Interaction Mechanism Between Passive Pile And Adjacent Tunnelling Considering Shearing Deformation Of Foundation

The rapidly increasing demand for public transport construction in congested urban areas will involve tunnel excavation adjacent to existing building foundations and services due to the lack of surface space.Adverse effects on nearby pile foundations may be induced due to changes in stress in the ground and hence,ground movements.Accurate predicting of effects of tunnelling on pile foundations poses a major challenge during design and construction.Methods to analyze this problem may be broadly classified into three categories:numerical analysis,simplified analytical method and laboratory test.The simplified two-stage analytical method is simple and clear,which has a wide range of applications,and is easy to be accepted by engineers.In this paper,the additional displacements and internal forces of adjacent pile caused by tunnelling are analyzed based on the Pasternak foundation model.Main work and study of this paper are as follows:?1?Two kinds of analytical methods are employed for estimating free field soil displacement induced by tunnelling which are based on cavity contraction theory in infinite medium including elastic solution?source image technique?and elastic-perfectly plastic solution?elastic-plastic cylindrical cavity contraction method?.Results show that elastic solution?source image technique?and elastic-perfectly plastic solution?elastic-plastic cylindrical cavity contraction method?can estimate the stress and displacement distribution of surrounding soil induced by tunnelling.?2?The free field soil displacement obtained in the first stage is transformed into the additional stress applied on the pile on Pasternak's foundation.Based on the principle of minimum potential energy,the pile equilibrium equation on Pasternak's foundation is derived.The equilibrium equation is solved through finite difference method combined with the boundary conditions and hence,the pile horizontal displacement and the bending moment caused by tunnelling are obtained.Considering the shielding effect of pile groups,the expressions for horizontal displacements and bending moments of pile groups are derived by introducing a transmission coefficient of soil displacement.The rationality of the method used in this paper is verified by comparing with the engineering cases,the integral numerical methods and the centrifuge test results.The influences of different boundary conditions on the bending moment and horizontal displacement of piles are discussed.Results show that there is an influence range for the effect of boundary conditions on the pile responese:the boundary conditions have great influence on pile displacement and bending moment near the boundary,while the horizontal displacement and bending moment at the middle of the pile are less affected.?3?An analytical solution for estimating pile response is presented in this paper considering three-dimensional effect of lateral soil deformation in Pasternak's foundation.The calculated results are compared with and without considering the 3D effects of pile-soil-tunnel interaction.It is found that results with considering the 3D effect of lateral soil deformation are closer to the monitoring data and centrifuge test data.A parametric analysis is performed to discuss the influences of various factors on the deflection and bending moment of pile groups,including the modulus of shear layer,pile diameter,ground loss ratio,pile-tunnel distance and pile spacing.?4?The cavity expansion theory is employed for estimating pile-tunnel interaction.The influences of pile jacking and tunnelling on the pile bearing capacity are analyzed.The pile total load capacity is determined as the summation of the contribution from ultimate end-bearing capacity and the ultimate shaft friction.The ultimate cavity pressure at pile tip is calculated using a spherical cavity expansion model in infinite medium,and then the ultimate end-bearing capacity of the pile is obtained.The equivalent average shear stress along pile length is estimated using modified?-method,and the ultimate pile shaft friction is obtained.The modified method considers the reduction of the shear stress of the pile shaft due to tunnelling.The soil stresses in plastic zone and elastic zone during pile jacking are calculated using a spherical cavity expansion model in infinite medium,the ultimate end-bearing capacity of the pile is calculated based on the limit pressure at pile tip.The influences of relative tunnel-pile positions on pile end-bearing capacity are analyzed.When the distance between the pile and the tunnel center is constant,it is more advantageous for pile bearing capacity by increasing tunnel depth.With the increase of tunnel-pile distance,the end-bearing capacity of the pile will not be affected by the tunnelling when the pile tip is outside the plastic zone.The ultimate pile shaft friction is estimated using modified?-method.The parameter?_m is modified in order to consider the effect of tunnelling on shaft friction.By introducing the pile capacity factor R_Q,the influence of tunnel depth on pile capacity is analyzed.When the tunnel volume loss is a constant,the total pile capacity increases with the depth of the tunnel,when the depth is large enough,the total pile capacity will not be affected by the tunnelling.