In October 1976 a 7-year old road embankment, constructed on side-long ground across a stream valley to eliminate a bend, failed after a period of exceptionally heavy rainfall. Site investigation identified slip surfaces at the weathered and softened top of the mudstone bedrock. Fluvioglacial deposits encountered beneath and up-slope of the embankment were considered the main source of the high pore-water pressures responsible for failure. The remedial measures adopted included drainage provision to prevent the future build-up of pore-water pressures beneath the reconstructed embankment. The original road, carrying the diverted traffic since the time of the slip, was supported by a temporary, rock-anchored, sheet-pile retaining wall during the excavation of slip debris and weathered bedrock.
The events leading up to the slip, the subsequent site investigations and resulting design are described, together with the construction of the remedial measures.