated based on limit equilibrium theory.The results show that dangerous rock blocks are very sensitive to connectivity of structural plane.With the increase of connectivity of structural plane,the stability of dangerous rock blocks rapidly deteriorates under the action of induced factors.According to the development characteristics and potential failure modes of overhead rock mass,the stability of overhead rock mass is evaluated comprehensively based on limit equilibrium theory and minimum safety thickness method.The results show that the overhead rock mass at the entrance is in the basic stable state at present,and according to the limit equilibrium theory,the rock mass thickness above the entrance is regarded as a"critical thickness",and the stability of overhead rock mass is calculated after the connectivity of rock mass structural plane is deduced,that is,most of the overhead rock mass is in the basic stable state.(4)ABAQUS finite element software is used to simulate the impact lining process of dangerous rock blocks,and the dynamic response characteristics of lining structures under different impact angles are analyzed.From the distribution of stress,displacement and damage of concrete element in the most dangerous section of lining structure,internal force of lining structure,stress of steel bar and impact force caused by falling dangerous rock block,it is concluded that concrete above the arch shoulder is destroyed rapidly under the impact of dangerous rock block,while there is no obvious plastic damage at the lining arch waist and below.(5)In view of the damage of overhead rock mass collapse to lining structure,it is necessary to consider the prevention and control of overhead rock mass.The deformation characteristics of lining structure under the measures of concrete pillar support on the top of the cavity,sand buffer layer laying on the top of the lining and concrete filling of the cavity are considered by numerical simulation.It is found that the stress concentration occurs at the end of the column and plastic deformation occurs at the top of the lining arch.The minimum principal stress of the lining concrete decreases rapidly after being impacted by dangerous rock blocks,but the maximum principal stress reaches the ultimate tensile strength of the concrete under the measures of laying sand buffer layer of 1 m,2 m and 3 m above the lining.After comparing the deformation characteristics of the lining structure under different density concrete measures,it is found that reducing the density of the filling concrete can effectively improve the stress state of the lining structure and enhance the safety margin of the structure.Based on this,concrete mix ratio test was carried out,and it was found that concrete with low density of 800kg/m~3 can maintain a certain strength and collapse ratio of cement:water:fly ash:ceramsite:ceramsite=1:0.5:0.05:0.96:0.64.Under this ratio,light concrete can effectively improve the safety factor of lining edge parts compared with ordinary concrete. |
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