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Discussion |
1 School of Earth Sciences and Geography, Centre for Earth and Environmental Science Research, Kingston University, Penrhyn Road, Kingston-upon-Thames KT1 2EE, UK (a.p.dykes{at}kingston.ac.uk)
2 Department of Geography, Durham University, Science Laboratories, South Road, Durham DH1 2LE, UK
3 North Wales Trunk Road Agency, Units 91–92, Bowen Court, St. Asaph Business Park, St. Asaph LL17 0JE, UK (dougnichol{at}northwales-tra.gov.uk)
4 Gwynedd Consultancy, Environment Directorate, Gwynedd Council, Shirehall Street, Caernarfon L55 1SH, UK
5 GroundSolve Limited, Adlink House, 86 The Highway, Hawarden CH5 3DJ, UK
A.P. Dykes & J. Warburton write: Nichol et al. (2007) present an interesting case study of a small shallow landslide that occurred in North Wales in November 2005. They describe the landslide and its local context in sufficient detail to conclude that it was caused by the combination of high rainfall and subsurface hydrological controls. They also conclude that a bio-engineering approach to remediation of the affected slope is necessary to protect the highway and will provide the most appropriate solution given the environmental sensitivity of the site. Although we agree fully with the general conclusions as inferred from the information presented, we feel that it would be beneficial to provide a broader context for this study in terms of recent research into peat instability on hillslopes. The purpose of this discussion is therefore to comment on some of the details provided in the study and, in doing so, to highlight some of the problems and misconceptions often associated with the study of peat landslides.
Nichol et al. (2007) begin by stating that ‘Peatslides are relatively uncommon in the UK’ and that ‘as far as can be determined, this is the first example of a natural peatslide event to be documented for Wales’. The latter comment is probably true, as we have not yet verified the reportedly similar event in January 2005 that affected the A470 road 45–50 km south of Llyn Ogwen (Mason 2005). However, peat landslides are perhaps more common in the UK than is often realized. Of all the recorded global peat mass movements, around 20% have occurred in mainland Britain (i.e. excluding Northern Ireland) and 60% in Ireland (Northern Ireland and the Republic of Ireland combined). Over 50 separate landslides involving blanket bog are known from Northern Ireland and at least 35 in northern England (excluding bog bursts involving raised bogs). In Scotland, Acreman (1991) reported a number of peat landslides among the many that occurred in 1983 near the English border, Bowes (1960) described one on the Isle of Lewis, and in Shetland five peat slides occurred on 19 September 1990 (Veyret & Coque-Delhuille 1993) followed by more than 20 peat slides on 19 September 2003 (Dykes & Warburton 2008a). More recently, two large peat landslides occurred in the North York Moors, northern England, following severe local flooding on 19 June 2005 (Fig. 1). Our comments here are broadly divided into three topics: classification and causal factors; shear strength and stability analysis; post-failure remediation.
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Post-failure remediation. Nichol et al. (2007) identified ‘displaced rafts of peat that were left behind at the head of the slide and were considered likely to fall towards the highway in the near future’. Among the more than 20 recent peat failures that we have regularly worked on, and the more than 80 others that we have also visited and examined, we have seen no evidence of post-failure movement of peat rafts left stranded within a landslide scar during the failure event. Indeed, such movement is not only extremely unlikely, it becomes even less likely with time. The break-up of the blanket peat mass covering a hillslope after the initial displacement during the failure event necessarily releases any confined water pressures that had developed at or below the peat–mineral interface to cause the failure. Furthermore, an isolated residual peat raft has no hydrological continuity with the undisplaced peat cover, and the surrounding substrate material is unconfined and will produce overland flow if saturated. There is thus no mechanism by which excess water pressures sufficient to cause further movement could develop beneath such a peat raft. Subsequent drying, oxidation and associated shrinkage, and surface erosion of the peat raft, progressively reduce the volume and total weight of the raft on the slope, thus reducing the shear stress and preventing further movement.
Further information on this topic can be obtained from several recent detailed reviews and syntheses of peat instability research. Together, these provide a comprehensive bibliography of peat mass movements and relevant peat engineering studies (Warburton et al. 2004; Dykes & Kirk 2006; Dykes & Warburton 2007a; Evans & Warburton 2007).
D. Nichol, G.K. Doherty & M.J. Scott reply: We are grateful for the commentary by Dykes and Warburton. The additional information is of considerable interest to engineering geologists working in areas of peatland. However, some further comment on certain aspects of their discussion is necessary to complete the picture.
The ‘event in January 2005 that affected the A470 road 45–50 km south of Llyn Ogwen’ actually involved failure of a stream channel at Bwlch Oerddrws, the highest mountain pass in Wales. We therefore believe that the description of this event as a peatslide may not be strictly correct. Doherty & Scott (2005) described the event and the associated remedial works.
The gravel subsoil at Llyn Ogwen does not generally exhibit high permeability as described by Dykes and Warburton. These upland drift deposits typically feature low permeability and so heavy rainfall is likely to generate only modest increases in porewater pressures. This low permeability is demonstrated by the slow infiltration rates, rapid runoff conditions and sudden increase in the number and size of streams when it rains heavily in this area.
The attempt by Dykes and Warburton to reclassify the peatslide at Llyn Ogwen is helpful. However, in this instance the natural pipe lies within the lowermost part of the peat profile, implying a failure of the peat rather than the mineral substrate. The development of artesian pressures within blocked soil pipes is an easily understood concept, and Pierson (1983) discussed the mechanism of failure in terms of the Mohr–Coulomb infinite slope theory.
The accuracy of the shear vane test in peat is clearly contentious. However, in the right hands, this simple field procedure proves exceedingly useful and should not be dismissed lightly. This was demonstrated by Radforth (1969, pp. 129–136), who assessed the merits of the test by review of vane shear testing at a series of engineering projects involving peat. He concluded that the test was perfectly acceptable in 15 out of 16 cases and for most peat types.
Dykes and Warburton imply that loose blocks of peat resting on steep slopes above a sensitive receptor do not need to be removed. However, notwithstanding their experience (which may be more or less representative of that of others), this is an issue that needs to be taken in the context of not just the potential hazards but also the potential risks. In this case, we concluded that the possibility of reactivation as a result of further heavy rainfall could not be dismissed; indeed, at Llyn Ogwen subsequent rainstorm activity did shift several isolated blocks. The risks when the slide is situated so close to a busy and important highway were thus deemed unacceptable, and removal of the blocks was specified in the interests of public safety. The responsibilities placed on highway or road engineers working for a public authority are onerous in respect of public safety, and this might not be fully appreciated by those not working in such an environment.
A.P. Dykes1, J. Warburton2, D. Nichol3, G.K. Doherty4& M.J. Scott5
1School of Earth Sciences and Geography, Centre for Earth and Environmental Science Research, Kingston University, Penrhyn Road, Kingston-upon-Thames KT1 2EE, UK (e-mail: a.p.dykes@kingston.ac.uk)
2Department of Geography, Durham University, Science Laboratories, South Road, Durham DH1 2LE, UK
3North Wales Trunk Road Agency, Units 91–92, Bowen Court, St. Asaph Business Park, St. Asaph LL17 0JE, UK (e-mail: dougnichol@northwales-tra.gov.uk)
4Gwynedd Consultancy, Environment Directorate, Gwynedd Council, Shirehall Street, Caernarfon L55 1SH, UK
5GroundSolve Limited, Adlink House, 86 The Highway, Hawarden CH5 3DJ, UK
Received for publication 5 September 2007. Accepted for publication 5 November 2007.
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