Study On The Construction Mechanics Of The Intersection Of The Existing Tunnel Shaft And The Main Tunnel In The In-situ Extension

With the rapid growth of China’s economy,the number of people traveling and the volume of cargo transportation have increased sharply.In order to meet transportation needs,in-situ tunnel expansion projects at home and abroad are increasing.During the in-situ expansion of the tunnel,the existing lining needs to be removed first,and then the surrounding rock excavation and lining construction are carried out.A relatively stable balance system has been formed between the original ground and the existing lining through coordinated deformation.The in-situ expansion of the tunnel will break this balance and face greater safety risks during construction.Based on the in-situ extension project of the existing Nanling Tunnel,a combination of on-site monitoring and numerical simulation was used to analyze the changes in the surrounding rock stress and construction mechanical characteristics during the in-situ extension of the intersection of the existing tunnel shaft and the main tunnel.The simulation analyses on the different extension types,excavation methods,and cycle footage of existing tunnels were carried out.The results show that the single-sided expansion is asymmetrical excavation,which easily leads to uneven stress release of the surrounding rock to form a bias effect.The surrounding rock stress caused by the single-sided expansion is bigger than that of the double-sided expansion.The surrounding rock deformation caused by the fullsection method is smaller than the step method and the short-step method.Based on the project,it is recommended to use double-sided and the full-section method to expand.At the same time,under the permission of construction conditions,the cyclic footage of excavation should be shortened as much as possible.The simulation analysis on the geometric construction parameters of the intersection of shaft and tunnel was carried out,the results show that the buried depth of the shaft and the width ratio of the shaft to the tunnel are all negatively related to the stability of the surrounding rock.When the shaft crosses the tunnel obliquely,the stability of the tunnel in-situ expansion is better than that of orthogonal.When the shaft has a large buried depth and a large section,strengthened support or local reinforcement measures should be implemented within a certain range of the cross section.Based on Midas/GTS finite element software,the construction process of the intersection of shaft and the tunnel was simulated.Under the premise of the on-site monitoring data to verify the calculation accuracy of the finite element model,the influences of in-situ expansion on surrounding rock displacement field,stress field,excavation plastic zone and initial support stress were studied.By monitoring the stress change value of the surrounding rock during the in-situ expansion process,the distribution of the surrounding rock stress field at the intersection of the shaft and the tunnel,which changed with the advancement of the tunnel face is studied.The results show that the shaft influences the arching effect of the tunnel and creates a penetrating plastic zone from the side wall of the tunnel to the bottom of the shaft.