Study On The Influence Of Twin Tunnels Construction On Existing Tunnel

The urban underground rail transit has become the main means to solve the urban traffic congestion with its advantages of safety,quickly,time saving,large capacity and so on.Multiple crossing tunnels are always seen in the central city,consequently the interaction between the existing tunnels and the later new tunnels has become a hot spot and difficult point in the research of urban rail transit construction.Accordingly the ground loss and unloading effect caused by tunnelling inevitably lead to additional internal force and additional deformation of existing tunnel,What is more,the tunnel structure safety and operation are seriously affected because the structure of existing tunnel is bound to cause cracking or leakage when the additional internal force and additional deformation exceed the certain limit,therefore,the influence of tunnel construction on adjacent tunnel deserve to be studied.Based on the existing research and taking the construction of twin tunnels on existing tunnel in Nanning as the background of the project,a three dimensional numerical model was established to verify the reliability of this simulation method compareing with the results of centrifuge model test which have been studied.Then,on the basis of confirmed method,a series of three dimensional numerical models were carried out to investigate two typical cases that parallel twin tunnels were excavated beneath or above the existing tunnel and overlapped tunnels were excavated crossing from one side or both sides of existing tunnel.Finally,the calculation results of the surface settlement,the vertical displacement along the crown,induced strain along the invert,radial displacement of the existing tunnel were analyzed.The related conclusions are obtained:(1)The influence of parallel twin tunnelling on existing tunnel:When the new tunnels were excavated beneath the existing tunnel,the ground settlement induced by tunnelling was more seriously affected by the shielding effect of existing tunnels than that when the new tunnels were excavated above.The settlement of existing tunnel caused by the parallel twin tunnels excavation underneath was significantly larger than the heave caused by the parallel twin tunnels excavation above,settlement decreases and heave increases with increasing the cover depth of new tunnels and existing tunnel.The tensile strain which caused by tunneling along the side of the existing tunnel that close to the new tunnel was induced within a distance of 3D(D is the tunnel diameter)from the center of the existing tunnel,and it was little affected by the tunnel depth.The radial deformation of the existing tunnel section is mainly reflected as "the vertical radius decreases and the transverse radius increases" when the new tunnels were excavated underneath,and the radial deformation of the existing tunnel section is mainly shown as "the vertical radius increases and the transverse radius decreases" When the new tunnels construction above.(2)Influence of overlapping twin tunnelling on existing tunnel:The maximum surface settlement caused by overlapping twin tunnels construction with the sequence of"lower-crossing first and then upper-crossing" is about 20%larger than that caused by overlapping twin tunnels construction with the sequence of "under crossing first then up"when the overlapping tunnels cross the existing tunnel,and the maximum are obtained near the centerline of the upper tunnel.There is a significant difference between the maximum vertical displacement of existing tunnel which caused by overlapping twin tunnels construction with the sequences of "under crossing first then up" and "lower-crossing first and then upper-crossing" respectively when the overlapping tunnels were excavated underneath the existing tunnel,the latter is about 69%larger than the former and the maximum are obtained near the centerline of the upper tunnel.The maximum vertical displacement caused by overlapping twin tunnels construction with the sequence of "lower-crossing first and then upper-crossing" is about 16%larger than that caused by overlapping twin tunnels construction with the sequence of "under crossing first then up" when the overlapping tunnels up-under crossing the existing tunnel,and the shape of the displacement curve has significant difference.It is meaningful that this article research results can provide theoretical guidance for the design and construction of the subway project in Nanning of GuangXi.