Research On Construction Mechanical Behavior Of Highway Tunnel With Super Large Cross Section

In recent years,with the rapid development of China's tunnel engineering construction,a large number of large-section tunnels have emerged in China.Among them,the construction technology of tunnels with large cross-sections from 140m~2 to250m~2 has become more mature.However,for the excavation of a super-large section tunnel with an area of 428.5m~2,the relevant design theory and construction experience are insufficient,and there are few engineering cases for reference.In addition,the design and construction standards for this excavated area tunnel have not been clearly defined in China's existing highway tunnel design codes.Therefore,it is of great significance to strengthen the research on the construction mechanical behavior of such super-large section tunnels.Based on the super-large-section tunnel of Liantang tunnel of Shenzhen eastern transit highway,this paper studies the surrounding rock loose pressure,construction method and supporting parameters of super-large-section highway tunnel through theoretical analysis,numerical simulation and on-site monitoring.The main research contents and results are as follows:(1)This paper summarizes the existing theory of the surrounding rock loose pressure calculation and the formation of the pressure arch,adopts the numerical simulation method,and further studies the characteristics of the stress change of the surrounding rock and the influence of different factors on the pressure arch range after the excavation of the super-large section tunnel from the angle of the pressure arch.And based on the concept of pressure arch,regression calculation formula is obtained to calculate the surrounding rock loose pressure of deep buried super-large section tunnel under the conditions of III and IV surrounding rock.(2)By establishing a three-dimensional numerical model of a super-large section tunnel,the deformation and mechanical characteristics of the surrounding rock after the tunnels of different surrounding rock levels and different cross-section sizes are excavated by the full-section method are firstly studied.It is concluded that the tunnel with the largest span in the context of this project is not suitable for excavation by the full-section method.Secondly,by analyzing the stress of surrounding rock and supporting structure after tunnel excavation,the two commonly used large section excavation methods of two-side wall drift heading method and Center Cross Diaphragm method are compared.After considering the deformation and force of surrounding rock,construction speed and construction cost,it is recommended that the Center Cross Diaphragm method be used for the excavation of the super-large section of the Liantang Tunnel.(3)A field monitoring test was carried out at the largest span of the Liantang Tunnel.After analyzing the deformation of the arch top and both sides of the tunnel after excavation,the contact pressure between the surrounding rock and the primary branch,and the stress monitoring results of the steel arch frame,it was found that the on-site monitoring results and numerical simulation results have a high agreement,which explains that The rationality of the numerical simulation shows that the numerical calculation results can provide a reference for the actual construction.(4)The method of orthogonal test and numerical simulation was used to further optimize the support parameters of the tunnel with a large section.The primary and secondary order of the impact of each support parameter on the evaluation index was determined under different evaluation indexes.According to the results of orthogonal tests,the optimal combination plan for the initial support of the largest span of the Liantang Tunnel is recommended to be 33cm for the thickness of the initial support,1m for the steel arch span,and I20a for the steel arch type.