PROPERTY CHANGES OF SOIL AROUND A ROTARY PRESS-IN PILE WITH WINGS AND LATERAL LOADING BEHAVIOR OF THE PILE

Abstract

 A rotary press-in pile with wings has advantages to get large vertical bearing capacity for its pile diameter and to reduce the amount of soil excavated for its installation. On the other hand, it has been concerned that the rotary pile might cause weakening of the soil around the rotary pile. Even in the case of clay, it has not been confirmed that such weakening of the surrounding soil would be caused by peripheral soil displacement during pile installation and/or the passage of pile wings. In this paper, a series of experiments was performed to grasp the behavior of a rotary press-in pile horizontally loaded, and the property changes of surrounding soil by installing a rotary press-in pile with wings. In the experiments, the execution of pile installation was managed with constantly maintaining the ratio of s/p in the range of 1.0±0.1, where s is the penetration length of pile rotated by one revolution and p is the helical pitch of its pile wing.
 The experiments were carried out at two different sites, where one was the site depositing an alluvium clay layer and the other with a diluvial loam layer. In the experiments, in order to investigate the properties of soil around a pile, the measurements of bulk density and water content were conducted for soil samples taken from the vicinity of the pile shaft with specially equipped apparatus. And also, the one dimensional consolidation test, and the tri-axial compression test were performed for samples taken in the vicinity of the pile were performed. The lateral loading tests, apparatus of one of which was directly equipped with pile shaft and the other of which was normal test, were carried out.
 In order to compare the differences in the pile installation methods, the laterally loading tests for the pile installed by rotation method and the buried pile were carried out.

 As a result of this study, the conclusions are as follows.
 1) In the soil around a rotary press-in pile with wings, density increase and stiffness decrease were caused by soil displacement and wing passage.
 2) The range where soil density increases depends on the compressibility of soil.
 3) The stiffness decrease of the soil in the vicinity of pile is more strongly affected by soil displacement than by wing passage.
 4) The installation method of the rotary pile constantly maintaining s/p can provide appropriate installation without the occurrence of void around the wing although there seemed the gap between the pile shaft and the ground in any case.
 5) The coefficient of horizontal subgrade reaction for the rotary pile with wings became 0.54-0.74 times as large as that for the buried pile due to the gap between pile shaft and ground, and the stiffness decrease of the soil in the vicinity of the pile.