Impacts of Pile and Bulb Spacings on the Under-Reamed Pile Group’s Capacity – A Physical Model Study

Abstract

Under-reamed piles are cast-in-situ concrete bored piles having one or two bulbs formed by enlarging the zone around the pile tip. By the under-reamed bulbs, the bearing capacity of pile increases significantly comparing to the one of typical pile. This research studied the influences of the vertical spacing of the bulbs and the horizontal spacing of the piles in a pile group on the bearing capacity of single pile and pile group by performing a series of physical model tests with a scaling factor of 1:50. A clean sand was used to model the soil strata in the field. A digital image correlation (DIC) technique was used to determine the contour of maximum shear strain, which was subsequently analyzed for the area of the influence zone. From the results of this study, the followings are found. The capacity of single pile increases more than three times when the pile is under-reamed because of the increase of the bearing area. Increasing of number of bulbs does not significantly increase the capacity of single pile and pile group. Increasing of horizontal spacing between the piles results in an increase in pile group capacity. The behavior in that the pile and pile group capacities increase is consistent with the increase of the area of the influence zone. That is, the influence zone area becomes significantly bigger when the pile is under-reamed. For pile groups, with increasing the horizontal spacing, the intersected influence zone area becomes less, while the unioned one becomes more. Thus, the capacity of pile and pile group can be explained by the influence zone described in this study.