Determination of post-liquefaction strength: Steady state vs residual strength

The post-liquefaction strength or the so called residual strength S_r of sandy soils determined through back-analyses of case histories reported in the literature are typically very low compared to the laboratory undrained steady state strengths (S_us) for many sands at comparable densities. This has raised concern regarding the use of S_us for post-liquefaction stability analyses. A careful analysis of S_us data for many sandy soils shows that fines content and relative density D_r play important roles affecting S_us. At the same D_r, relatively clean sands show the highest S_us. Silty sands show intermediate strengths. Sandy silts show the lowest S_us. Further, most of the soils involved in the case histories are primarily silty sands. The corresponding back-calculated S_r data are found to be similar in magnitude with S_us data for silty sands at comparable relative densities. While there may be other contributing factors, it appears that fines may have significantly contributed to the low back-calculated S_r values. Based on the laboratory S_us data and back-calculated S_r data approximate lower bound estimates are proposed for post-liquefaction strength as a function of D_r and fines content.