Peat slope failure in Ireland

Abstract

Recent peat failures in Ireland in the autumn of 2003 at Pollatomish, County Mayo and Derrybrien, County Galway have focused attention on such events. However, peat failures are not a recent phenomenon with possible evidence of peat failures in Ireland having been identified as far back as the Early Bronze Age. This paper summarises the issues surrounding peat failures in Ireland that would be of interest to an engineer\engineering geologist assessing this geohazard. The distribution of peat throughout Ireland, its formation, and its typical characteristic properties are discussed. A review of historical failures shows that there is a relationship between run out distance and failure volume and that the majority of the failures are clustered at slope angles between 4º and 8º. It seems that the risk of fatalities from peat slides is relatively low. The likely casual factors attributed to peat failures are presented using examples of failure, including the recent failures at Pollatomish and Derrybrien both of which have been investigated by the authors. Particular attention is paid to shear strength properties of peat and the applicability of traditional soil mechanics. Given the uncertainties which exist about peat strength, a cautious approach to slope stability assessment is advocated together with identification of potential causal factors to mitigate against this geohazard.