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

Abstract

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 directly compare estimates of peat shear strength from Direct Simple Shear (DSS) and tensile strength (TS) 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 southwest, so the composition of the peat at the study site is inferred to be dominated by sedges (particularly Eriophorum vaginatum) with some heathers (Ericacae) 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. Key wordsFabric/structure of soils, Landslides, Strength & testing of materials Notation F factor of safety Ksat saturated hydraulic conductivity p'c preconsolidation stress su undrained shear strength σ'v consolidation stress τh-max peak horizontal shear stress φ friction angle

Landslide investigations during pandemic restrictions: initial assessment of recent peat landslides in Ireland

“…Table 1 presents data obtained from lower catotelm peat, i.e. the assumed failure zone, at five bogflows and three bog slides plus a recent ‘peat slide’ in Co. Antrim (Dykes 2019 ). It shows that the characteristics of lower catotelm peat that may relate to in situ strength seem to be broadly consistent throughout the island of Ireland (Dykes 2019 ), and are probably representative of the peat at all of the three recent landslides.…”

“…The thickness of the peat is not known but measurements at other failure sites (e.g. Dykes and Jennings 2011 ; Dykes 2019 ) suggest that the base of the peat is likely to be up to around 3 m deep, and possibly more, across much of the affected area at BBM-20. Based on the video of Flynn ( 2020 ) and the (pre-failure) GE view of the site, the source area is around 450 m long and up to 250 m wide, with a minimum volume of 200,000 m 3 if the peat is slightly less than 3 m deep throughout.…”

“… a A = Yang and Dykes ( 2006 ), B = Dykes ( 2008c ), C = Dykes et al ( 2008 ), D = Foteu Madio and Dykes ( 2018 ), E = Dykes ( 2019 ) b Data from a peat slide in Co. Antrim …”

MPeat2D – A fully coupled mechanical-ecohydrological model of peatland development in two dimensions

“…The landscape variations, together with the feedback from mechanical, ecological, and hydrological processes, affect the stresses on the peat body that control the occurrence of failure conditions on the peatland. The peatland failure involving mass movement (Dykes and Selkirk-Bell, 2010;Dykes, 2022;Dykes, 2008) influences the estimation of carbon accumulation on the peatland because it might result in the formation of water channels that facilitate the drainage and oxidation processes (Warburton et al, 2003;Evans and Warburton, 2007). Potentially, this phenomenon could determine the maximum limit to peatland carbon accumulation in a landscape (Large et al, 2021).…”

Editorial

“…Differences in botanical constituents for various peat materials may partly explain differences in their geotechnical properties, although there is a dearth of reported work on this aspect. The next two papers, by Dykes (2022) and Foteu Madio and Dykes (2022), investigate botanical composition and fibre affects towards gaining improved understanding of instability and failure in upland (blanket) bogs. Here there is heightened interest arising from the dual concerns of (a) landslide hazards associated with more likely high-intensity rainfall events and (b) environmental impacts of peat failures arising from access road or track construction for wind farms or other purposes on peatlands.…”