Study On Bearing Failure Mechanism Of Uplift Piles And Progressive Evolution Law Of Pile-soil

In recent years,the development of underground space has gradually become one of the important ways to alleviate urban congestion.However,for areas with high groundwater level and greater permeability soil,the buildings will sustain large vertical stress causing by the buoyance of water,which increases the risk of structural instability.The uplift piles,which were used to resist the stress from the pile top,has become an important measure to control deformation for the large underground space structures,the super-high buildings,and the tidal quay platforms.However,there is no sufficient designing experience of the uplift piles for underground structures in China at present,including the failure mode,the deformation response of pile body to limiting stress,and the synergistic effect and the optimized layout of pile groups.Based on the indoor model test and numerical simulation,this paper focuses on the failure characteristics of uplift piles under loads and the progressive evolution law of pile and soil.Indoor model tests are designed to analyze the influence of pile length,pile-soil interface characteristics and soil pressure on bearing capacity of uplift piles,and the force law and synergistic deformation characteristics of pile groups are studied.Combining with the theoretical analysis,the bearing failure mechanism of uplift piles can be obtained.It is concluded that the ultimate bearing capacity of uplift piles can be improved by increasing pile length,the friction coefficient of pile surface and soil pressure in a certain range.Compared with the deformation differences of central pile,side pile and corner pile,the central pile reaches the ultimate failure state first.Based on FLAC3 D software,numerical simulation technology is used to validate and compare with the test results.The calculation model of uplift pile group is established,and the progressive evolution law of pile and soil during the excavation of foundation pit and the uplift of piles.By comparing the differences of stress and displacement of uplift pile and soil,the weak links of pile group are found.The result shows that the corner pile has the best uplift resistance,the side pile is the next,and the center pile is the weakest.Setting the pile spacing to 3-7 times the pile diameter,it is concluded that the optimum pile spacing is 6 times the pile diameter.Finally,taking Chegongmiao Station of Shenzhen Metro as a demonstration project,effective control measures are proposed to increase the ultimate bearing capacity of pile groups.It is suggested that the base-enlarged uplift piles should be installed along the central axis of the main part of the station,and that the spacing between the base-enlarged uplift piles and the conventional equal-section uplift piles should be arranged at 2:1.The conclusions obtained will provide an important basis for optimizing the layout and type selection of uplift piles and provide guidance for uplift pile engineering.