The shear strength of liquefied soil, su(LIQ), mobilized during a liquefaction flow failure is normalized with respect to the vertical effective stress (σ 'vo) prior to failure to evaluate the liquefied strength ratio, su(LIQ)/σ 'vo. Liquefied strength ratios mobilized during 33 cases of liquefaction flow failure are estimated using a procedure developed to directly back-analyze the liquefied strength ratio. In ten cases, sufficient data regarding the flow slide are available to incorporate the kinetics, i.e., momentum, of failure in the back-analysis. Using liquefied strength ratios back-calculated from case histories, relationships between liquefied strength ratio and normalized standard penetration test blowcount and cone penetration test tip resistance are proposed. These relationships indicate approximately linear correlations between liquefied strength ratio and penetration resistance up to values of qc1and (N1)60of 6.5 MPa and 12 blows/ft (i.e., blows/0.3 m), respectively.Key words: liquefaction, flow failure, liquefied shear strength, stability analysis, kinetics, penetration resistance.