Universal Criterion for Interpreting Capacity from Load Tests on Piles

Abstract

Interpretation of field load tests on piles has many important practical and financial implications in foundation engineering practice. However, identifying the ultimate capacity or resistance on a continuous load settlement curve remains elusive. The geotechnical engineering literature abounds with over 40 criteria to aid in interpreting the ultimate capacity from static load tests on deep foundations. These include settlement, settlement rate, offset, and creep criteria, among others. Since the 1990s criteria proposed by Davisson, AASHTO, and New York City Building Code as well as the 5% and 10% of diameter criteria have evolved to become the most routinely specified for all foundations, without assessing their performance or what they have been intended for. This study provides a side-by-side comparison of the performance of these methods in relation to usability, accuracy, precision, and length and diameter effects. In addition, the performance is also compared with the recently developed NYU criterion where the capacity is taken as the smallest of (i) a settlement corresponding to the elastic compression plus 0.75 in.; (ii) the capacity at plunging or strain-softening; or (iii) settlement corresponding to 5% of the pile diameter, unless the settlement threshold is modified by the structural engineer of record. Assessment is made possible using a database of 350 load tests conducted on square and round concrete piles, open and closed steel pipe piles, as well as H-piles. Comparison suggests that the NYU criterion is the most versatile, precise, and accurate among the examined methods.