2. Liquefaction-Induced Flow Failure of Gentle Slopes of Fines—Containing Loose Sands by Case Histories and Laboratory Tests

In two unprecedented and very similar case histories during recent earthquakes in Hokkaido, liquefied sand strangely flowed underground in very gentle man-made fill slopes of a few percent, leaving large ground depression behind. In both of them, a large amount of non-plastic fines was involved in fine sands that presumably destined the sand highly contractive and flowable on the contractive side of SSL. This particular sand has been tested in undrained triaxial tests, demonstrating that the sand of original grading (F_c ≈ 35%) is far more contractive with strain-softening behavior in the post-peak stage and easier to flow than the sand deprived of fines in the same relative density. This strongly suggests that the high content of non-plastic fines is the major cause of the flow failure observed in Hokkaido. A series of torsional simple shear undrained cyclic tests on contractive sands with fines were addressed to examine liquefaction behavior under different initial shear stresses indicating that the flow failure was confirmed to initiate even in a low gradient slope when the effective stress path comes across a straight line from the origin on the \(\tau \sim \sigma_c^{\prime}\) diagram uniquely defined irrespective of stress paths.