Modeling debris slide geometry with balanced cross sections: A rigorous field test

Cross sections of slopes subjected to progressive displacements can be constructed from arrays of deeply embedded survey poles aligned in the directions of slip. Changes in the spacing and plunge angles of the poles can be used to construct vertical sections that show positions of slip surfaces and the changing shapes and/or positions of stratigraphic boundaries by applying the principles of cross-section balancing. Balanced models of such progressive failures can be used as the basis for determining field locations, data needs, and mitigation strategies associated with more rigorous geotechnical investigations. A slow debris slide in the coastal bluffs of Lake Michigan has been monitored bi-weekly to tri-weekly beginning in 1996 with balanced cross sections constructed annually. When the debris slide was drilled in 47 different places in 2003 as part of a bluff de-watering study, surfaces of rupture and stratigraphic contacts observed in drill cores matched with strong correlation the predicted depth positions of slip surfaces from balanced section models. Without access to the balanced geometric models of the debris slide as guides to drilling sites and geotechnical analyses of soils, the drilling program would not have produced the level of knowledge necessary for a well planned mitigation strategy.