Unprecedented liquefaction flow of gentle slopes containing non-plastic fines

In two recent earthquakes in Hokkaido, Japan, liquefied sand unusually flowed underground in very gentle man-made fill slopes, creating large ground surface depressions. In both cases, a significant amount of non-plastic fines was present in the loose fine sands. When tested in undrained triaxial tests, these sands were found far more contractive and easier to flow than the sand of the same density without fines. This strongly suggested that the high fines content was the major cause of the strange flow failures because it made the sand flowable on the contractive side of the steady-state line. Another series of cyclic torsional simple shear tests on contractive sands with fines under initial shear stress indicated that the flow failure can initiate even in a gentle slope. This is when the effective stress path comes across a straight yield line drawn from the origin on the τ − σ _c′ diagram uniquely defined irrespective of stress paths. A scenario to replicate the unusual flow failures was developed based on the field study and test results.