Study On Mechanical Properties Of Existing Tunnel Caused By Construction Of New Tunnel Underpass

The improvement of the level of urbanization has brought the rapid development of underground space.The emergence of the underground interchange have solved the problem of heavy traffic.Underground interchange usually located in the busiest section of the city.They have complicated structure form and there are more buildings and more people around them.The tunnel route is criss-cross.Thus this bring a series of problems related to construction.Such as the construction of tunnel near building,construction of two parallel tunnels,new constructed tunnel on cross existing tunnel,new constructed tunnel beneath the existing tunnel and so on.This article is standing on the shoulders of predecessors.Rely on planned two river tunnel project in chongqing province.Theoretical analysis and numerical simulation are adopted to study the changing rule of the existing tunnel in a series of mechanical behavior during the process of a new tunnel orthogonal beneath existing tunnel and under the condition of three kinds of middle rock thickness.The main research contents and conclusions are as follows:(1)This paper introduce the basic principle of the mechanics of tunnel excavation in the near construction.Theoretically analyse the change of stress and displacement during tunnel excavation,stress zoning with support condition,effect of support on the displacement of rock around tunnel.Vault subsidence displacement is greater than the lateral wall displacement.Rock conditions is an important factor in the impact area division of tunnel excavation.(2)Finite element simulation of the dynamic construction process during new constructed tunnel orthogonal beneath existing tunnel is adopted.Mainly analyze the change law on the typical section of tunnel longitudinal including vertical displacement of vault and arch bottom,horizontal displacement of arch waist and the change of surrounding rock stress and bending moment of retaining structure around cross position.Tensile stress of surrounding rock appear at arch bottom.When the new constructed tunnel excavated to through cross position,the tensile stress is the biggest.Rock compressive stress at haunch of existing tunnel increase during new constructed tunnel through cross position.After through the cross position,the tensile stress reduce.The section of existing tunnel is more closer to cross position,the greater effect existing tunnel will suffered.Extrusion and pushing force because of new tunnel excavation is the main reason that cause horizontal displacement of waist of existing tunnel and formation loss is anther reason that cause settlement of existing tunnel.(3)Process of a new constructed tunnel orthogonal beneath existing tunnel is simulated by finite element under the condition of three kinds of middle rock thickness.Analyze the displacement、axial force、bending moment of typical section on existing tunnel and the change of rock stress around the cross position.Vertical displacement of the vault on existing tunnel is more influenced by middle rock thickness between two tunnels.As the middle rock thickness is less,change of axial force is more on the every section of existing tunnel.When the middle rock thickness is large,area of influence on the existing tunnel by new constructed tunnel is big and the stength is small.when the middle rock thickness is small,area of influence on the existing tunnel by new constructed tunnel is small and the stength is big.(4)Discuss consolidate technique of cross tunnel and focuse on reinforcement technology and its mechanism incloud small pipe grouting、bolt system、level through prestressed anchor,and finnaly cross tunnel reinforcement measures for middle rock is recommended.