Study On Uplift Mechanism Of Pile Foundation Of Shiyangtai No.1 Bridge In Wujing Expressway

The mechanism of bridge pile foundation deformation has always been the top priority of bridge disease research at home and abroad.Due to the particularity of the bridge pile foundation structure,it is very easy to be affected by many factors to produce uplift or settlement deformation,resulting in pier deformation,bridge deck cracking,bridge structure instability and tilt,and even irreversible damage to the bridge,which seriously affects the stability of the bridge and the safety of driving operation.Based on this,this paper relies on the project of abnormal uplift of some pile foundations of Shiyangtai No.1 Bridge on Wujing Expressway,analyzes the influencing factors of abnormal uplift and deformation of pile foundations of bridges at home and abroad,and combines the actual situation of the bridge.The possible influence mechanism hypothesis of the uplift of some pile foundations of Shiyangtai No.1 Bridge is proposed.Through the monitoring and analysis of the slope displacement on the north and south sides of the bridge,the experimental study on the physical characteristics of the core soil samples of the overlying soil under the bridge pile foundation,and the numerical simulation of the bridge pile foundation in the coal mining area,the formation mechanism of the abnormal uplift of some pile foundations of Shiyangtai No.1Bridge is discussed in depth.The main research contents and research results are as follows:(1)Through on-site drilling geological exploration,the engineering geological survey of the bridge research area is studied.Based on the analysis of the abnormal lifting mechanism of bridge pile foundation at home and abroad,combined with the existing research,three basic hypothesis theories of bridge pile foundation uplift are proposed: ’landslide side push’,’expansive soil expansion’ and ’the coal mining area collapse arch extrusion’ theoretical conjecture.(2)Through the displacement monitoring of the north and south slopes of the bridge,it is found that the slope on the south side of the bridge slides towards the bridge site,and the slope on the north side slides back to the bridge.Combined with the monitoring value of deep horizontal displacement,it is found that the displacement deformation of the slopes on both sides is relatively small,that is,the deformation of the slopes on the north and south sides of Shiyangtai No.1 Bridge belongs to the normal deformation range.The slopes on both sides of the bridge are in a stable state,and the lateral thrust generated by the sliding deformation cannot make the bridge pile foundation uplift.(3)Through the macro and micro analysis of the core soil samples of the overlying soil layer under the bridge pile foundation and the experimental study of the physical characteristics,it is found that there is weak expansive soil in the overlying soil layer under the bridge pier.After the axial compression is applied,the soil sample is calculated according to the equal proportion under the ideal condition.After the water expansion,it is impossible to generate enough force to make the bridge pile foundation uplift.(4)FLAC3D is used to simulate the influence degree of bridge pile foundation in the coal mining area.It is concluded that after coal mining,the pile foundation is uplifted,and the maximum uplift displacement reaches 255 mm,which is almost the same as the change of309 mm in the actual monitoring of the elevation of the 8# pier of the bridge.Therefore,it can be considered that the coal mining area is the main reason for the abnormal uplift of some pile foundations of Shiyangtai No.1 Bridge.(5)Based on the analysis of the formation mechanism of the influence of the coal mining area on the uplift of the pile foundation,the roof of the coal mining area lost its bearing capacity and collapsed.Under the influence of the weight of the surrounding rock such as the upper mountain,the rock and soil in the upper loose area collapsed to the bottom of the goaf to produce the side thrust of the accumulation body facing the direction of the bridge pile foundation.After the soil in the loose area collapsed and fell,the overlying rock and soil bent to form a collapse arch.The vault rock structure can be regarded as a composite arch composite beam bearing the weight of the surrounding rock of the upper mountain.The vault bearing the upper surrounding rock sinks,and the arch foot produces the side thrust of the collapse arch facing the bridge pile foundation.Under the action of the double lateral thrust generated by the accumulation body and the collapse arch,the interlayer dislocation of the sandstone soil layer is driven,and the dislocation bending uplift between the soils produces vertical upward extrusion pressure,the maximum reached 2.5Mpa,which was1.42 Mpa higher than the initial stress value of the pier.It can completely cause the abnormal uplift of the soil around the pile foundation and the bridge pile foundation.