Research Of The Response Of The Pile-soil-cap System Under Loading

This paper used transparent soil technology and the particle image velocimetry test method to study the responses of the single piles with cap under the vertical load. On the premise of the same or similar as the natural properties of soil, transparent soil was used in the experiment instead of natural soil particles, to achieve the goal of internal soil visualization technology. Transparent soil consists of two parts:skeleton material and pore fluid. The transparent soil model was made by the fused quartz sand with white oil and Dodecane mixed pore fluid. The particle image velocimetry technology achieved by capturing and analyzing the speckle images induced by laser light sheet into the transparent soil model. This techno-logy solved the problems that the traditional test in the research of pile foundation on the stress response cannot achieve full field and continuity measurement of the displacement field inside the soil mass. Through the self-made vertical loading equipment for the transparent soil model, a series of tests were conducted on the piles grouped by cap size, pile length and pile diameter separately. The deformation of soil was analyzed at the ultimate load.The study found that deformation of the soil at the vicinity of the pile can be divided into three deformation area of pile head, pile body and pile base. Scope of pile head lateral deformation only affected by size of pile cap, has nothing to do with the pile length and pile diameter. The lateral deformation zone abound the pile body began in the pile head, and with the increase of pile cap size, the coverage along the pile body increases, until it merges with the pile bottom deformation zone. Pile bottom deformation area is mainly irregular circular region centered at the bottom of the pile, and the influence range is not affected by the cap size, pile length and pile diameter, only related to the settlement. The cap can improve the ultimate bearing capacity of pile-soil system, at the same time, this paper found that when reaching the ultimate bearing capacity, increasing the pile length can effectively reduce the deformation of the soil at the deep layer, then make the soil to fail mainly in the shallow layer. In this case, we can just reinforce the shallow soil to improve the bearing capacity of pile foundation and reduce the economic cost on the premise of making reasonable choice for pile caps, pile length and pile diameter.Based on the above research, in this paper, the response of single piles with cap under lateral load was numerically studied with FLAC3D. The conclusions are as follows:the weight of pile caps and its friction contact with the soil will offset the lateral load, and the caps can improve the overall bearing capacity of single piles. Under the same lateral load, the greater the cap size, the smaller the displacement of pile. The increase of caps size will improve overall the side friction of pile, also the pile with larger cap could have a stronger ability to resist deformation. It is also found that moderate increase in the pile length, can effectively reduce the horizontal displacement of pile body. In pile foundation design, we not only should consider the increased level of resistance with the increase of pile length, but also consider whether the pile can withstand the maximum bending moment. It is worth mentioning that the maximum bending moment is constants at a quarter of the pile length. When reaching the maximum allowable load, the pile caps only have distinct influence to the soil resistance at the first quarter of the pile length. The usual p-y curve calculation methods tend to be conservative when applied to the single pile with pile caps under the condition of elastoplastic ground. In the absence of caps, the horizontal resistance coefficient of soil basically agrees with the K method except for the inflection point and the pile bottom. And the existence of pile caps will change the resistanceof soil at the vicinity of the pile.