Static Load Induced Liquefaction, Steels Corners Road Embankment Failure

A simple, roadway widening became significantly more expensive when geologic conditions conspired to complicate the project. The widening was to be accomplished by the construction of a 25-foot wide and 15-foot high embankment. The embankment was located adjacent to a “wetland” area and pond. Fill placement begin and a height of 7 feet was reached when operations ceased for the week. Work began two days later. A 400-foot long scarp formed and the fill “moved” nearly 4 feet vertically and 1-foot laterally. An initial suspicion of settlement due to compression of peat was dismissed when no organic deposits were found beneath the embankment. The exploration encountered glacial outwash deposits consisting of alternating layers of thick, relatively loose, hydrostatically-charged sand confined between thin, dense clay strata. Consolidation settlement analysis could not account for the magnitude of vertical or the lateral component of the movement. We concluded that the embankment construction caused the liquefaction of the sand layers beneath the fill; effectively reducing the shear strength to zero. Slope stability analysis confirmed the only means of producing the failure was to introduce positive pore pressure into the sand deposits. The resulting shallow, circular failure surface was a near-perfect match of the head scarp and toe bulge observed in the wetland at the edge of the pond. To increase pore pressure, three criteria must be met; the availability of water with sufficient head to completely saturate and “charge” the strata, the prevention of free drainage, and the increase of soil strain through dynamic or static loading. The buried glacial valley, the presence of the ‘kettle’ lake, and rapid construction of the embankment fulfilled these criteria