| In the construction of tunnels in the southwestern mountainous areas of China,it often encounters unfavorable geological conditions such as shallow burial and surrounding rock,which brings difficulties to the tunnel construction,especially in the tunnel portal section.How to tunnel the tunnel section under shallow buried and broken surrounding rock conditions Excavation and support to ensure the stability of surrounding rock during tunnel construction is of great significance to avoid the collapse of the tunnel and the smooth development of the entire project.At present,the new Austrian method is adopted for tunnel construction in China.The new Austrian construction emphasizes the common bearing capacity of the surrounding rock and the initial support,and the choice of initial support has a great relationship with the surrounding rock level of the tunnel.It is necessary to study the division and modification of the surrounding rock grade of the tunnel in the buried section,the design of the support scheme and the construction monitoring.In addition,the reasonable construction scheme is also the key to ensure the stability of the surrounding rock during the tunnel construction of the shallow buried section of the tunnel.Therefore,this paper takes the Mawu Tunnel of Shijie Expressway in Chongqing as the engineering background,and studies the classification of surrounding rock grade,optimization of support parameters and optimization of construction scheme for the shallow-buried and fractured tunnel of Dongkou.The specific research results are as follows:(1)Conducting on-the-spot investigation and data collection on the geological conditions of the shallow-buried and fractured section of the Mawu tunnel,considering that there is no high ground stress in the surrounding rock of the tunnel,and adopting BQ according to the degree of surrounding rock fracture,structural surface state and groundwater distribution characteristics.The method and RMR method are used to divide and correct the surrounding rock grades.The results of the surrounding rock classification of the two methods are compared,and the surrounding rock grade of the shallow-buried and fractured section of the Mawu tunnel is clarified,which lays a foundation for the support design and construction scheme optimization.basis.(2)According to the results of surrounding rock classification,the pre-reinforcement scheme for the shallow-buried and fractured section of the Mawu tunnel is designed.The support parameters are designed.The rationality of the support parameters is checked according to the convergence-constraint method.The calculation results show The tunnel design displacement is 1.2cm,which is much smaller than the reserved deformation,indicating that the support parameters used are reasonable.(3)According to the characteristics of the shallow-buried section of the Mawu tunnel,the construction of the typical section was monitored,and the monitoring data was collected and processed.The deformation law of the surrounding rock caused by the tunnel constructionwas analyzed,combined with the regression analysis method.The stability time of the surrounding rock was predicted.The prediction results show that the self-stabilization time of the V-level surrounding rock of the shallow-buried tunnel is about 25 days to 30 days,which is about 5-7 days later than the stability of the surrounding rock of Grade IV.(4)In order to ensure the rationality of the on-site construction plan,the FLAC3 D software was used to optimize the construction of the shallow-buried and fractured tunnel of the tunnel,and the excavation and support process of the step method,the CD method and the three-step seven-step method were carried out respectively.In the simulation study,the deformation law of surrounding rock caused by the three construction methods,the surrounding rock and supporting stress,and the distribution range of surrounding plastic zone were analyzed and compared.The step method was finally selected as the shallow buried section of the Mawu tunnel.The program,the on-site monitoring results are basically consistent with the numerical simulation results,indicating that the construction optimization results based on numerical simulation are feasible.. |
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