The paper describes a detailed investigation carried out at Abersychan near Pontypool, South Wales, to determine the stability of a valley side prior to multi-storey development. A routine investigation revealed the presence of deeply weathered and disturbed shales beneath variable drift deposits, and despite the absence of visual evidence of slip at that time, initial calculations indicated a case of marginal stability. A rigorous investigation was then authorized to examine the stability of the site in detail, and this confirmed a case approximating to limiting equilibrium. The shear strength of the shale was found to be related to its mineralogical content and to be independent of its flocculated structure, which had not been destroyed by weathering. The use of standard classification tests to describe the true nature of such a soil was found to be unreliable. The information obtained from the two investigations was used to formulate a scheme of anchored reinforced concrete diaphragm walls capable of supporting multi-storey buildings and stabilizing the slope.