The Effect Of Asymmetric Water Pressure In Shield Tunneling Construction

Shield tunneling,as one of the important methods to develop underground space,has become the main means of urban underground engineering.The asymmetric water pressure on both sides of shield tunnels built beside rivers results in the change of stress on lining segments.The water pressure on both sides of tunnel lining segments has a certain influence on the stress of lining structures.Firstly,this paper introduces the background of the topic selection and the development of shield construction and lining structure at home and abroad.Combined with the structural and mechanical characteristics of segment lining,it expounds the influence of shield construction process and the matters needing attention.Secondly,based on the judgment of classical theoretical formula,the influence of some parameters on the stress of lining is analyzed.Finally,based on the first phase of a Metro Line 2 project in a city,this paper systematically studies the influence of asymmetric water pressure on the deformation and internal force of lining segments,as well as the stratum displacement and surface subsidence.The main research contents are as follows.(1)Using ANSYS finite element software and beam-spring model,the two-dimensional finite element model of segment is established.The load-structure calculation of single-ring segment is carried out,and the deformation and internal force variation of lining segment under asymmetric water pressure are analyzed.(2)The three-dimensional finite element model is established to simulate the tunnel excavation process,and the effects of asymmetric water pressure on the surface subsidence are studied.The main results and conclusions of this paper are as follows:(1)Under the action of asymmetric water pressure,the position of segment joints should be avoided as far as possible at the vault and the upper right 30 ~45.If not,flexible joints should be used as far as possible.(2)With the increase of the water pressure ratio on both sides of the segment,the deformation of the same joint of the lining segment shows an upward trend,especially on the left side of the segment,that is,on the back side of the water surface.That is,the greater the difference of water pressure between the two sides,the larger the deformation is.(3)As the ratio of water pressure on both sides of the tube increases,the shear force on the sheet surface is not affected.The shear force value on the side of the back surface is reduced by 33.8%.The shear force value on the water side is increased very small,almost negligible.With the increase of the water pressure ratio on both sides of the segment,the position of the maximum axial force of the segment does not change significantly,but thevalue of the axial force of the node becomes smaller on the side of the back surface of the segment,and becomes larger on the water-facing side of the segment.The increase reached4.4%.There was no significant change in the position of the maximum positive and negative bending moments of the segments,but the values of the bending moments increased slightly on the water-facing and back-water surfaces,and the increase on the water-facing side was greater than that on the back surface,with the maximum increase of 7.8%.(4)Because of the existence of asymmetric water pressure,the ground settlement on both sides of the tunnel axis also presents an obvious asymmetric distribution,and the ground settlement on the side facing the water surface is obviously larger than that on the side behind the water surface.With the increase of the ratio of water pressure on both sides of the segment,the maximum value of surface subsidence also increases.When the ratio of water pressure on both sides changes from 1:1 to 1:1.25,the maximum value of surface subsidence increases more than double.