Botanical and geotechnical characteristics of blanket peat at three Irish bogflows

Abstract

Systematic investigations of instability and failure of peat covered hillslopes began in the late 1990s and quickly identified the potential importance of botanical controls on the properties and behaviour of the blanket peat involved in the failures. However, attempts to unravel some of these controls did not begin for several years. During 2010-12 investigations of the blanket peat at three relatively recent bogflows in northwest Ireland were done with the aim of establishing some form of relationship between botanical or paleoecological characteristics and standard physical and geotechnical properties, assuming the latter to be meaningful but recognising that this may not be the case. In-situ measurements and investigations at all three sites were followed by extensive laboratory characterisation of small core, block and monolith samples. The botanical composition of the peat could not be fully determined due to the very high degree of decomposition. However, analysis of macrofossils allowed distinct depth-related patterns of several key botanical indicators to be determined. In particular the monocotyledon fragments, dominated by Eriophorum vaginatum, showed distinct and potentially useful distributions throughout the peat profiles. Overall results showed that the basal peat at one of the sites was discernibly different from the other two sites having fewer monocotyledons, fewer fibres, higher dry bulk density and higher saturated hydraulic conductivity. This approach therefore offers a potential basis for developing a means of assessing peat mass characteristics from small auger samples. Key words Fabric/structure of soils, Landslides, Strength & testing of materials Notation F fibre content Fm fine fibre fraction of the peat (fragment of plant tissue 0.15-1.00 mm in any dimension) Rm coarse fibre fraction of the peat (fragment of plant tissue > 1 mm in any dimension) Ft total fibre fraction of the peat (all fibres > 0.15 mm in any dimension) Fh humus fraction of the peat (all particles < 0.15 mm in any dimension) FVS field vane strength MFt mass of the total fibre fraction (g) MFh mass of the humus fraction (g) Ms mass of oven-dried (105°C) peat sample-mass of ash (g)

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

“…The depth relationships between some parameters that have not previously been found so clearly within a single peat profile (or indeed at a single site) enhances confidence in the consistency of results from the different approaches to the determination of the strength of the peat. In particular, the values of the measured properties are entirely in line with those obtained from three large bogflows in northwestern Ireland that were used for the first systematic investigation of botanical controls on peat properties (Table 4 in Foteu Madio and Dykes 2018).…”

“…The peat was dominated by monocotyledon plants, particularly Eriophorum vaginatum and other sedges, but with other plants common to the present living vegetation cover (heathers -Ericacae, including Calluna vulgaris, and mosses including some Sphagnum) also represented. The results from the first phase of the Croaghan study suggest that the peat properties here most likely result from the same botanical composition as established by Foteu Madio and Dykes (2018). The limitations of these Croaghan results for more detailed analysis arise from the lack of replication of the peat profile investigation.…”

“…Foteu Madio and Dykes (2018) found that the blanket bogs at three entirely separate uplands comprised essentially the same assemblages of plant species, as determined by analyses of macrofossil contents. The peat was dominated by monocotyledon plants, particularly Eriophorum vaginatum and other sedges, but with other plants common to the present living vegetation cover (heathers -Ericacae, including Calluna vulgaris, and mosses including some Sphagnum) also represented.…”

“…Foteu Madio and Dykes (2018) also showed a basal layer much weaker than overlying peat, around 150 mm thick, characterised by distinctly fewer fibres (particularly coarse fibres). Fig.…”

“…A secondary hypothesis is that the peat characteristics at the study site in Co. Antrim are consistent with the upland blanket peat in NW Ireland (Counties Sligo, Leitrim and Cavan), indicating broadly the same botanical composition (c.f. Foteu Madio and Dykes 2018). The landslide can then be explained in terms of peat characteristics (i.e.…”