Research On The Group Effect Of Jacking Pile Group In Sand Under Axial Loads

Pile foundation has been widely used in the construction of high-rise buildings, offshore oil drilling platforms and buildings constructed under adverse ground conditions over the last decades. The pile foundation supporting the above structures was usually exposed to the tremendous pulling force or even axial cyclic loading. Structures supported by pile foundation may tend to topple due to the failure of pile foundation. Therefore, it is significant to study the mechanical behavior of pile foundation under axial load using experimental and theoretical methods.This research will explore the mechanical behavior of jacking pile subjected to axial load in sand by model pile tests and numerical simulation. Firstly, five tests of single pile subjected to compression, uplift and cyclic loading were conducted to study how buried depth of the pile, pile side roughness and pile material influence the mechanical properties of a single jacking pile in sand. Secondly, ten axial loading tests of2×2pile groups, including two kinds of pile side roughness and five kinds of pile spacing, were conducted. These tests aimed to measure the group effect coefficients of pile groups with different spacing. Finally, based on the displacement coordination algorithm of the load transfer method (the so called t-z method), a MATLAB program was written to simulate pile reaction under axial load. An elastoplastic t-z model considering the Possion Effect and a hyperbolic q-z model were employed in the program. The results illustrated that the program using t-z model and q-z model was feat for the forecast of axial resistance of pile in sand. The conclusions are as following:The load-displacement curves of a jacking pile under compression and tension are very different. It proves that the failure of jacking pile under compression and tension are ductility and brittle respectively. The bearing capacity of axial load increases with the increase of the buried depth and the pile roughness. However, these methods can hardly change the ultimate displacement. By contrast, the bearing capacity of compression load increases if the selected material is relative softer and this may lead to a greater ultimate displacement. The maximum compression load and the maximum tension load will increase with the cyclic index under cyclic axial displacement. The reason may be that the sand under the pile tip and on the side of pile becomes dense with cyclic index. The results of single jacking pile and jacking pile groups tests demonstrated that the ultimate uplift friction resistance ft is less than the ultimate compression friction resistancefc and the measured values of ft/fc is between0.1and0.17. When a2×2pile group is subjected to compression load, to get the maximum resistance the optimal spacing of piles is4D, the corresponding group effect coefficient is1.23approximately. When the pile group is subjected to tension load, the optimal spacing of piles is between4D and5D, the corresponding group effect coefficient is1.2. The group effect can be neglected when the pile spacing is greater than7D.Through numerical analyzing results and test results, it can be conclude that the numerical model can reflect the function of three foregoing factors to bearing capacity. It can reflect the difference between the tension friction and the compression friction and the hysteresis property of the load-displacement curve. Moreover, the function of ft/fc and the elastic modulus of pile material has been calculated by the program, the group effect factors in the numerical model have been obtained by fitting the results of pile group tests.