Study On The Axial Bearing Behaviors Of Squeezed Branch Pile

Squeezed branch pile is a kind of variable section pile, which is formed by setting branch or bearing plate on the pile body in the original basis of bored pile. It has been widely used in the projects because of its characteristics of high bearing capacity, small settlement, and high economic benefits, ect.Because of the complex mechanism of the squeezed branch pile, current understanding of its bearing behaviors is not sufficient, the theoretical studies on squeezed branch pile are now far behind the engineering practices. In this paper, study of the axial bearing behaviors of squeezed branch pile is made on the basis of existing research results at home and abroad and using finite element analysis and field test. The main contents are as below:Firstly, the author establishs a two-dimensional axisymmetric finite element model of squeezed branch pile and straight pile to analyse the bearing behaviors of squeezed branch pile. The results show that compared to straight pile, the branch pile can greatly reduce the pile top settlement and improve the ultimate bearing capacity; branch and part of the soil on the top slope under load will form a "free surface". which will reduce when calculating the side friction; the existence of the branch makes significant changes on the bearing behaviors of squeezed branch pile when compared with the straight pile.Secondly, the author establishs two-dimensional axisymmetric finite element models of squeezed squeezed branch pile with different branch spacing, different branch quantities, different branch diameters, and different branch angles for systematic analysis their impact on the bearing performance of squeezed branch pile. The results indicate that the optimal branch spacing is 2.5 to 3 times the branch diameter, and the optimal branch diameter is 2 times the diameter of the main pile. The amount of branch will improve the ultimate bearing capacity of squeezed branch pile, but too much branch will lower the economy of squeezed branch pile. Increasing the squeezed branch angle and reducing the branch height can not only improve the ultimate bearing capacity of squeezed branch pile, but also its economic efficiency. The distance between the branch and soft substratum should be maintained at 2.5 to 3 times branch diameter or more.The bearing behaviors of squeezed branch pile is analysed under the condition of self-balancing in-situ test. The test results show that the bearing capacity of uplift squeezed branch pile should be a deduction of that obtained by self-balancing in-situ test. Under the upward load acting at the pile tip. the effects of squeezed branch are obvious and so are the time-lag and sequential effects of the involvements of the branch resistance and pile shaft friction. The pile shaft friction and the branch resistance should be corrected separately when the self-balancing vertical compression test is taken for determining the ultimate bearing capacity of squeezed branch pile. Replacing the over-feeding straight pile segement by the squeezed branch pile, the drawbacks of low reaction force contributed by the upper pile part in a self-balance pile test can be overcome.