Recent large-scale slides occurring during the hydraulic placement of an artificial island berm in the Beaufort Sea resulted from the liquefaction of the berm sand. Subsequent laboratory tests and back analyses have led to advancements in the understanding of the liquefaction potential of sand. Analyses of undrained triaxial tests, undertaken to measure steady state parameters, suggest that there is a "collapse surface" in three-dimensional void ratio – shear stress – normal stress space. A necessary condition for liquefaction is that the soil state lie on this surface. This collapse surface concept is fundamentally an extension of the steady state concepts proposed by others, and in many respects follows the principles of critical state soil mechanics. Replotted published tests support the concept. Parameters used to describe the position of the surface are termed collapse parameters. These can be converted into parameters analogous to Mohr–Coulomb failure parameters and can therefore be used in conventional limit equilibrium stability analyses. Utilizing these parameters overcomes limitations inherent in previously proposed undrained steady state analysis methods. These concepts also provide a basis for a rational explanation of the Beaufort Sea hydraulic fill slides. Key words: liquefaction, sand, hydraulic fill, slope stability, steady state testing.