Multi-Parameters Inversion And Engineering Application Of Surrounding Rock Of Highway Tunnel In Weakly-Cemented Sandy And Gravel Section

The tertiary weakly-cemented sand and gravel are widely distributed in central and western areas of China,and the surrounding rock is sensitive to the disturbance of excavation.After excavation,the stability of the surrounding rock is extremely poor,and engineering hazards such as collapse and large deformation may occur.Due to the uneven distribution of rock and soil in weakly-cemented sand and gravel formations,it is difficult to accurately obtain the parameters of surrounding rock,and thus construction and design often rely on engineering experience.This paper relying on G340 Zhaigou Tunnel,adopting the methods of on-site deformation monitoring and numerical calculation,establishes the back analysis model and uses the intelligent displacement back analysis method to obtain the parameters of surrounding rock.The parameters of back analysis that meet the accuracy can be applied to the optimization of shotcrete anchor supporting parameters and the comparison of construction methods in the shallow buried section of the tunnel.The main research contents and conclusions are as follows:(1)The monitoring section is selected to conduct deformation monitoring on the tunnel under the surrounding rock with weakly-cemented sandy cobble.The logarithmic curve function is used to fit the monitoring datas,and the final settlement of the arch crown of the prediction section is 21.91 mm,and the final around convergence value is 13.85 mm.The deformation of the tunnel is concentrated within 20 days after excavation,and the settlement of the arch crown and surrounding convergence gradually stabilize from rapid deformation.The predicted values can provide a basis for back analysis.(2)The parameters and range of the surrounding rock to be inverted were determined,and learning samples were constructed through orthogonal experiments and numerical calculations.BP and DE-BP neural networks were constructed,and a back analysis model was established to invert and calculate the parameters of surrounding rock.According to the posterior difference test results,the computational stability and prediction accuracy of the DE-BP model are superior to traditional BP neural network model.The DE-BP model performs well in dealing with multi-parameter inversion problems with few inputs and multipled outputs.The maximum error between the calculated displacement values obtained by backdating the inversion parameters and the measured values does not exceed 6.70%,and the inversion parameters can meet the required accuracy.(3)The effects of anchor rod length,spray layer thickness,steel frame spacing,and anchor rod spacing on the horizontal displacement,arch settlement,and tensile stress of the support structure of the tunnel were evaluated through orthogonal experiments,range analysis,and variance analysis.Based on the trend of changes in various evaluation indicators with the level of factors,the optimized spray anchor support plan was determined to be an anchor rod length of 4.0m,a spray concrete thickness of 26 cm,a steel frame spacing of 0.7m,and a anchor rod spacing of 1.2m.(4)Analyzing the effects of different construction methods and excavation footage on the settlement,horizontal displacement,plastic zone distribution,and internal force of the support structure of the section arch,considering safety,engineering cost,and time limit for a project,it is recommended to use the circular excavation method to reserve core soil for the shallow-buried section of the tunnel,with a excavation footage of 0.8m per cycle.The optimized spray anchor supporting parameters and excavation method have a certain safety reserve and achieve good engineering applied results.