Study On Negative Skin Friction Of Squeezing Expanded Pile In Collapsible Loess

In collapsible loess drop load caused by negative skin friction on pile side is extremely considerable, which will produce serious security hidden danger for pile foundation and engineering, so it should be taken full account of in the design of pile foundation. But now methods to reduce or eliminate the negative skin friction are difficult to attain both safety and economy in collapsible loess. A squeezing expanded pile is a special pile and its soil squeezing effect will have d ifferent influences on negative skin friction. Based on theoretical analysis and laboratory test, this paper has studied the effect of squeezing soil into hole on the negative friction in collapsible loess.The compaction mechanism, soil squeezing effect and change of negative skin friction in a squeezing expanded pile have been studied by theoretical analysis. With squeezing soil, a certain range of soil around the pile gradually enters the plastic state, and plastic zone is expanding. The stress and degree of density of the soil around the pile are decreasing along the radial direction. When a squeezing expanded pile is immersed by water, soil between piles which is not squeezed will produce collapsible settlement and shear stress is generated at the soil and soil interface. In the process of the shear stress passing along the radial direction, the vertical circular ring will gradually consume shear stress, and when the thickness is enough, pile side can eliminate the negative skin friction.In order to get closer to the actual results, the laboratory scale model test has been designed, which treats squeezing expanded piles as the main research object and transforms pore forming method and pile diameter. The characteristics and distribution of skin friction of squeezing expanded pile after water immersion, the distribution law of the collapsible deformation of soil between piles in water immersion, and soil squeezing effect are systematically studied in collapsible loess. Test results show that: in the case of squeezing expanded piles are unloaded and immersed, negative skin friction along the pile down, basically increase s at first and then decreases, shows a single peak, and finally transforms into positive skin friction. The settlement of soil between squeezing expanded piles gradually becomes greater along the radical direction and the fast increasing range enlarges with the increase of the pile diameter. The degree of density of soil between squeezing expanded piles gradually decreases along the radical direction and the effective soil squeezing thickness enlarges with the increase of pile diameter. The change curve of degree of density and the change trend of settlement are basically consistent. With the increase of ratio of the effective soil squeezing thickness and pile diameter, negative skin friction increases at first and then decreases. When the ratio exceeds the ―minimum ratio‖, the negative skin friction is decreasing, and the consumption of soil squeezing effect on the negative friction is shown. As the ratio continues to increase, the negative skin friction continues to decrease, which indicates that the consumption of soil squeezing effect on the negative friction strengthens with the increase of pile diameter.