Peak Undrained Resistance of Loose Sands

The peak undrained resistance (σ_d,upeak) of a loose sand during liquefaction under monotonic loading does not necessarily occur on or near the Mohr Coulomb failure envelope. Instead, it occurs as that combination or product of a decreasing effective confining pressure (${\bar{\sigma }}_3$) and an increasing stress level (SL). The condition in which this product is a maximum occurs at a particular point on a plot of the deviator stress (σ_d) versus volumetric strain (ε_v) as assessed from a drained triaxial test. A method is presented whereby a single drained test with volume change measurement can be used to assess this undrained peak. This is a refinement of a more complete effective stress method of liquefaction analysis that requires only drained triaxial testing.