Mixed-grid Finite Element Analysis On Penetration Effect Of Steel-pipe Pile In Saturated Soft Soil

The penetration of steel-pipe pile in soft soil has a significant effect on surrounding soil,which not only changes the surrounding environment,but affects the penetration resistance and pile set-up.The penetration effect is mainly manifested in the disturbance of soil structure,soil deformation and excess pore water pressure(EPWP)response.The current research mainly focuses on the change of pile bearing capacity after penetration,but the mechanism of penetration effect of steel-pipe pile and its influence on subsequent bearing capacity of pile foundation should not be ignored.The key to study the mechanism of penetration effect of steel-pipe pile is the analysis of large soil deformation and pore water pressure response.The traditional Lagrange method adopts the constitutive of effective stress form,but it cannot solve the mesh distortion caused by large deformation.CEL or ALE method ignores pore water pressure effect.Therefore,it is necessary to propose a finite element method to solve the problems of mesh distortion and EPWP calculation,and then to deeply study the mechanism of penetration effect of steel-pipe pile in saturated soft soil.In this paper,the Hybrid Lagrangian-ALE method was introduced to simulate the penetration of steel-pipe pile based on effective stress analysis.The influence of different construction and soil parameters on the soil deformation and EPWP response around pile was analyzed,and the mechanism of penetration effect of steel-pipe pile in saturated soft soil was revealed.Then the evaluation method of penetration effect of steel-pipe pile was determined.Finally,the influence of soil parameters on soil consolidation effect after the steel-pipe pile penetration was studied,and the correlation between pile set-up and EPWP dissipation was discussed.The main contributions of this dissertation are as follow:(1)A Hybrid Lagrangian-ALE finite element method considering the stress continuity between different types of mesh was proposed to simulate the penetration of steel-pipe pile in saturated soft soil.In this method,the soil model of the area near the pile-soil contact surface was set as Non-Lagrange grid,and soil parameters under the condition of total stress analysis were adopted.The rest soil model was set as Lagrange grid,and soil parameters under the condition of effective stress analysis were adopted.Considering that the shear moduli of isotropic soil at the elastic deformation stage were equal under the two stress analysis conditions,the correlation of soil model parameters under the two stress analysis conditions was established to ensure the continuity of soil stress at the interface of the mixed grid.The total process of steel-pipe pile jacking(the penetration process and the soil consolidation stage after penetration)and the process of steel-pipe pile driving in saturated soft soil were simulated,and good agreements were obtained between the measured and calculated values,which verified the feasibility and reliability of Hybrid Lagrangian-ALE method.(2)For the close-ended jacked steel-pipe pile,the influence of different construction and soil parameters on the soil deformation and excess pore water pressure(EPWP)response around pile was analyzed.The maximum value of EPWP(?u_max)around pile was affected by R_p,z_p,c_u,E and OCR;The radial extent of EPWP and lateral soil deformation were affected by R_p,c_u,E and OCR,and had no relationship with z_p after penetration into a certain depth;The radial extent of lateral soil deformation was approximately 15 to20R_p and the radial extent of EPWP was approximately 10R_p.(3)The steel-pipe pile was set as an open-ended,and the influence of different construction and soil parameters on the soil deformation and excess pore water pressure(EPWP)response outside the steel-pipe was analyzed.?u_max and the radial extent of lateral soil deformation outside the steel-pipe were affected by R_p,z_p,c_u,E and OCR;The radial extent of EPWP was affected by z_p,c_u,E and OCR,and had no relationship with R_p when pile radius exceeds a certain value;The radial extent of EPWP was approximately 5 to 15R_p and the radial extent of lateral soil deformation was approximately 20 to 25R_p.For small diameter steel-pipe pile,the soil deformation induced by open-ended pile was more obvious than that induced by close-ended pile,but with the increase of z_p,the penetration mode of open-ended pile was similar to that of close-ended pile due to being blocked of the steel-pipe;For large diameter steel-pipe pile,the soil deformation and EPWP response induced by close-ended pile was more obvious.(4)The influence of different construction and soil parameters on the distribution of EPWP inside and outside the steel-pipe during the penetration of super-large diameter open-ended steel-pipe pile was analyzed.For the jacked open-ended steel-pipe pile,the radial extent of EPWP was affected by R_p,z_p,c_u,E and OCR,and was approximately 3R_p;?u_max of soil inside and outside the steel-pipe was affected by z_p,c_u and E,and had no relationship with OCR;?u_max of soil adjacent to the inside and outside the steel-pipe has no relationship with R_p,and?u_max of soil core center was affected by R_p.For the driven open-ended steel-pipe pile,?u_max of soil inside and outside the steel-pipe was affected by N_h,F_h and c_u,and had no relationship with E;?u_max of soil adjacent to the inside and outside the steel-pipe has no relationship with R_p,and?u_max of soil core center was affected by R_p;The radial extent of EPWP and the EPWP of far field soil were affected by R_p,N_h,F_h,c_u,E and OCR.(5)Based on parametric analyses,the evaluation methods of the distribution of EPWP around pile induced by close-ended and open-ended steel-pipe pile jacking were determined by multiple regression analysis.The research on the distribution of EPWP along depth during close-ended steel-pipe pile jacking under different construction and soil parameters was simplified to study the influence of different construction and soil parameters on the proportion coefficients associated with the EPWP distribution curve in depth and the normalized maximum EPWP(?u_max/c_u).The formula of the maximum EPWP based on the expansion theory of spherical cavity was modified to calculate?u_max/c_u during close-ended and open-ended steel-pipe pile jacking,which took into account the influence of R_p,z_p,c_u,E and OCR.(6)The whole process of close-ended steel-pipe pile jacking in saturated soft soil was simulated to study not only the influence of k_s,c_u,E and OCR on soil consolidation effect after the pile penetration,but also the correlation between pile set-up and EPWP dissipation.The results of research showed that:k_s and OCR affected the rate and duration of EPWP dissipation;OCR also affected the value of EPWP at the end of pile jacking;However,c_u and E only affected the value of EPWP at the end of pile jacking.In the penetration phase,the negative EPWP near ground surface around pile and at a certain depth below the pile tip would increase the penetration resistance.At the early stage of consolidation,the negative EPWP increased gradually to positive value,which could cause the decrease of bearing capacity of pile and even led to the static load test pile sank suddenly;When the consolidation time reached a certain value,the dissipation of positive EPWP would increase the bearing capacity of pile to a certain extent.