Load distributions and settlement performances of foundation systems with large diameter bored piles

Abstract

Foundation systems that include large diameter bored piles in excess of 2m diameter bored piles are commonly used in Hong Kong. These piles are designed to be end bearing and founded in rock, with small side resistance, and tied together by a pile cap. These foundation systems are generally used to support high-rise building structures. Previous studies had always been focused in piles with smaller diameters and in the behaviors of single piles. Load distribution and settlement performances of these foundation systems that involve superstructure, substructure, and large diameter bored piles are difficult to obtain as the scale of loads required for field data measurement are beyond any load cells could achieve. Without actual field data, load distribution within the foundation systems and performances are based on extrapolation from empirical methods and case studies. This research focuses on developing a better understanding of the load distribution and settlement performances of foundation systems with interaction of superstructure, substructure, and large diameter (>2m) end bearing bored piles. The methodology was based on utilizing field measurement to calibrate the stiffness of the pile. The stiffness then is used in 3 dimensional numerical models to carried out parametric studies of foundation systems with varying pile, soil, and substructure properties. The field study was carried out to obtain stress-strain relationship of an instrumented pile within the foundation system under the construction loads. The field data allowed the development of a bi-linear pile stiffness model to analyze the behavior of the foundation systems, which include load distribution, settlements, building tilts, and differential settlements. Results indicated that the effects of underground soil conditions have significant impact on the load distribution and settlements. For load distribution, it is found that loads are transferred to piles with high relative stiffness. Relative stiffness, k₁/k₂, is defined as the stiffness of a structure, k₁, over the stiffness of another structure, k₂, to quantified its effect on load distribution. Pile cap with higher relative stiffness distribute load evenly to piles by compressing the range of the percent of total load being distributed to piles. Soils underneath pile raft reinforce the pile raft allowing even load distribution which could reduce differential settlement in the analyzed case by up to 65%. For settlements, while the total settlement of individual piles are governed by stiffness, differential settlements for the entire foundation system is determined by the relative stiffness of individual piles. The difference in stiffness caused piles to settle at a different rate resulting differential settlement. In addition, different pile lengths created a foundation systems that consists of piles with large variance in stiffness, which contributed additional differential settlement. For total settlements, while long piles founded in sound rock had little movements at the base, elastic shorting along the pile shaft contributed settlements. Utilizing shorter piles in a weaker rock could be used to reduce elastic shortening. Increasing pile cap thickness has no benefit in improving settlement performances. For capacities, pile raft can contribute significantly to the total capacities between 20% and 70% of additional capacities. Minimum settlements of at least 10mm is required to mobilize capacities of 10% of the total capacities. Generally, foundation systems have a self regulating system to redistribute load to stiffer piles to prevent individual piles from failing. This is due to the reduction in relative stiffness of yielding pile, and any additional applied load will be transferred to stiffer piles rather than the yielding piles.

Date of Award	2014
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology