Model Tests On Vertical Bearing Characteristics Of Variable-section Pile

Variable-section pile is a new type of pile. The pile use different parts of soil besides itself to install enlarging plates, and make friction pile variable-section multi-supports frication and end-bearing pile. The enlarging plates change the stress mechanism of the pile, and make bearing capacity improved while settlement deformation decreased. Compared with pi1es with the same diameter, it has more carrying capacity and less deformation and displacement. If promoted in every field of civil engineering, it will receive great economic and social benefit. Based on the exiting research results of the variable-section piles, the paper studied carrying capacity of pile under pullout loads, load transferring mechanism, stress distribution of piles and soil around piles, through indoor model tests. And it mainly studied the influences on carrying capacity and deformation of pile with enlarging plates from different distances of enlarging plates.Through the date measured from indoor model tests, drew Q-S curve and relation curve of pile axial force, from which we can find that variable-section pile has good load-bearing characteristics. When the load is comparatively smaller in the initial stages, axial force in the place where the enlarging plates are set did not change significantly, axial force of the pile is basically decreased linearly, show that the carrying capacity of the pile is provided mainly by the lateral friction of piles. As the load increases, in the places where the enlarging plates are set, pile axial force changes significantly, which means that the enlarging plates come into play, increasing its share of the load, which supports the reason that variable-section pile has stronger carrying capacity than the standard diameter pile. At the same time, due to the impact enlarging plates, the role played by lateral friction within a certain range of the enlarging plates will be affected, even negative friction can exist.Modeling tests were carried out in laboratory, and results showed that for a variable-section pile with one enlarging plate or two enlarging plates which were wide apart, the soil mass under the plates was compressed, while the soil mass between the plates tended to collapse in a pear shape. When the plates were close to each other, the soil mass between them integrated with them. The settlement curve of uniform-section pile was similar with that of variable-section pile; however, the side friction (including pile side friction and plate bottom resistance) of the latter was much higher than pile end resistance, regardless of the quantity of plates or interval between them. For the variable-section pile with one enlarging plate, bearing capacity of the pile was concentrated around the plate, especially when the plate was constructed in the middle of the pile and the bearing capacity was relatively large. For the variable-section pile with two enlarging plates, the plates in the upper section had higher bearing capacity than those in the lower section, and so they should be constructed in the upper section; interval between the plates influenced the bearing capacity in an insignificant way. For the variable-section pile with three enlarging plates, its bearing capacity was a bit higher than that with two plates. All in all, the bearing capacity of variable-section pile has been greatly enhanced to the degree of being equivalent with the uniform-section pile of the same diameter as the enlarging plate. Distribution of axial stress along variable-section pile is different from that of uniform-section pile. With the increasing of load level, the axial stress under the plates decreases sharply due to the bearing capacity of the plates, and the result is irrelevant with the quantity or location of the plates.