Study On Deformation And Load-bearing Characteristics Of Segmentally Variable Cross-Section Pile

In recent years，with rapid development of the high speed railway bridge project，bridge engineering puts forward higher requirements for pile foundations.It is a newchallenge for scholars, engineers and technicians to develop and improve new pilefoundations according to pile foundation technology and theory innovation, which cansatisfy the strict request of high quality,quick progress and low cost. The step taperedpile foundation is a new kind of form of structure. Its engineering practice hasdeveloped before theory research. It is proved that it is more reasonable and moreeconomy form of structure. Its development and application has the good economicvalue and technological advantages. However, there still are some problems, that theresearch on the step tapered piles are not systematic and imperfection. Therefore, it isvery important to carry out the test and theoretical research on the deformation andload-bearing characteristics of this type of piles. This paper studys on the deformationand load-bearing characteristics of this type of piles systematically, by the physicalmodel test, numerical simulation, theoretical analysis and program development. Themain contents and results are as follows:①Through the large-scale model test, we obtain the pile body deformation andstress characteristics of piles with four types of variable cross sections under verticalstatic load as well as the P-S curve of the pile top and skin frictional distribution curveof the pile body. On this basis, the bearing capacity of unit volume of piles with variablecross section and the relation between bearing capacity ratio and variable cross-sectionratio are deduced.Through the analysis we reach the following conclusions: with theincrease of variable cross-section ratio, the vertical bearing capacity of unit volume ofconcrete first increases and then decreases. And the ratio of the ultimate bearingcapacity of variable cross-section piles of different standards to uniform cross-sectionpiles is significantly related with variable cross-section ratio. The smaller the variablecross-section, the more considerable the loss of the vertical ultimate bearing capacitywill be. Variable cross-section pile has the optimal variable cross-section ratio, thevalue being about0.8.②The compressive and tensile stress/strain curve of pile body with four types ofvariable cross sections under transverse static load as well as load-displacement curveand load-rotation curve of the pile top are obtained through large-scale model test. On this basis, we derive the bending moment distribution of the pile body. The analyticalresults are as follows: pile body with variable diameters at different positions of variablecross section exhibits different deformation characteristics. Generally, the stress onlarge-diameter segment above the variable cross section is greater, whereas that onsmall-diameter segment is small. Such stress dustrution is more resonalbe than that invertical piles. Variable cross-section piles have the optimal position of variable crosssection, its value being about46.7%of the total pile length.③9numerical analysis models are established in total.According to Flac3Dnumerical analysis, we obtain variable cross-section piles with different variablecross-section ratios and postitions of variable cross section. The distribution of stressfield and displacement field under vertical and transverse load, the deformation curve ofpile body, load-displacement and load-rotation curves of the pile top are obtained. Theconclusions from the model test are thus complemented and verified. The analysisindicates that as the variable cross-section ratio and the position of the variable crosssection vary, the distribution pattern of displacement field and stress field also vary. Butwhen the variable cross-section ratio is larger than0.8, the influence of variablecross-section ratio on the distribution of dispalcement field and stress field is relativelysmaller; when the position of variable cross section is less than46.7%of the pile length,the influence of variable cross section on dispalcement field and stress field undertransverse load is significant. However, the influence depth along the pile body islimited.④The calculation theory of allowable bearing capacity and settlement of variablecross-section piles under vertical load is established in the critical state; the computationprogram is formulated to verify the accuracy of the algorithm and versatility of theprogram using the computational example.⑤The theoretical calculation method under transverse load based on structuralmechanics and theory of elastic piles is analyzed and summarized. The internal forceand displacement curve of piles under transverse stress with different positions ofvariable cross section and stress boundary conditions of the pile top are theoreticallycalculated. The results are compared with the those in model test and numerical analysis,which show good consistency. Meanwhile, the computation program is formulated tolay the foundation for the application and promotion of variable cross-section piles.⑥Based on strain energy theory, the expression for equivalent stiffness and theformula for equivalent width of the foundation of variable cross-section pile groups are established. Then the design and calculation methods of the foundation of variablecross-section pile groups is deduced.⑦The condition and the factors that affect the adaptability of the variablecross-seciton piles are analyzed. Construction condition and mechanics analysis are thenecessary and sufficient conditions for the using of the variable cross seciton pile. Theshear stiffness of interface and the ratio of shear modulus between pile and soil is themain factor affacted on the vertical deformation and load-bearing characteristics of thistype of piles. The diameter of the pile and m is the main factor affacted on thehorizontal deformation and load-bearing characteristics of this type of piles.