Assessment of Several Nominal Resistance Interpretation Criteria for Drilled Foundations

Abstract

Agreeing on the nominal resistance (aka. capacity) derived from any load-settlement curve requires the use of an interpretation criterion. Few of the currently available criteria have been originally designed for drilled foundations despite the increasing use of such piles for supporting infrastructure projects. The performance of 16 interpretation criteria used in current geotechnical practice was assessed using a database of 194 load tests conducted on drilled shafts. Their performance was evaluated in terms of: (1) applicability; (2) correlation among each other; and (3) the effect of drilled shaft diameter, length, and soil type. Eight of the 16 methods could not be reliably used. Capacities interpreted from the remaining eight were consistent but resulted in excessive settlement in a few cases. Performance was also evaluated in terms of serviceability. A new criterion is proposed, where the nominal resistance is defined as the load corresponding to the smallest of (1) a settlement equal to the elastic compression of a free-standing column plus 0.75 in. (20 mm); (2) the load at plunging or strain-softening; or (3) settlement corresponding to 5% of the pile diameter, unless modified by the structural engineer of record. The proposed method showed good correlation with several established criteria while including a built-in serviceability safeguard against excessive settlement.