Recent peat slide in Co. Antrim extends the known range of weak basal peat across Ireland

Peat instability and failure became an important topic of study following a series of damaging landslides in 2003. A peat slide in August 2014 at Croaghan, in the Orra Mountains of Co. Antrim, Northern Ireland, provided the first opportunity to compare directly estimates of peat shear strength from direct simple shear and tensile strength testing. Physical properties of peat from throughout the depth profile were obtained to provide reference characteristics for the strength test results and to facilitate comparisons with results from locations where detailed botanical compositions have been analysed. Both methods showed that the basal peat at the study site had a shear strength much lower than 6 kPa. The physical properties are the same as at sites 150 km further south-west, so the composition of the peat at the study site is inferred to be dominated by sedges (particularly Eriophorum vaginatum) with some heathers (Ericaceae) and mosses (including Sphagnum). The weak basal layer appears to be similarly lacking fibres, which allowed failure to occur in response to adverse water pressures arising from extreme rainfall. Basal shear strength of <5 kPa is a reasonable starting assumption for planned engineering works on intact blanket peat in Ireland.