Slope Failures in Tertiary Expansive OC Clays

This paper deals with slope stability in a oddly behaved, expansive, overconsolidated, stiff Tertiary clay of São Paulo, Brazil. The properties were not fully recognized until a recent major failure occurred during excavation on a 26-m-high, 250-m-wide slope constructed for the new Carvalho Pinto Motorway. A comprehensive site investigation was carried out and involved laboratory and in situ tests and successfully employed the Marchetti dilatometer. Back analyses of failures showed two different mechanisms, one leading to shallow failure in tertiary clays owing to expansion, followed by surface degradation or slaking. The other mechanism was lack of shear strength in the clay to resist stresses imposed by a high and steep slope. This paper gives site investigation data, discusses the causes, and back analyses of failures, and presents the final slope design.