Study On Construction Method Of Strongly Weathered Granite Section Of Huangchagou No.2 Tunnel

The Huanji Expressway is a new transportation artery built to improve the comprehensive transportation system in Northeast China,strengthen the highway network,and construct external channels.The No.2 Huangchagou Tunnel of the Huanji Expressway is located in the jurisdiction of Ji’an City,Tonghua City,Jilin Province,and is a key project of the Huanji Expressway.The strongly weathered granite section of the tunnel accounts for over 50% of the total length of the tunnel.The highly weathered granite rock mass has a severe degree of weathering,with broken surrounding rock,poor cementation,and easy disintegration when softened by water,exhibiting low strength and poor engineering characteristics.When tunnel construction passes through strongly weathered granite formations,improper selection of construction methods can easily lead to engineering accidents such as falling blocks,roof falling,instability of the face,and collapse,resulting in casualties and property losses.Therefore,conducting in-depth research on the construction methods of Huangchagou Tunnel 2 has important engineering significance for ensuring the safe construction of Huangchagou Tunnel 2and for tunnel construction in the same type of strata.Based on the analysis of the tunnel engineering and geological conditions,this article summarizes relevant information on tunnel construction in similar strata and preliminarily determines the construction methods available for selection;Using numerical simulation method,simulate the excavation process of four available construction methods,and select displacement and stress values of monitoring points to compare and analyze the four construction methods.Based on the relevant regulations on tunnel deformation and stress,and after safety verification,the construction method of the tunnel is finally determined;On this basis,further study the construction step distance of the selected construction method;The feasibility of the selected construction method was verified through monitoring and measurement.The main research conclusions of this article are as follows:(1)The common construction methods are compared and analyzed from the aspects of advantages and disadvantages,scope of application,economy,construction period,etc.In combination with the stratum lithology and hydrogeological conditions of Huangchagou Tunnel 2,the construction methods adopted in the actual project under similar stratum conditions are investigated,and four construction methods are determined as the primary selection scheme of tunnel construction methods,namely,bench method,annular heading with core soil method,middle diaphragm method,and double side heading method.(2)Based on indoor experiments,tunnel geological conditions,and support parameters,the physical and mechanical parameters of the surrounding rock required for numerical simulation considering the influence of advanced support measures were determined.Establish a numerical simulation model to simulate the excavation process of the four initially selected construction methods.Through the comparative analysis of the displacement,plastic zone and stress of the monitoring points,combined with the strength reduction method for safety verification,it was determined that the tunnel was constructed by the bench method.(3)Qualitatively analyze the impact of the upper step height on tunnel construction,and select a 5m upper step height for construction.Numerical simulation analysis of tunnel displacement and stress under different construction steps of 5m,10 m,15m,20 m,25m,and 30 m.It was found that for the settlement of the arch crown and the convergence of the side walls,the deformation growth rate is faster during the construction step distance of 5m～15m;During the stage of construction steps ranging from 15 m to 30 m,the growth rate of deformation slows down.Based on the comprehensive construction efficiency and tunnel safety,it is determined that the reasonable lead distance between the upper and lower steps of the tunnel is 15 m.(4)The maximum value of arch settlement measured by monitoring was 38.1mm,which increased by 0.3% compared to the simulation results;The minimum value is33.21 mm,which is reduced by 12.5% compared to the simulation results.The maximum convergence value of the monitoring measurement chamber is 43.43 mm,which is reduced by 7.8% compared to the simulation results;The minimum value is41.7mm,which is 11.4% less than the simulation results.The main reason for the difference between the numerical model calculation results and the tunnel monitoring measurement data is that after the tunnel excavation is completed,the tunnel begins to deform rapidly,and at this time,the monitoring measurement system has not been established and the deformation at this stage has not been recorded.Secondly,there are a large number of joint fissures in the strongly weathered granite rock mass,and there is a certain deviation between the numerical model parameter values and actual engineering.