Study On The Displacement And Stress Characteristics Of Pile Foundations Due To Shield Tunnel Construction

With the rapid development of economy and society, urban subways have been being built to relieve the ground traffic. For the subway is usually built along the city’s main road, it may inevitably traverse the existing bridges’foundations, which may lead to pile’s displacement and internal force redistribution, and then affect the regular service of the structure. In this paper, based on Beijing Metro Line10which traverses the existing bridge, adopting the finite element method to simulate tunneling effects on piles under different working condition, such as geological conditions, pile form, position between piles and tunnel, tunnel construction parameters, then the tunneling influence on the pile foundations’displacement and stress characteristics will be obtained. The main achievements and conclusions are as follows:1) The settlement of pile decrease with the soil’s compressibility, Poisson’s ratio, shear strength enhance. When the inserting depth of the pile is greater than the depth of tunnel, the piles’settlement decrease with its length increase and the differential settlement between two piles under one cap also decrease as well as the gradient of the cap. When tunneling in silty soil under the pile, its inclination departs from the tunnel in the case where piles are just above the tunnel excavation face, to the tunnel in the case where distance between tunnel and pile is greater than2.5m, and keeping standing in the case where piles are just above tunnel axis. When tunneling at the side of pile, horizontal displacement along the pile depth varies nonlinearly, and pile produces curvature. The weaker the soil shear strength, the greater the bending deformation. The pile’s bending direction from the same with tunnel lining to the opposite with soil Poisson’s ratio increases. In the condition tunnel tranverse two-pile foundation, pile bolck’s inclination direction is the same with the cap.2) The axial force of the pile decreases when piles are located just above the tunnel structure, and increase in the other conditions. The extent of influence reduces with the distance between tunnel and pile get longer. The soil’s shear strength, compressibility, Poisson’s ratio varations have little effect on pile’s axial force. In the condition of two-pile foundation, the pile load near the tunnel reduces, that part of load bears by the other pile. The extent of influence reduces with the pile’s inserting depth getting longer.3) The additional bending moment is smaller when tunnel is under the piles, relatively larger when tunneling at the side of pile and the maximum value is generally on the level the same with tunnel axis. The extent of influence is related to the distance between tunnel and piles, in silty soil it reaches the maximum when the distance is6m. The pile’s normal stress increase at the side near the tunnel, decrease at the side far away from the tunnel, and the greatest effect is on the level the same with tunnel axis. The additional positive moment value and normal stress variation decrease with the shear strength and Poisson’s ratio of the soil enhance. When tunnel tranverses two-pile foundation, the bolck of the pile near the tunnel is subjected to greater bending moment due to the connections of piles and caps, and then concrete of the pile bolck on the side far from the tunnel is under tension.4) The surface subsidence value has a linear relation with the pile settlement and horizontal displacement, and has a nonlinear relation with the pile’s axial force, bending moment, normal stress. As the surface subsidence increases, the effects on internal force increase, as well as its variation rate.Based on above analysis, some functions are given, through these functions, the pile’s settlement and horizontal displacement can be calculated by surface subsidence caused by tunnel construction, horizontal distance between the tunnel and pile and the pile’s length.