Axial tensile load tests on deep foundations are increasingly significant because of the expansion in their usage for a myriad of projects requiring support of tensile loads as well as for being a less costly alternative to compression load tests. A variety of interpretation criteria for compression load tests is available, which are often also used for tensile tests, but a few have been proposed specifically for tension load tests. In this study, the performance of seven of the available widely used interpretation criteria was evaluated using a data set of 80 tensile load tests. These criteria are (i) original Davisson, (ii) AASHTO, (iii) New York City building code, (iv) French building code, (v) 5% diameter, (vi) 10% diameter, and (vii) New York University (NYU) compression criteria. Their performance was evaluated for applicability, accuracy, precision, and the diameter and length effects. The AASHTO criterion exhibited the best accuracy and a moderate precision, which is promising. The study also presents efforts to identify a better performing criterion, which yields the introduction of the NYU tension criterion where capacity is defined as that corresponding to the smallest displacement of (i) half the elastic elongation of the pile plus 0.25 in.; (ii) pile head displacement corresponding to the first incidence of pull-out or strain-softening resulting in loss of more than 5% of capacity; and (iii) pile head displacement of 0.5 in., unless modified by the structural engineer of record. The NYU tension criterion achieved a similar accuracy to the AASHTO criterion and a better precision, which is optimal.

A variety of design methods for determining piles’ axial compressive load capacity are routinely employed in current practice. These range from methods specified in building codes to proprietary methods developed and employed by an assortment of engineering firms. In this study, the performance of eight commonly used design methods was evaluated using a database of 505 load tests and associated geotechnical design parameters compiled from Professor Olson’s database and that of the Iowa State Department of Transportation. The methods investigated included standard penetration test (SPT)-based methods, such as those recommended by the Federal Highway Administration (FHWA), U.S. Army Corps of Engineers, American Petroleum Institute, and Revised Lambda; and cone penetration test (CPT)-based methods such as those recommended by the Norwegian Geotechnical Institute, Imperial College, Fugro, and University of Western Australia. Pile capacities were calculated using APILE software and were compared with the measured capacities interpreted using the standard Davisson criterion and stored in the databases. The performance of all design methods was evaluated in relation to accuracy, precision, effect of soil type, diameter, and length. Both SPT and CPT methods exhibited similar accuracies, however, CPT-based design methods exhibited significantly better precision compared with SPT-based methods. All methods had shortcomings and appeared to work best under certain conditions, which are documented in this paper. The authors believe that this evaluation will permit practicing engineers and regulating bodies to better understand the efficacy of various design approaches in common use.