Pile length optimization by using shaft friction and end bearing curves developed from instrumented pile load test

Abstract

Most high-rise buildings or bridges are supported by drilled cast in-situ piles. Construction project often start with pile capacity design, followed by installation of instrumented ultimate test piles to determine pile-soil response under load. A piled foundation is typically designed with consideration of capacity and various safety factors based on the assumption that the pile will not settle beyond the allowable limit. However, many practitioners have neglected the importance of pile settlement analysis and omitted the instrumented ultimate test pile in construction programs. Lack of knowledge on pile-soil response could lead to overdesign or underdesign of the piled foundation system. To address the research gap, this paper describes the current practice where the pile capacity and optimization analysis is carried out based on SPT-N value correlated to the ultimate load test result. The optimized pile length is then analyzed using another pile optimization approach (T-z method) that applies the nonlinear pile load response curves for shaft friction (t-z curve) and end bearing (q-z curve) derived from the actual test result. Comparative results show that the computed pile head settlement is in good agreement with the measured pile load test result with approximately 86% accuracy. Further optimization can be performed on the working piles by using the T-z method. Additionally, the construction-induced variability, particularly the pile base response in optimization, is also examined. The optimization has economic and environmental benefits as fewer construction materials are needed to ensure the safe performance of the piled foundation.