Impact Analysis Of Pile Foundation In Metro Tunnel Underneath Existing Bridge

With the development of urban traffic in China.the subway project in urban rail transit developed rapidly,but the formation of deformation owning to underground excavation and the stress release are bound to a certain degree of disturbance to the surrounding environment.Because of the complexity of the urban environment,the tunnel construction may through a large number of existing bridge foundation inevitably,affecting the normal use of bridge.Therefore,it is of great significance to study the sedimentation patterns of the strata near the bridge pile foundation and the influence of the internal force and bearing capacity of the pile foundation.Based on the excavation of Beijing subway line 14 and numerical analysis,the paper analyzed strata displacement and stress of bridge pile foundation,combined with the actual monitoring data.The research work and the results are as follows.Tunnel excavation process is accompanied by the expansion of the surface subsidence trough.The expansion of the surface subsidence trough accords with the normal distribution law of peck formula.The largest settlement is on the axis in tunnel excavation and the distribution is symmetrical on both sides.The main influence scope is from 0 to 3D(D represent the tunnel diameter)horizontally and vertically.Tunnel excavation has obvious effect on nearby existing bridge's bearing capacity.The bearing capacity of pile foundation weakens and the neutral point falls down.The same tunnel excavation has different influence on different length of pile.The longer the pile is,the greater the side friction resistance of pile is.Because long pile has larger contact area with the soil.But the damage of bearing capacity of longer pile is bigger,owning to its buried depth.The soil around the lower of long pile sinks obviously compared with short pile,making side friction of long pile bigger.As a result,the damage of bearing capacity of longer pile is great.Protection scheme of pile foundation can effectively reduce the settlement of tunnel excavation.Deep hole grouting maintain stability better for pile side soil among two kinds of soil protection plan.In the pile protecting grouting,the sedimentation of pile side soil is little.The side friction of upper part of pile is large and the side friction of lower part of pile is small.Therefore the pile foundation stability of deep hole grouting is good.