Reassessment of Drilled Shafts in Alluvial Soils Using LRFD and ASD: The Motilones Bridge Case Study

Abstract

This paper presents a retrospective comparative analysis of Load and Resistance Factor Design (LRFD) and Allowable Stress Design (ASD) methodologies for drilled shaft foundations in Amazonian alluvial soils. Using the Motilones Bridge in Peru as a case study, originally designed following LRFD specifications, the foundation is reassessed with contemporary approaches including both updated LRFD standards and ASD methodology. This retrospective perspective allows comparison of design methodologies not considered together during the original design phase. The analysis reveals systematic differences between design approaches, with ASD consistently producing more conservative results than LRFD methodologies. The study also identifies significant variability in results from different software implementations of the same design methodology, highlighting the importance of software verification in geotechnical practice. Additionally, the evolution from earlier to more recent LRFD specifications demonstrates how design codes incorporate improved understanding of geotechnical uncertainty. All methodological approaches confirm the adequacy of the original foundation design, while providing insights into how methodological choices influence design outcomes. The study demonstrates the value of retrospective analysis for understanding methodological evolution and offers practical guidance for foundation design in similar tropical alluvial environments. Recommendations include explicit documentation of methodological choices, multi-software verification for critical designs, and consideration of methodological implications when transitioning between design approaches.