Optimization Analysis Of Excavation And Support Parameters For Large-span Ultra-shallow Buried Construction Of Jiefangbei Phase Ⅲ Ring Road

With the rapid development of China’s economy,the urbanization process continues to promote,people for the way to travel to put forward higher requirements.Due to the limited urban space,the surface roads are far from being able to meet the growing traffic demand,many cities are developing underground space on a large scale,and the number of large-span ultra-shallow buried municipal road tunnel projects is increasing.Urban large-span super-shallow buried tunnels generally have large cross-sectional areas,supershallow burial depth,poor geological conditions,strong sensitivity to the surrounding environment,etc.If improper design theory,excavation methods,and support parameters may lead to destabilization and damage of the surrounding rock during the tunnel construction,and even collapse of the phenomenon.Therefore,how to choose a reasonable design theory,excavation method,and support parameters for urban largespan ultra-shallow buried tunnels to ensure the quality,safety,and economic benefits of tunnel construction is a scientific problem that needs to be solved in the design and construction of urban large-span ultra-shallow buried tunnels.In this paper,using the third phase of the Jiefangbei Ring Road Tunnel as the engineering background,theoretical analysis,numerical simulation,and monitoring and measurement methods are used to revise the definition of deep and shallow buried tunnels and the formula for calculating the surrounding rock pressure,explore a reasonable excavation method for large-span ultra-shallow buried tunnels in cities,optimize the initial support parameters,and analyze the force characteristics of lining sections:(1)the principles of tunnel classification for large span and super shallow buried and the formula for calculating the envelope pressure were analyzed,and the existing highway tunnel design code was amended to address the deficiencies that the definition method of deep and shallow buried does not apply to large span tunnels and the formula for calculating the envelope pressure does not apply to super shallow buried tunnels.(2)The finite element software Midas/GTS was used to study the effects of construction excavation by CRD method,Three-Bench Seven-Step method,and Doubleside heading method on the stability of the surrounding rock of large-span super-shallow buried tunnel.By analyzing the effects of different excavation methods on surface settlement,surrounding rock,and initial support stress and deformation,the most reasonable excavation method was selected with the allowable ultimate displacement and lining safety internal force as indicators,and the modified surrounding rock The results show that the Double-side heading method can more effectively reduce the impact on the stability of the surrounding rock during construction,and play an obvious effect on improving the mechanical properties of the surrounding rock and enhancing the stability of the tunnel,and the modified pressure calculation formula for the surrounding rock is consistent with the numerical simulation results.(3)Comparing and analyzing the effects of different anchor lengths,anchor spacing,and initial spray concrete strength grade on the displacement and stress of the surrounding rock and initial support,studying the change law of the stability of the surrounding rock with the support parameters,selecting the best support parameters,and suggesting the optimization of support parameters suitable for large-span super shallow buried tunnels.(4)The load-structure method was used to compare and analyze the stresses of the secondary lining of large-span super-shallow buried tunnel under different span and section types,and the change law of internal force of secondary lining structure with section form was studied,and the suitable secondary lining section form for large-span super-shallow buried tunnel was proposed to provide a reference for design.(5)The monitoring and measurement data of surface settlement,vault settlement,and horizontal convergence value around the tunnel construction site are collected and analyzed,and compared with the numerical simulation results to verify the correctness of the model and provide a basis for guiding the construction.