Cut slopes failures on the Triassic beds in Coimbra, Portugal

An analysis is presented of several cut slope failures which occurred in the Triassic Gr,s de Silves, in the area of Areeiro in Coimbra, Portugal. They developed along thin mudstone layers dipping at some 16A degrees towards the face of the excavation. The failures mainly occurred after periods of intense rainfall, sometimes immediately after the excavation of the slope, and in some cases have continued for the 7 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 they dilate following excavation. This dilation allows the ingress of water further reducing the shear strength of the mudstones.