Preliminary loess undrained shear strength resistance factors for use in load and resistance factor design


As of October 1, 2007, State Departments of Transportation are required to design substructures of bridges including shallow and deep foundations, earth retaining structures, and buried structures using the AASHTO Load and Resistance Factor Design (LRFD) methodology. Full implementation of LRFD for design of slopes is currently not possible because there is a need to establish a baseline of the variability and uncertainty of soil properties in a given geographical region. Therefore, the primary objective of the study described herein was to begin the process of collectively analyzing soil data of loess in the Memphis area. A second objective was to provide geotechnical engineering practitioners with a general overview of LRFD in design of soil slopes. The two methods available to determine soil strength resistance factors, which are required for LRFD, consist of calibration based on current Allowable Stress Design (ASD) factors of safety and calibration based on probabilistic analysis. An overview of these two methods is provided. Although calibration of resistance factors to ASD yields similar designs as current ASD methods, this approach does not provide consistent levels of safety and reliability, which is the ultimate goal of LRFD. Therefore, this study focused on obtaining soil strength resistance factors for loess based on probabilistic analysis. Soil resistance factors back-calculated from ASD are higher than those where the calibration factors are determined probabilistically because the resistance factors obtained by matching to ASD procedures may include an inherent bias based on level of site investigation, quality of sampling and testing, interpretation of test results, and judgment of the designer, that is not included in the resistance factors obtained using probabilistic calibration methods. Consequently, probabilistic calibrated resistance factors may yield more conservative designs until further study can be performed to quantify this bias.

Publication Title

41st Symposium on Engineering Geology and Geotechnical Engineering 2008

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