Residual Strength of Liquefied Sand: Laboratory vs. Field Measurements

Determining the residual strength of liquefied sand is essential for estimating post-earthquake stability of vulnerable earth structures, or calculating runout of liquefaction flow slides. Current practice is to select values from a database of back-calculated residual strengths from failure case histories, which have been related to representative penetration test resistance numbers in the failed materials. Given the uncertainties involved, it is desirable to compare the field data with laboratory tests under controlled conditions. This paper describes residual strength measurements for a uniform fine sand using two recently-developed tests designed to impose large strains and strain rates: a modified triaxial test in which a metal coupon is dragged through the liquefied sample by an external dead weight, and a ring shear device which can impose constant rates of strain on the liquefied sand. In all cases, a stress-thinning behavior is observed; however, coupon movement through the liquefied sand is basically laminar, representing conditions in the interior of a flowing mass, while the rotating ring creates a well-defined contact shear band and higher resistance, which might be considered more representative of flow at the base of a sliding mass. Comparison with back-calculated field values shows that coupon residual strengths plot at the lower bound, and ring shear results at the upper bound, of backcalculated field values