Available at: https://digitalcommons.calpoly.edu/theses/3434
Date of Award
10-2022
Degree Name
MS in Civil and Environmental Engineering
Department/Program
Civil and Environmental Engineering
College
College of Engineering
Advisor
Robb Moss
Advisor Department
Civil and Environmental Engineering
Advisor College
College of Engineering
Abstract
Current state of practice for calculating liquefaction triggering and deformations is outlined in multiple published articles (Seed 1971, Youd et al. 2001, Idriss and Boulanger 2008, National Academies of Sciences 2016, etc.). These methods use a factor of safety analysis with the cyclic stress ratio (the cyclic stress caused by the earthquake normalized by the effective vertical stress in the soil) and the cyclic resistance ratio (the resistance of the soil normalized by the effective vertical stress in the soil). These cyclic ratios are calculated first for free-field sites (no external forces on the soil), then correction factors are used to account for external forces. Specifically, the Ka factor accounts for static (or driving) shear stresses caused by additional force at the surface through slopes or buildings. The current literature that addresses driving shear stresses in potentially liquefiable material estimates Ka and quantifies the driving shear stress using the initial static shear stress ratio ( where ts is static shear stress, and s’v is vertical effective stress, where a = 0 for free-field sites).
The current standard of practice suggests that Ka should be neglected due to the lack of consensus on its use. This study explores the research and methods in current liquefaction analysis pertaining to driving shear stresses and how these influence the probability of liquefaction. This study compares three direct simple shear testing programs by exploring the testing parameters’ impact on the results. The testing parameters between the three comparison studies are normalized, and results at different shear strain values are compared. The results show differing trends within the data; however, the magnitude of cyclic shear stress ratios varies between the three. The overall consensus from this study is that there should likely be a Ka < 1 applied to the cyclic resistance ratio for low relative density soils and a Ka > 1 for higher. However, the specific thresholds of relative densities (and other parameters) warrant further investigation. This comparison study leads to suggested further research, including more load-controlled direct simple shear tests and further testing on a range of relative densities.