Design Of Anti-friction Device And Theoretical Study On The Characteristics Of Skin Friction Of Pile

Negative skin friction occurs when the piles undergo displacement caused by the sedimentation of soil. Currently, studies regarding the properties and mechanism of negative skin friction are still not comprehensive. During the engineering practice, problems still exist for the calculation of negative skin friction and the evaluation of carrying capacity of pile foundations. Structure damage or dysfunction often happen due to the negative skin friction, and which can cause unnecessarily loss. This study put forward as well as optimized a drag reducer based on the previous research. The aim is to reducing the negative skin friction which could be caused by the sedimentation of soil body and the effect of loading, and at the same time preventing the weakening of positive skin friction. Mathematic model for the drag reducer theoretical calculation has also been built up; by combining the negative skin friction calculation theory, the feasibility of theoretical study was verified. The influence of drag reducer on pile foundation was analyzed by finite element software. The foundation soil was simplified as homogeneous soil layer plus lateral soil and bearing soil layer, in order to eliminate the soil physical property effect on the finite element analysis result. Major research outcomes are as below:(1) An instrument for eliminating negative skin friction of piles was invented, and patent was applied. The positive effect of the patent is: positive skin friction could be sufficiently used, while the negative skin friction is efficiently reduced. The instrument is structurally simple and highly feasible.(2) Carrying drag reducer skin friction calculation method was proposed for the first time. By combining the soil sedimentation and solidification analysis theory, as the reference for theoretical study of drag reducer.(3) Working properties of carrying drag reducer was analyzed by numerical simulation method. Based on the combination of the simulating data in the situations of lateral loading, decreasing of groundwater level, and solidification of filling soil, negative skin friction can be reduced by 33.4%- 56.3%, while 95.3%- 110.0% of the positive skin friction can be retained above the neutral point, when placing drag reducer at the middle of the piles; the depth of neutral point decreases by 8.3%- 30.3% when comparing with regular piles. However, when solidification occurs to lateral filling of central piles for multilayer soil, the neutral point increases by 3.8%; when there is no negative skin friction, sedimentation of the pile top is 2.5%- 9.8% less than regular piles. When lateral filling was applied to the central piles for multilayer soil, the sedimentation of the pile top is 3.1% larger than regular piles, when the groundwater decreases, the sedimentation is 2.7% larger than regular piles.In conclusion, the positive skin friction of pile with the device was the same with ordinary pile. The device can effectively isolate the influence of negative skin friction that caused by the changes of load around pile, underground water levels and soil consolidation. But the neutral point will deeper than ordinary pile in most cases, the bottom of the device is still under the action of the negative skin friction. Overall, pile with anti-friction device can effectively reduce the negative skin friction compared with ordinary pile, it has significant advantages compared to traditional asphalt coating method. The research has certain references value to eliminate the negative skin friction of pile foundation.