Peat slope failure in Ireland

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.