The liquid limit of peat and its application to the understanding of Irish blanket bog failures

Abstract

Catastrophic failures of blanket bogs, involving the escape and outflow of large volumes of semi-liquid basal peat, are well-known phenomena in Ireland but have only very rarely been reported from elsewhere in the world. Their precise causes and mechanisms are as yet unclear. The liquid limit (w L ) was identified as a potentially useful indicator of the susceptibility of peat to such failure because peat has extremely high natural water contents and, as an index property, w L takes no account of the properties or structures of highly heterogeneous intact peat. However, the usual procedure for determining the w L of peat is not fully standardised. Prepared samples will normally include potentially highly reactive particles of disrupted fibres and wood fragments that would not be present in such freshly disintegrated form in the field. This paper presents results from w L determinations of peat obtained from the scar margins of three bog failures in northwest Ireland, using four different test procedures including a method involving wet-sieving of the peat to separate the humified <425-μm fraction for testing without incorporating artificially fragmented particles of fibres. The sampled peat was classified as H 8 -H 10 according to the von Post humification scale. The fibre contents varied between the sites, but the ash contents were <3% in all but one test sample, and bulk densities (dry and field-wet) of the peat from all three sites were almost identical. w L results from the wet-sieving method were 708-785%, compared with 633-980% from the standard method. The highest measured field water contents exceeded the wet-sieved w L for all three of the field sites. Tests of cone penetration into intact peat cores demonstrated the influence of the reinforcing effect of in situ fibres. The results strongly suggest the need to adopt a fully standardised procedure for determining the w L of peat. Additional shear vane measurements of intact and remoulded peat from a bog failure in Northern Ireland indicated a very high 'strength sensitivity'. This leads to the suggestion that a slight disturbance of basal peat can lead to a loss of strength that rapidly propagates as local stresses change and cause further remoulding as water contents exceed w L .

Geomorphological maps of Irish peat landslides created using hand-held GPS

“…up to 3 m above the failure surface adjacent to the east side of the channel, indicating that the failure gained a very large momentum within just a few metres of downslope movement. The failure dating from around 1997 (southeast) is described by Yang and Dykes (2006), is described by Yang and Dykes (2006), and the plan is reproduced from that paper.…”

“…An account of the failure is being prepared for publication elsewhere. Yang and Dykes (2006) presented a plan of this bogflow that can be compared with the vertical aerial image in Figure 2, as well as a description and interpretation of the failure.…”

“…Slieve Anierin, c.1998 (SAR-98) This was one of the first surveys to be undertaken using this GPS method, and dense fog limited visibility to as little as 30 m at times. The bogflow was first described and interpreted, and its geomorphological map presented, by Yang and Dykes (2006).…”

Use of fall cones to determine Atterberg limits: a review

“…testing of fine-grained soil in its natural condition or of the homogeneous soil paste produced using wet (preferred) or dry sample preparation techniques), and the chemistry and pH of any water added to the soil sample in preparing the soil paste for testing (Jang & Santamarina, 2016), can also influence the deduced values of LL and PL. For instance, the LL and PL values measured for peats and other highly organic soils are invariably strongly dependent on these factors (Hanrahan et al, 1967;Hobbs, 1986;Yang & Dykes, 2006;Asadi et al, 2011;O'Kelly, 2015). In the case of fibrous peat material, preloading (which gives the organic solids some stress history because of their compressible nature) produces lower LL values (O'Kelly, 2015).…”

An Investigation of Two Peat Slope Failures in the Wicklow Mountains

“…Quantitative analysis of the geotechnical properties of the slide material is limited to a small handful of the reported slides that have occurred in recent decades (i.e. Alexander et al (1986);Carling (1986); Long and Jennings (2006); Yang and Dykes (2006)). However, the geotechnical tests used in some of these studies were primarily developed for mineral soils and may not be applicable in peat soils which have significant microstructural and structural differences (Boylan et al, 2008).…”