Study On Soil Plugging For Jacked Steel Pipe Pile In Clay

Some of the soil is squeezed into pipe pile to form the"soil plug"during the penetration.The soil plug not only affects the penetration resistance,but also is closely related to the soil compaction effect.This study focuses on soil plugging effect in clay.The main research is as follows:1.The mechanism of soil plug during jacking the steel pipe pile in clay is investigated by the Coupled Eulerian-Lagrangian(CEL)approach to simulate the large deformation penetration process.The evolution and mechanism of soil plug is presented with soil velocity field inside and outside steel pipe pile,stress field,height of soil plug and incremental filling ratio.The influencing factors of friction coefficient,pile diameter,soft or hard soil interlayer and the shape of pile shoe on the formation of soil plug is discussed.The results show that the evolution of soil plug is mainly divided into three stages including the upwelling stage,the transition stage and the downward sliding stage during the penetration of pipe pile.As the pile penetrating,the continuous concave plastic arch forms at the axis under the pile tip.The vertical stress increment here up to 7-8 times the undrained shear strength indicates the initial formation of soil plug.With the increase of friction coefficient between pile and soil and the decrease of pile diameter(same wall thickness),the plugging effect enhances.Pipe pile is prone to a fully plugged mode when the hard soil is over the soft one,while the hard soil is prone to squeeze into pile when the soft soil is over the hard one.2.The soil compaction effect of pipe piles with different diameters during the penetration is investigated by CEL method to study the compaction stress,soil yield and penetration resistance.The theory of cavity expansion is combined with the soil plugging effect.The solution of cylindrical cavity expansion based on soil plugging effect is obtained.The results show that the radial influence range and vertical influence range of compaction stress are about10D(pile diameter)and 7D respectively.The soil yield in the radial range of 6~7R_o(outer radius of pipe pile)at the pile tip.The proportion of tip resistance is the smallest(<10%).With the increase of pile diameter,the proportion of internal shaft friction increases and the proportion of external shaft friction decreases.When the pipe pile is close to the soft and hard soil interlayer,a large radial squeezing stress is formed in the hard soil.3.The soil plugging effect under nonhomogeneous clay condition is analyzed.The variation of clay strength with depth and strain softening are obtaind by the secondary development of ABAQUS.The problem of mesh distortion in jacking pile process is solved by remeshing technique with the combination of python program.The height and displacement of soil plug,penetration resistance and stress field are studied.The results show that when the penetration depth is more than 3m(3.3D),the influence of strain softening on the height of soil plug gradually appears.During the downward displacement of the soil plug,there is a inverted U-shaped area with small displacement at the pile tip.The influence of soil softening on internal shaft friction is greater than external shaft friction.The phenomenon which soil softening reduces internal shaft friction mainly occurs at the bottom of soil plug(within the range of 3.8D from the pile tip).The strain softening will weaken the compacting effect and the soil arch effect at the pile tip.