Application of SPT Rod Energy Loss to Liquefaction Evaluation of Deep Alluvium beneath an Earthfill Dam

Standard penetration test (SPT) data is typically adjusted for energy transmission effects using energy measurements at the top of the drill string. This energy ratio (ER) measured at the ground surface is assumed to also be applicable at the sampler at the bottom of the drill string. However, several studies since the 1970s have shown that there is appreciable energy attenuation along the drill string. For deep borings, the accumulated energy loss can be significant, and if ignored, may lead to incorrect conversion of the measured blow counts to energy-normalized N₆₀ values. As a result, the computed N₆₀ values may be erroneously high. This paper presents the results of a study on Stone Canyon Dam, a 225-foot high zoned earthfill dam located in Los Angeles, California. Downstream portions of the dam rest on up to 75 feet of alluvium consisting of interbedded gravely sands and clays. Liquefaction evaluation of the alluvium was based on side-by-side SPTs and cone penetration tests (CPTs) to depths of up to 200 feet. Initial SPT-based and CPT-based liquefaction analyses were found to produce drastically different cyclic resistance ratio (CRR) values for the alluvium. However, this discrepancy between the SPT- and CPT-based CRR values was largely resolved when energy loss along the SPT drill string was taken into account.