Initiation of a multiple peat slide on Cuilcagh Mountain, Northern Ireland

In October 1998 a multiple peat slide occurred on the northern slopes of Cu ilcagh Mountain, Co. Fermanagh, in response to a high-magnitude rainfall ev ent. Few peat slides have been recorded in Ireland, and a detailed field su rvey and investigation of the failure was undertaken within four weeks of t he event. The morphological evidence indicated a distinct sequence of event s which appeared to begin with the failure of a small segment of slope abov e a degraded transverse drainage ditch which was cut less than ten years pr eviously. This segment of slope was no more than 42 m wide and 25 m long, w ith 0.7 m of peat overlying up to 0.5 m of a pale coloured clay, the latter containing small pipes and resting on the surface of a darker coloured loa my material. The failure surface was located at or near the base of the pal e clay layer. Finite element software was used to model hydrological condit ions within the upper segment of slope and to calculate factors of safety f or different slope configurations including the presence or absence of a dr ain or a subsurface pipe. Using the peak shear strength of the pale clay, a s determined in the laboratory, both the drain and subsurface pipes were re quired to obtain a factor of safety of 1.0 or less. Allowing for the uncert ainties associated with the hydrological modelling of the pipes, it is sugg ested that the cutting of the drain and the hydrological impacts of its sub sequent degradation are ultimately responsible for the failure. Copyright ( C) 2001 John Wiley & Sons, Ltd.