Research On Surface Settlement Characteristics During Construction Of Tunnel Portal Section In Mountain Areas

During the construction of tunnels in mountainous areas,the rock and soil may be disturbed,leading to a redistribution of stress in the surrounding rock and causing surface subsidence.However,the soil at the entrance of the tunnel is loose,the stress distribution in the surrounding rock is uneven,and the self stability ability is poor,which is prone to settlement.If the settlement is large and control measures are not taken in a timely manner,it can lead to safety production accidents.Therefore,in order to ensure construction safety,it is necessary to focus on studying the upper surface settlement law of the tunnel entrance section,providing scientific basis for controlling surface settlement of the tunnel entrance section.At present,many scholars at home and abroad have studied the surface settlement of tunnel openings in mountainous areas through tunnel burial depth,tunnel clearance,and other aspects.However,there is relatively little research on construction methods and the impact of overlying rock and soil layers on surface settlement.This article takes the Hejiapo,Gaopo,and Taojiashan tunnels of Section 1 of Guiyang Jinsha Line Expressway as the engineering background,and studies the surface settlement law of the tunnel entrance section through on-site measurement,theoretical analysis,and numerical simulation.Firstly,through on-site monitoring and measurement,analyze the law of measured surface settlement at the tunnel entrance section.Secondly,based on the measured surface settlement data of the portal section,the Exponential function model of the cross section surface settlement is established;Afterwards,the Peck formula was revised,and the range of correction coefficients was analyzed to obtain the revised Peck formula.Finally,through Midas/GTS NX numerical simulation,the impact of different construction methods and mechanical parameters of different overlying rock and soil layers on surface settlement was analyzed.The main research work and results are as follows:(1)The Total station method was used to measure the surface settlement of Hejiapo,Gaopo and Taojiashan tunnel portal sections.The results show that the maximum measured surface settlement is located at the central axis of the tunnel,and it shrinks from the central axis to both sides,and is symmetrical with the central axis of the tunnel.(2)Based on the measured surface subsidence data,the relationship between the surface subsidence and the abscissa of the observation point is studied,and a Exponential function model of the cross section surface subsidence is proposed.By comparing the measured surface subsidence curve with the surface subsidence curve calculated by the model,the linear correlation between the two is analyzed.The results show that the settlement troughs of Hejiapo,Gaopo and Taojiashan tunnel portal sections conform to the law of Exponential function curve within the range of model parameters.Through verification,the Exponential function model can describe the characteristics of ground settlement at Hejiapo,Gaopo and Taojiashan tunnel portal sections,and the description degree is better than Peck formula.(3)By introducing a correction coefficient and considering the construction method and the overlying rock and soil layer,the Peck formula is modified to obtain the range and variation pattern of the correction coefficient in the modified Peck formula.By analyzing the variation pattern of the correction coefficient,it was found that compared to the double wall method,the three step method has a better correction coefficient a The average should be 0.091 higher than the correction factor β The average is 0.119 higher;When the overlying rock and soil layers are respectively dolomitic limestone and limestone,the correction coefficient of the former a The average is 0.019 higher than the latter,with a correction factor β The average is 0.064 higher.(4)By comparing the measured and corrected Peck formula calculated surface settlement curves,it was found that the settlement grooves of the Hejiapo,Gaopo,and Taojiashan tunnel portal sections of surface settlement conform to the curve characteristics of the corrected Peck formula.The rationality of the modified Peck formula is verified by predicting the surface settlement data of the un excavated section.It is found that the introduction of the correction coefficient can improve the accuracy of the Peck formula,that is,the modified Peck formula can describe and predict the characteristics of the surface settlement of Hejiapo,Gaopo and Taojiashan portal sections.(5)Through numerical simulation using Midas/GTS NX finite element software.the impact of construction methods on surface settlement was analyzed.It was found that the settlement generated by the step method was the largest,followed by the three step method,and the double wall method was the smallest.By comparing the numerical simulation results of Hejiapo and Gaopo tunnels,the predicted results of the modified Peck formula,and the measured settlement curve,it was found that the modified Peck formula had a small error with the measured and simulated settlement,indicating that the modified Peck formula could better predict the surface settlement of the excavated section;Through numerical simulation,the influence of mechanical parameters of the overlying rock and soil layer on surface settlement was explored.It was found that as the values of elastic modulus,cohesion,and internal friction angle continued to decrease,the amount of surface settlement continued to increase.The variation amplitude of the three parameters from large to small was elastic modulus.internal friction angle,and cohesion.Based on the orthogonal Design of experiments.through the sensitivity analysis method,it is found that the sensitivity of the elastic modulus of the tunnel rock and soil layer is greater than the internal friction angle,and the sensitivity of both is far greater than the cohesion.