A Theoretical and Numerical Study on Effects of Overburden Stress on the Capacity of Under-Reamed Pile

Abstract

Under-reamed piles are the cast-in-situ concrete bored piles having one or two bulbs formed by enlarging the pile tip of the stem. By providing the under reamed bulbs, the bearing capacity of piles increases significantly when comparing to the one of typical piles. End bearing capacity and friction capacity are all components of bearing capacity. This research studied the influences of the overburden stress of on the bearing capacity of single pile by analyzing the influence zones to determine the maximum shear strain. Results of physical model test from the past study, ultimate bearing capacity theoretically analyzed, and results from FEM analysis were used and the contours of maximum shear strain obtained by the experiment and FEM, which defines the influence zone, were compared. The followings are found. Pile capacity ratio (PCR) of single pile increases to around 2 to 3 if 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. When the surcharge load is increased from 0 to 100 kPa on the ground surface, the PCR of single under-reamed pile becomes 2 to 9 and the PCR of multi-layered under-reamed pile is 3 to 13. If there is no surcharge, the ultimate pile capacities of multi-layered under-reamed pile determined from the experiment and FEM are close to the ones of single-layered under-reamed pile. On the other hand, increasing surcharge significantly increases the PCR of multi-layered under-reamed pile. Thus, the overburden stress significantly influences to the soil strength. The increase of the pile capacity can be explained by the increase of the area of influence zone. That is, it becomes significantly larger when the pile is under-reamed.