Install Effects And Bearing Capacity Of Drilled Displacement Piles With A Screw-shaped Shaft

In this paper,numerical computation,model test,field investigation and theoretical derivation were used to systematically study the install effects and bearing capacity of drilled displacement piles with a screw-shaped shaft(DDS piles).Firstly,a numerical method for simulating the installation of displacement piles was proposed,and the installation process of DDS piles was implemented successfully by using this method,the characteristics of the soils closed to the drilling rod were analyzed.Then,the bearing mechanism and the effects of pile-soil interface,soil strength and pile geometrical parameters on the ultimate bearing capacity were investigated in the model test and numerical studies.Based on the numerical results,two failure modes,?cylindrical shear failure? and ?individual bearing failure? and two methods for calculating bearing capacity corresponding these two failure modes were proposed and verified.The main contents and conclusions of this dissertation are as follows:(1)The VUMAT subroutines of modified Cam-Clay model and Hypoplasticity model were developed and implemented in Coupled Eulerian-Lagrangian(CEL)method of ABAQUS/Explicit.A numerical method for simulating the installation of displacement pile was proposed.Then the installation process of DDS piles was successfully implemented and the characteristics of the soil stress,soil compactness and soil displacement during the drilling process were analyzed.In addition,the influences of geometrical parameters of drilling rod(screw pitch,screw thickness and screw width)on the installation effects were also studied.A filed test was conducted to investigate the characteristics of soil horizontal displacement and pore pressure during the test piles installation and subsequent soil consolidation duration.Based on the measured data,an empirical method for prediction the horizontal displacement was proposed.(2)A self-designed testing apparatus and a test method were used in the model test to study the bearing capability.A numerical method was proposed based on the back-analysis of the model test.Then the effects of pile-soil interface,soil strength and pile geometrical parameters on the ultimate bearing capacity were investigated by using this numerical method.Based on the numerical parametric study,two failure modes,?cylindrical shear failure? mode and ?individual bearing failure? were proposed.The results of the model test showed that: the unit shaft resistance values of the DDS model piles are respectively 4.6 and 3.5 times greater than that of the straight wall model piles which respectively has a diameter equals to the inner and external diameter of the DD model piles.The results of the numerical parametric study showed that: the limit shear stress on the pile-soil interface,the friction angle of surrounding sand,screw pitch,and thread width significantly influence the bearing capacity of DD piles with a screw-shaped shaft,whereas,the friction coefficient on the pile-soil interface and the thread thickness have little effect.With the increase of the screw pitch from 0 to 1000 mm,the ultimate bearing capacity increases to the maximum and then decreases gradually.The best range of the screw pitch is from 250 mm to 500 mm which can be as a reference for the design works of DDS piles.(3)Two mechanical models corresponding to ?cylindrical shear failure? mode and ?individual bearing failure? mode were created,and two calculation methods and criteria standard of these two modes were proposed.The theory of the ultimate bearing capacity of foudations proposed by Meyerhof(1951)was used in the analysis of ?individual bearing failure? mode and soil shear strength was considered in the ?cylindrical shear failure? mode.Finally,two calculation methods and criteria standard were verified by numerical and test results.