Experimental Study On Stepped Variable Cross-section Pipe Pile Driving

As a new type of pile,the stepped variable cross section pile is inspired by the slub pile.Through the design of the stepped variable cross section,the ultimate bearing capacity of the tube pile per unit volume can be improved without significantly reducing the ultimate bearing capacity of the single pile.So as to achieve the advantages of economic benefits.Moreover,the variable-section pile also has many advantages such as reducing the influence of the pile body on the soil around the pile,simple process,saving materials and so on,which has attracted more and more scholars’ attention.In this paper,through the indoor static pressure pile experiment and the pile top static load experiment,the data of the penetration resistance of the prefabricated pipe pile during the penetration process,the ultimate bearing capacity of the single pile and the displacement of the soil particles around the pile are obtained.Through PFC2D numerical simulation,the changes of the soil particles during the sinking process of the pile and the changes of the porosity and displacement field of the particles around the pile were observed in detail.Finally,based on the theory of circular hole expansion and the empirical parameter method in "Technical Specifications for Building Pile Foundations" JGJ94-2008,the penetration resistance during pile driving,the soil displacement around the pile and the ultimate bearing capacity of the single pile are predicted,and the indoor model experiment and numerical simulation curves are compared and analyzed.The main conclusions of this article are:1.It is found that with the increase of the variable diameter ratio(small diameter/large diameter),the maximum uplift and total penetration resistance of the surface soil around the pile increase,and the lateral earth pressure increases continuously in the process of static penetration.However,if the penetration resistance per unit volume is calculated,the variable diameter ratio zero point six seven and zero point eight three The pile types of are larger than that of the pipe piles,and the diameter ratio is larger than that of the pipe piles zero point eight three The penetration resistance per unit volume is the largest.2.Through the static pressure stacking experiment,it can be found that the axial force distribution curve of the pile body of the pipe pile is relatively gentle,while the axial force distribution curve of the variable-section pile has a sudden change at the end of the variable-section.The slope of the lateral friction resistance curve of the variable-section pile is greater than that of the pipe pile,and the lateral friction resistance curve reaches its peak when the pile length is about 0.7m.3.Both the indoor model experiment and the empirical parameter method calculation in"Technical Specifications for Building Pile Foundation" JGJ94-2008 can be found that,according to the ultimate total bearing capacity of a single pile,the pipe pile is larger than the variable cross section pile.If calculated according to the ultimate bearing capacity of a single pile per unit volume,its ranking from large to small is in order of variable diameter ratio 0.83>0.67>1>0.5.The pile type with a diameter reduction ratio of 0.83 is the most economical benefit,which is consistent with the conclusion in conclusion 1.4.Numerical simulation found that the variable cross-section pile can significantly reduce the lateral and vertical displacement of soil particles around the pile.The smaller the reduction ratio,the smaller the porosity change and stress concentration of the soil around the pile due to pile disturbance.If the construction site is restricted,the pile type with a smaller diameter ratio can be selected as far as possible.5.The circular hole expansion theory can be used to predict the change curve of end resistance,friction resistance,and radial tangential particle displacement with the increase of pile penetration depth.Due to the poor compactness of the artificially tamped sand and the smaller plastic strain caused by the pile squeezing the soil,the theoretical value curve obtained by the analytical solution is slightly larger than the indoor model test and numerical simulation curve.However,the curves of the three methods have little difference,which verifies the reliability of the conclusion of this article.