The Influence Of Pile Loading On Adjacent Existing Metro Tunnel Forces And Deformation

The stability of the tunnel structure will be affected invevitably by the pile foundation built adjacent to existing tunnel. While the load on the top of the pile increases gradually, the soil displacements around the pile extend to the tunnel, of which the force state will be affected. The pile loading leads to the diaplacements of the tunnel structure and the uneven convergence of the tunnel section, causes a series of problems such as the crack and water seepage of the tunnel lining, the tilt of the track. This will have some influence on normal operation of the subway. Obviously it is especially important to study the influence of pile loading on adjacent existing metro tunnel.In this paper, finite element program MIDAS/GTS is adopted and three-dimensional model is created. In order to study the influence of pile loading on adjacent existing metro tunnel, a series of factors are considered to get the force and deformation features of the tunnel, finally the force state, deformation models and control measures are summarized. The contents of this paper are outlined in details as the following:1. The tunnel force and deformation at the case of single pile loading are analyzed, the influence of the tunnel on the force state and displacement fields of the pile are discussed, the variation of maximum displacement of tunnel and the deformation features of tunnel lining are analyzed with parameters, such as length of the pile, diameter of the pile, load on the top of the pile, distance between tunnel and pile, thickness and stiffness of the lining and forms of the section. It shows:①The stress fields of tunneling and the stiffness of the tunnel lining have influence on the development of the side friction and the displacement fields of the pile.②While the length, diameter,and load of the pile increasing, the displacements of the tunnel increase. The influence of the pile length is related to the burying depth of tunnel, while the ratio of pile length and burying depth being 1, the displacement reaches the maximum. The displacements the tunnel increase in the form of exponent with the increasing of the distance between tunnel and pile. And while the thickness and stiffness of the lining increasing, the maximum settlement becomes smaller and the minimum settlement becomes larger.③While there are piles on both sides of the tunnel, the increment of maximum displacement is large, but the differential value is small. The settlements of round section are slightly larger than three-centered circular section, but the differential value of tunnel bottom is smaller, for operational tunnel, the round section is better than three-centered circular section.④When pile length is less than burying depth, the utmost pressure-point of tunnel section is between hance and vault, it shifts down with the increasing of pile length; when pile length is between burying depth and minimum depth of tunnel, the utmost pressure-point shifts down with the increasing of pile length; When pile length is over burying depth, the utmost pressure-point shifts up with the increasing of pile length. The distance between tunnel and pile is larger, the utmost pressure-point is lower. With the increasing of the load on the top of the pile, the utmost pressure-point becomes higher.2. The force state and deformation of the metro tunnel under the load of the pile group foundation are simulated and analyzed, the influencing factors which is considered includ distance between the tunnel and pile, pile distance along the tunnel, pile distance perpendicular to the tunnel, the row number of tunnel and so on. At the same time, the longitudinal deformation of the tunnel is considered. The shielding effect of isolation piles is simulated, and the force state, displacements and deformation features of the tunnel are obtained from the simulation. The influencing of the different isolation pile length and distance between the tunnel and pile is analyzed. The research shows:①The tunnel displacements increase when the distance between the tunnel becoming larger, and the rate of the displacement decrease becomes smaller while the the distance between the tunnel becoming larger. The maximum tunnel displacement decrease when the pile distance along the tunnel increasing, and decrescent rate becomes smaller. The influence of the vertical pile distance is less than the parallel pile distance. The tunnel displacement decrease with the increasing of the pile distance perpendicular to the tunnel, and the decrescent rate is smaller relativly.②The maximum tunnel displacement decreases with the increasing of the longitudinal rigidity and the curve of longitudinal settlement is very gentle.③The displacements of pile top and the maximum tunnel displacement drcrease 10%～40% because of the isolation piles. The utmost pressure-point of the tunnel section is deflected due to the influence of the isolation piles, so the settlement difference of track is decreased. The tunnel displacements drcrease while the distance between tunnel and pile increasing, and the tunnel displacement decreases rapidly when the isolation piles is the same or longer than the adjacent pile.The influence of the isolation piles decreases when the isolation piles is over 1.25 times of the adjacent pile. The reduction of the tunnel displacements become larger while the distance between the tunnel and piles increasing, and the influence of the isolation piles increases. The maximum tunnel displacement increases when the isolation piles is close to the pile foudation。3. Finally, the deformation model of tunnel section under the influence of external load is proposed, then some control measures of tunnel displacements are summed up, On this basis, two of the measures are supplemented and modified using the conclusions which are gained by the simulation and analysis in this paper.