Cut slopes failures on the Triassic beds in Coimbra, Portugal

Abstract

Several cut slope failures that occurred in the Triassic “Grés de Silves”, in the area of Areeiro in Coimbra, Portugal, are analysed. They developed along thin mudstone layers dipping at some 16º towards the face of the excavation. The failures mainly occurred after periods of intense rainfall, in some cases immediately after the excavation of the slope and in some cases have continued for the seven years since, even where the slope height was only a few metres. The conditions leading to the failures were investigated in the field and in the laboratory. It was concluded that the main factors inducing the slope failure are a conjugation of the geological structure and the reduction in the shear strength of the mudstones when the excavation results in dilation, such that fissures open allowing the ingress of water.