Improved CPT-based Liquefaction Assessments with the I_B Parameter: A Case Study in the San Francisco Bay Area

Abstract

Traditional liquefaction triggering evaluations using cone penetration test (CPT) data are generally conducted with simplified methods that employ the Soil Behavior Type Index (I_c) to differentiate between "sand-like" and "clay-like" materials. However, we observe that the commonly used I_c = 2.60 cutoff often misclassifies materials, particularly in cases where intermediate soils exhibit high friction ratios (F_r > 2-3%). This misclassification can result in overpredictions of liquefaction potential. To address these limitations, we propose using the Modified Soil Behavior Type Index (I_B) as an alternative for selecting "sand-like" materials for liquefaction evaluations. By applying a cutoff of I_B = 28 to paired CPT and laboratory classification data from sites in the San Francisco Bay Area, we achieve significantly improved material classifications, particularly for soils with high F_r values. Our findings suggest that the I_B parameter offers a more consistent and accurate material classification approach, ultimately enhancing the reliability of CPT-based liquefaction assessments.