Peat Slides Research Articles

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.

Geomorphic impact and system recovery following an extreme flood in an upland stream: Thinhope Burn, northern England, UK

Quantitative assessments of the impacts of extreme floods on channel morphology are rare. Real Time Kinematic (RTK) GPS surveys of a 500-m reach of the Thinhope Burn, an upland gravel-bed stream in the UK, taken in 2003 and 2004 permitted an assessment of geomorphic work whilst the channel was at steady-state. A large flood that occurred on 17 July 2007 resulted in a catastrophic impact to the Thinhope Burn valley floor. The reach was re-surveyed after the event in 2007, and again in 2008 and in 2011. Digital elevation models were produced from the survey data, which allowed the spatial patterns of erosion and deposition and volumetric changes between surveys to be established. A total of 5202 m 3 of deposition and 2125 m 3 of erosion was recorded in the reach following the flood event. Field walking of the catchment and comparison of aerial photographs for 2003 and 2007 suggested that most of the material mobilised had originated from existing sediment held in terraces and paleoberms on the valley floor. Although slope failures were evident, including peat slides in the headwaters, delivery of sediment from coupling zones to the channel was thought to play a secondary role in the geomorphic response shown by the channel. Similarly, large volumes of erosion and deposition were found after resurveys in 2008 and 2011, suggesting that the system was still in its relaxation phase. The results obtained in this investigation coupled with historical information on the flood history of Thinhope Burn dating back to 1766 suggested that rare large floods are the geomorphically effective flows in the catchment.

Landslides in blanket peat on subantarctic islands: Causes, characteristics and global significance

Blanket peat on hillslopes in the British Isles is known to be susceptible to mass movements. A few such peat landslides had also been reported from southern hemisphere islands with climates that favour the formation of blanket bogs. Previous studies of peat landslides on subantarctic islands are re-examined to address two primary hypotheses: (i) that landslides in blanket peat are primarily controlled by natural intrinsic thresholds, and (ii) that the general characteristics and failure mechanisms of the peat landslides on subantarctic islands do not differ significantly from British and Irish examples. Peat slides on Macquarie Island and Marion Island are analysed to establish whether quantitative comparisons can be made with the stability of blanket peat in Ireland. The inventory surveys of the subantarctic islands indicate that there may be several hundred peat landslides, perhaps three times more than in the British Isles. They appear to be common because of the combined effects of several factors including the constituent vegetation and the frequent moderate rainfall that promotes formation of smooth, impermeable iron and organic pans at the base of the peat. The absence of intense convective rainfall, droughts and anthropogenic disturbance leads to the conclusion that the failures result from peat accumulation reaching natural maximum thicknesses with respect to intrinsic stability thresholds. Different vegetation on different slopes seems to influence the shear strength of blanket peat on Macquarie Island, and in general the subantarctic peat landslides are shown to be visually similar but quantitatively different from Irish examples.

A peat slide at Glenfiddich, East Grampian Highlands

Synopsis A natural peat slide event at Glenfiddich, NE Scotland in August 2004 occurred during a period of rainstorm activity. The mass movement entailed some 700 m 3 of peat debris that obstructed a path, intersected a stream, caused the collapse of a bridge and encroached upon a designated Special Area of Conservation. Four principal factors contributed to the event; (1) prolonged wet weather and heavy rainfall, (2) a steeply dipping rockhead surface of weathered Dalradian psammite, (3) a sheltered north-facing slope and (4) a natural drainage pipe at the point of initiation of the slide. Remedial measures include erosion mitigation by placement of armourstone on parts of the rupture surface and replacement of a bridge across the stream with a ford. A programme of geological surveillance was implemented to monitor any variations in site conditions and identify any additional opportunities for environmental engineering works.

Discussion of ‘Peat slope failure in Ireland’ by N. Boylan, P. Jennings & M. Long, Quarterly Journal of Engineering Geology and Hydrogeology , 41 , 93–108

M.G. Winter, K.J. Barker & J.M. Reid write: Boylan et al. (2008) are to be congratulated on their excellent contribution to the debate that is currently driving our understanding of peat slope failures in Ireland, the UK and beyond. Their work adds some useful context and detail to the interesting case study of a peat slide in Wales presented by Nichol et al. (2007). One of the most striking elements of the paper by Boylan et al. (2008) is the change in apparent risk over time. On pages 98 and 99 they state that ‘Over the period 1600 to the present the estimated number of fatalities is 36’ and accept that the exact number of deaths is unlikely to be accurate from the records available. One might expect any variation to reflect an underestimate in the number of deaths; the authors seem to concur, as they round the calculated death rate up and equate this with a probability of fatality of around 0.1 (i.e. 10−1) per annum. However, they go on to state that ‘The [current] risk of a fatality …, taking into account the exposure of the population …, is of the order of a probability of 10−7 fatalities per year.’ The most recent fatalities shown in their figure 7 occurred in the early part of the last century and one can easily imagine how the exposure of the population to such risks may have altered by virtue of rural depopulation and emigration, for example. However, the difference in the annual probability of fatality as suggested by the historical data is six orders of magnitude greater than the calculated value given. It would be helpful if the authors could explain how this low current probability of fatality was obtained, particularly the assumptions implicit in their calculations, …

Subsidence and associated ground movements on the Pennines, northern England

The Pennines form a range of hills in northern England. The elevated parts are known locally as ‘the moors’, and these run from the Scottish border in the north, southwards to the Midlands. The area is dominated by elevated moorland plateaux, which have been incised by glacial and alluvial valleys. The plateaux are capped by strong, massive, cross-bedded Kinderscout Grit of Late Carboniferous (Namurian) age (known also as the Millstone Grit). Interbedded weak shales, mudstones and siltstones crop out on the valley floor, lower slopes and middle slopes, which have been eroded by landslides. The moorland is covered by a veneer of head and peat deposits that may be up to 4 m thick. Two types of subsidence, with associated ground movements, have been observed: (1) the large-scale regional tilting, landsliding and apparent lateral spreading of periglacial moorland plateaux with associated fault scarps and fissures; (2) ‘sinkholes’ in peat, which occur as distinct subsidence depressions up to 2 m in diameter. As these ‘sinkholes’ are not generated by mining subsidence or by the dissolution of (for example, karst or gypsiferous) bedrock, the term pseudo-sinkhole has been introduced, for the purpose of this paper. Pseudo-sinkholes may be occasionally associated with peat slides, bog bursts, subsidence depressions and concentric ring fissures on peat scars. The extent and magnitude of the subsidence and ground movements vary considerably. These may affect small, localized peat-covered slopes, no more than a few metres wide, or influence the morphology of entire moorland slopes. Because of the relative remoteness of ‘the moors’ these types of ground movements do not influence structures, or affect people, and therefore tend to have not been previously investigated, documented or reported. Similar features have been reported in the South Wales Coalfield, where there is a long, complex mining legacy. Since the Kinderscout (Millstone) Grit and associated interbedded sedimentary rocks do not contain any minerals of economic significance, the observed ground movements cannot be attributed to mining subsidence. The Pennine moors therefore provide a unique opportunity to investigate subsidence (tilt), scarps and fissures in the absence of mining (or other human) influences. Subsidence in peat is likely to have been generated by the subsurface fluvial erosion of layered fibrous and amorphous peat deposits. This leads to the generation ofsubsurface voids (pipes), which subsequently undergo collapse, followed by the migration of the collapsed zone towards the ground surface. This results in the generation of crescent-shaped, concentric fissures, subsidence depressions, graben and pseudo-sinkholes. The mechanism for the large-scale regional tilting and subsidence of moorland plateaux is more difficult to determine and still not fully understood. These ground movements are complex and are associated with deep-seated landslides, complex fissure networks and, in the study area, a reactivated fault-scarp that is up to 4 m high and over 700 m long. This paper suggests that these movements were possibly generated under conditions of periglacial erosion and weathering, during glacier retreat, deglaciation and gravitational stress relief of valley sides. This is most likely to have occurred during the closing stages of the Pleistocene ice age. This may have initiated the lateral spreading of moorland plateaux, subsequently resulting in fissuring, fault reactivation, tilting and subsidence. The aim of this paper is to document and draw attention to the different types of subsidence and associated ground movements on Pennine moorland, and to suggest causal mechanisms.

The Romeriksporten railway tunnel — Drainage effects on peatlands in the lake Northern Puttjern area

This study presents the results of an investigation of drainage effects of the Romeriksporten railway tunnel on peatlands in the Lake Northern Puttjern area. Drainage impacts on peatlands occurred in marked depressions reflecting faults or weakness zones in bedrocks. The impacts were most evident near the tunnel trace, but dry peatlands and streams were observed as far as 600 m from the tunnel trace. Several types of drainage effects were observed on peatland surfaces after tunnelling; surface subsidence and formation of local depressions, peat slides, peat cracking, dry peat holes, and death of vegetation. The effects depended clearly on peatland properties. The greatest and most damaging effects were associated with thick peat layers around open water surface. Here lowering of the water table caused peat slides. Such areas will generally be extremely vulnerable to lowering of the water level. The magnitude of subsidence of peatlands depends on peat thickness, peat decomposition and groundwater lowering. Even after seven years with artificial infiltration in bedrock wells in dry periods to counteract water table lowering, peat subsidence and peat slides were evident, demonstrating that temporary draw down of water can result in irreversible impacts on peatlands. To better be able to avoid damaging effects of tunnel drainage on environment, investigations of properties of peatlands above possible tunnel traces should be integrated in the planning procedure for tunnel constructions. In mires above such traces, the occurrence of floating peat mats, degree of peat decomposition, peat thickness, deliveries of water from surrounding catchments in dry periods and possible pathways of leakage through subsoils and bedrocks should be examined as basis for assessment of vulnerability of peatlands to tunnel drainage and selection of tunnel traces that evade the most vulnerable peatlands. When such areas can not be avoided, requirements should be set to the tightening of the tunnel to prevent damaging water draw downs in dry periods. Observations of drainage effects on peatland surfaces can complement hydrological information from measurements of tunnel leakages and groundwater levels and thus contribute to provide a broader picture of hydrological impacts and bedrock hydrogeology.

Landslides in blanket peat on Cuilcagh Mountain, northwest Ireland

The northern and eastern sides of the Cuilcagh Mountain upland, in northwest Ireland, are mantled with over 50 km 2 of blanket bog that has experienced an unusually high spatial and temporal frequency of peat mass movements. In all, 29 peaty-debris slides, nine bog slides, two peat slides and five more peat landslides of uncertain type have been recorded within this study area. More than 27 km 2 of this peatland has been afforded several levels of statutory protection as well as international recognition of its geo-environmental importance. Field and laboratory investigations of the peat at several of the more recent failure sites showed it to be typical of Irish and Pennine (northern England) blanket bogs in most physical and hydrological respects. Field geomorphological evidence and modelling of stability thresholds indicate that the particular susceptibility of the Cuilcagh Mountain blanket bog to failure arises from two local factors: (i) the attainment of threshold maximum peat depths on the East Cuilcagh plateau, and (ii) the unconformable deposition of thin layers of glacial till (in places) and blanket peat over the pre-existing topographic surface formed from the major shale formations that underlie the northern slopes. With two exceptions, there is no conclusive evidence that human activities and management strategies for the area have had any significant influence on the occurrence of the peat landslides. The high frequency of large rainfall events since 1961 that did not trigger landslides suggests that failures are unlikely to become more frequent in response to climate change effects because they are controlled by slowly changing internal thresholds.

Peat slope failure in Ireland

Recent peat slope failures in Ireland in the autumn of 2003 at Pollatomish, County Mayo and Derrybrien, County Galway have focused attention on such events. However, slope failures involving peat are not a recent phenomenon, and possible evidence of peat failures in Ireland has been identified as far back as the Early Bronze Age. This paper summarizes the issues surrounding peat slope failure in Ireland that would be of interest to an engineer or engineering geologist assessing this geohazard. The distribution of peat throughout Ireland, its formation, and its typical characteristic properties are discussed. A review of historical failures shows that there is a relationship between run-out distance and failure volume, and that the majority of the failures are clustered at slope angles between 4° and 8°. It seems that the risk of fatalities from peat slides is relatively low. The likely casual factors of peat slope failure are presented using examples including the recent failures at Pollatomish and Derrybrien, both of which have been investigated by the authors. Particular attention is paid to shear strength properties of peat and the applicability of traditional soil mechanics. Given the uncertainties that exist about peat strength, a cautious approach to slope stability assessment is advocated together with identification of potential causal factors to mitigate against this geohazard.

Characteristics of the Shetland Islands (UK) peat slides of 19 September 2003

An extreme rainfall event over the southern Shetland Islands in northern Scotland, UK, on 19 September 2003, triggered at least 20 significant peat slides and at least 15 smaller landslides of varying types. The peat slides were examined and surveyed to characterise and explain the distinctive morphological features that were produced. The failures varied in size from 0.4 to 7.3 ha (2,300 to 59,000 m3 displaced volumes of peat) and involved blanket peat up to 3 m deep and slope gradients as low as 4°. Almost all of the failure surfaces were located at the peat–mineral interface. The morphological features included large areas (up to 0.5 ha) of intact peat that moved without breaking up, linear compression and thrust features and unusual occurrences of mineral debris. These features suggest peat of high tensile strength throughout its depth and the generation of high and sometimes artesian water pressures at the base of the peat during the event. However, the variations between peat slides highlight some of the difficulties of trying to assess the susceptibility of blanket peat to failure without full knowledge of the local peat geotechnical properties and structural features within the peat mass.

Geomorphology of upland peat: erosion, form and landscape change

Series Editors' Preface. Acknowledgements. 1. Introduction. 1.1 The aims of this volume. 1.1.1 Thematic coverage. 1.1.2 Geographical context. 1.2 Terminology, definitions and peatland geomorphology. 1.2.1 Definitions of Peat. 1.2.2 The Physical and geotechnical properties of peat. 1.2.3 Peatland classification. 1.3 The geography of blanket mire complexes. 1.4 Patterns of peat erosion in space and time. 1.4.1 The onset of peat erosion. 1.4.2 Direct observation of the onset of erosion. 1.5 Causes of peat erosion. 1.6 A brief history of the evolution of peatland geomorphology. 1.6.1 Accounts of erosion in the natural science tradition. 1.6.2 Descriptive accounts of widespread peat erosion. 1.6.3 Quantitative observations of blanket peatlands. 1.7 Structure of this volume and the peat land system model. 2. The Hydrology of Upland Peatlands. 2.1 Introduction. 2.2. Controls on water movement in peatland systems. 2.2.1 Hydraulic conductivity of upland peat soils. 2.2.2 The diplotelmic mire hypothesis. 2.2.3 Groundwater flow in upland peatlands. 2.2.4 Evaporation. 2.2.5 Runoff generation. 2.2.6 The Water balance of ombrotrophic mires. 2.3 Geomorphology and the hydrology of upland peatlands. 3. Sediment Production. 3.1 Introduction. 3.1.1 Monitoring sediment production using erosion pins. 3.1.2 Sediment trap data. 3.2 Sediment production as a control on catchment sediment flux. 3.3 Evidence from field observation. 3.3.1 Climate correlations with trap data. 3.3.2 Direct observations of surface change. 3.4 Evidence from controlled experiments. 3.5 Timescales of sediment supply. 3.6 Conclusion. 4. Fluvial Processes and Peat Erosion. 4.1 Introduction. 4.2 Gully erosion of blanket peat. 4.2.1 Gully morphology and topology. 4.2.2 Processes of Gully erosion. 4.3 Erosion and transport of peat in perennial stream channels. 4.3.1 Production of peat blocks by fluvial erosion. 4.3.2 Transport of peat blocks in stream channels. 4.3.3 The fate of fine peat sediment in channel. 4.4 Sediment yield. 4.4.1 A conceptual model of sediment dynamics in eroding blanket peatlands. 4.4.2 Sediment yield, sediment supply and assessing catchment erosion status. 4.5 Conclusions. 5. Slope Processes and Mass Movements. 5.1 Introduction. 5.2 Peat covered hillslopes. 5.2.1 Limits to the stability of peat on slopes. 5.2.2 Creep on peat hillslopes. 5.3 Morphology of rapid peat mass movements. 5.3.1 Source zone. 5.3.2 Rafted peat debris. 5.3.3 Runout track. 5.3.4 Secondary tension and compression features. 5.3.5 Bog burst and peat slides - are they different?. 5.4 Mechanism of peat failure. 5.4.1 Speed of failure and movement. 5.5 Significance of surface hydrology in peat failures. 5.5.1 Water content and pore pressures. 5.5.2 Rainfall. 5.5.3 Slope drainage. 5.6 Stability analysis and modelling of peat mass movements. 5.7 The changing frequency of peat mass movements over time. 5.8 Summary and overall framework. 6. Wind Erosion Processes. 6.1 Introduction. 6.2 Wind erosion in the uplands and the significance in peatland environments. 6.3 Mechanisms of wind erosion. 6.4 The significance of wet or dry wind erosion processes. 6.5 Direct measurements of wind erosion of peat. 6.6 Aeolian landforms. 6.7 The significance of wind erosion for landscape development in upland peatlands. 6.8 Conclusions. 6.8.1 Further research - an agenda for pluvio-aeolian studies of upland peat. 7. Peat Erosion Forms - from Landscape to Micro-relief. 7.1 Rationale and introduction. 7.2 Macroscale - Region / Catchment Scale. 7.3 Mesoscale - Slope-Catena Scale. 7.4 Microscale - material structure scale. 7.5 Linking the geomorphological and the ecohydrological. 7.6 Conclusions. 8. Sediment Dynamics, Vegetation, and Landscape Change. 8.1 Introduction. 8.2 The effect of peatland dynamics on long term sediment budgets. 8.3 Mass balance evidence of patterns of long term erosion. 8.4 Re-vegetation of eroding peatlands. 8.4.1 Artificial re-vegetation of bare peat surfaces. 8.4.2 Natural revegetation of eroded landscapes. 8.5 Controls and mechanisms of natural re-vegetation. 8.5.1 Extrinsic controls on re-vegetation. 8.5.2 Intrinsic controls on re-vegetation. 8.5.3 Eriophorum spp. as Keystone Species for Re-vegetation of Eroded Peatlands. 8.5.4 Geomorphology, ecology, and 'erosion-regeneration complexes'. 8.5.5 Re-vegetation dynamics and long term patterns of erosion. 8.6 Stratigraphic evidence of erosion and re-vegetation. 8.7 The future of blanket peat sediment systems. 8.8 Changes in pollution climate. 8.9 Climate change impacts. 8.9.1 Increased summer drought. 8.9.2 Increased summer and winter storminess. 8.9.3 Changes in the growing season and vegetation. 8.9.4 Overall response of the peat land system. 8.10 Relative importance of peat erosion in wider upland sediment budgets. 8.11 Conclusions. 9. Implications and Conclusions. 9.1 Implications of widespread peat erosion. 9.2 Upland peatland erosion and carbon budgets. 9.2.1 Case Study Example: the Rough Sike sediment-carbon budget. 9.3 Release of stored contaminants from eroding peatlands. 9.4 Restoration of eroded upland peatlands. 9.4.1 Frameworks for restoration. 9.4.2 Approaches to restoration. 9.4.3 Implications of the landsystems model and sediment budget work for restoration. 9.5 Conclusions. 9.5.1 The nature of upland peatlands. 9.5.2 Geomorphological processes in upland peatlands. 9.5.3 The future of upland peatlands. 9.5.4 Representativeness of the peat land system model. Index.

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

Please click here to download the map associated with this article. Peat mass movements are relatively common geomorphological phenomena in the uplands of Ireland and parts of northern Britain. Geomorphological mapping of many peat landslides has led to the identification of different types of failures based on repeatedly occurring morphological and geometric characteristics. Furthermore, much of the recently enhanced understanding of mass failures in peat deposits, published in several recent reviews, has been obtained through detailed analysis of the geomorphological maps produced from the new field surveys that are herein presented together for the first time. The map that is the focus of this paper is a composite figure that shows geomorphological maps of the majority of extant bogow and bog slide types of peat mass movements in Ireland, together with two peat ows and a large peat slide that has not previously been described. The paper examines the methodology used to create the maps, and highlights the value of the maps in presenting potentially critical evidence that may not be discernible by field inspection alone. Creating geomorphological maps of peat landslides from field sketches provides highly satisfactory results if selected features are geo-referenced using a hand-held GPS at an average frequency of around once for every 30 m of source area perimeter or other linear component of a landslide. In the case of very large failures and very old and degraded failures, lower frequencies of GPS points can be tolerated because critical morphological evidence appears to scale-up with failure size and smaller-scale morphological details are lost with increasing failure age due to natural degradation of the disrupted peat.

Failure of peat-covered hillslopes at Dooncarton Mountain, Co. Mayo, Ireland: Analysis of topographic and geotechnical factors

Abstract Shallow landslides involving loss of blanket peat are relatively uncommon but can nevertheless be environmentally and geomorphologically significant. This paper describes a cluster of 40 shallow landslides, including significant peat failures, which occurred within 2.5 km 2 on Dooncarton Mountain (Republic of Ireland) in the evening of 19 September 2003 during an exceptional rainfall event. It examines two hypotheses: (i) that the combination of topography and local soil catena characteristics were the primary site factors that led to the occurrence of most of the failures, and (ii) that anthropogenic disturbance to the slopes in the form of boundary ditches or peat-cutting activities may have contributed to three of the largest peat failures. All of the landslides were mapped and sampled within 4 months of the event. Physical, hydrological and geotechnical properties of the slope materials were determined from samples obtained from nine representative failures. Possible controls on the stability of the slopes were investigated by modelling seven of the landslides and a series of hypothetical slopes using standard limit equilibrium methods. The humified blanket peat was underlain by a dominantly sandy mineral material, including a buried soil horizon, derived from weathering of the mainly schist bedrock. Direct shear tests established the respective shear strengths to be ϕ ′ = 50°, c ′ = 0 (buried soil) and ϕ ′ = 21–25°, c ′ = 8–11 kPa (basal peat), although back-analyses suggested that the average peat cohesion was lower than this on the failed slopes. Analyses of separate slope segments of the landslides showed that the main slope convexity, where the peat cover gives way to thin peaty soil, defined the zone of minimum stability. Failure of the slope segment immediately above the convexity was controlled by the depth of peat and the hydrostatic or possibly artesian water pressures within the slopes. Thus catena characteristics coupled with local topography (gradient and peat depth) were important determinants of slope instability. Two of the failed slopes were crossed by boundary ditches but, contrary to accounts from elsewhere, stability analyses suggested that these ditches did not contribute significantly to the landslides. Stability analysis of a slope affected by peat extraction also suggested no direct causal association, but the hydrological conditions developed in the vertical tine cuts did probably contribute to a large peat slide. At other locations, therefore, similar anthropogenic factors should be incorporated in landslide hazard assessments.

Significance of geomorphological and subsurface drainage controls on failures of peat‐covered hillslopes triggered by extreme rainfall

AbstractOn 19 September 2003, 40 landslides of 140–18 000 m3 volume occurred within 2·5 km2 on the slopes of Dooncarton Mountain (Republic of Ireland) during a storm that may have exceeded 90 mm within 90 minutes. The landslides were investigated to determine the reasons for such a high density of slope failures. All of the landslides were surveyed within four months, and nine of them were investigated in detail. The six largest landslides, all peat failures, accounted for 57% of the more than 100 000 m3 of material displaced during the event. A consistent sequence of superficial materials was found on the failed hillslopes, including an extensive iron pan at the base of a buried soil horizon 0·3 m below the base of the peat. Morphologically, almost all of the landslides occurred on steep planar slopes or around sharp convexities, with the latter failures developing retrogressively upslope. The only significant relationship found from analysis of 371 subsurface pipes and 142 seepage cracks (defined here as contiguous fissures conducting concentrated subsurface flow) across all the failures was that the thinner the peat cover, the deeper the pipes and seepage cracks occurred below the base of peat. It is concluded that most of the landslides were probably caused by a combination of excess water pressures in the buried soil horizon and the thinner overburden of peat or peaty soil associated with the steeper slope segments. Pipes and seepage cracks formed on the iron pan probably existed prior to the failure event and may have contributed to the high water pressures as rainwater inputs exceeded their discharge capacities. One large peat slide was probably triggered by excess water pressures developed within and between artificial tine cuts. The properties of the blanket peat were generally of little consequence in the occurrence of the landslides, but relict desiccation cracks and other structural weaknesses through the peat mass were probably highly significant. Although several aspects of the peat failures correspond to previously published examples, the context of these failures in terms of the topography and upland catena is distinctive. Copyright © 2007 John Wiley & Sons, Ltd.

Mass movements in peat: A formal classification scheme

Published accounts of peat mass movements throughout the last 500 years reveal common characteristics among the failures; however, there has been no consistency in the terminology used to describe them. Given the apparently increasing frequency of peat failures in the British Isles and the possibility of more peat failures elsewhere in the world as a consequence of climate change, there is a need for a specific classification scheme. This paper proposes a scheme that uses clearly defined terms that can be readily applied and reliably used to understand and assess future hazards from peat mass movements. The paper begins with a general definition of what should constitute a ‘peat failure’, then presents a precisely defined classification scheme for peat landslides (i.e. excluding creep), using type of peat deposit and failure morphology as the key criteria. The new scheme for peat failures is discussed in the context of existing landslide classification systems. Definitions are provided for bog bursts (flow failure of raised bogs), bogflows (flow failure of blanket bogs), bog slides (shear failure and sliding of blanket bogs), peat slides (shear failure at peat–mineral interface in blanket bogs), peaty-debris slides (shear failure within mineral substrate beneath blanket bogs) and peat flows (natural failures of other types of peat deposits including flow failure caused by head-loading). Some practical problems of classifying peat failures are discussed.

Analysis of the peat slide at Pollatomish, County Mayo, Ireland

A major landslide event occurred at Pollatomish, County Mayo, Ireland in September 2003, during a period of intense rainfall. It comprised about 30 significant individual longitudinal planar type slides of peat and weathered rock. Relatively simple limit state stability analyses, using the method of slices and an infinite slope analysis, were used to model the slide and it was found that the features observed on site could easily be reproduced. These included confirmation that thin layers of peat could be stable on steep slopes but the margin of safety reduces rapidly under elevated pore pressure conditions. As was observed in the field, the analyses suggested the most vulnerable zone was the upper layer of weathered rock but that slides could occur in the peat if its thickness was appreciable. Careful site characterization is vital in such studies. Here efforts have been made to understand the effect of fibers on the peat strength and some sensitivity analyes have been performed to assess the critical engineering parameters of the peat.

Hydrological controls of surficial mass movements in peat

Peat deposits occur in areas where water logging is common. Large areas of blanket mire occur throughout the temperate and cold regions of the Northern and Southern Hemispheres and are particularly extensive in the UK Uplands. Mass movements of peat, reported usually as peat slides and bog bursts, have been well documented for over 150 years. However, the fundamental controls of this form of shallow instability are still poorly understood. The aim of this paper is to review critically the available evidence linking peat hydrology and slope instability. Then, using data from the North Pennine uplands in Northern England, we examine the importance of rainfall, macroscale drainage conditions and peat hydrological processes in initiating slope instability. Shear failure by loading, buoyancy effects, basal liquefaction and surface or marginal rupturing have all been proposed as potential failure mechanisms. Data from Northern England demonstrate that hydrological processes are fundamental in determining the spatial and temporal occurrence of peat slides. Most instabilities are associated with convective summer thunderstorms, although instances of rapid snowmelt and intense winter rainfall have also triggered these events. All instabilities occur in association with distinct drainage features. Slides are initiated along natural drainage lines or in association with artificial drainage often brought about by mining activity or agricultural practices. At the scale of the soil profile the special hydrological properties of peat, in particular shallow water tables and low soil hydraulic conductivity, offer important clues to failure mechanisms. Pore water pressures are generally low and vary little in the peat profile and throughout the year. It is clear from observations of subsurface flow that significant volumes of runoff reach considerable depths in the soil. Macropores (flow occurring in pores greater than 1 mm in diameter) are important in delivering surface runoff to deeper parts of the profile and in some instances high pipe water pressures may develop. These findings define several important hydrological controls for surficial mass movements in peat but the prospect of predicting the location and timing of such events is still a long way off.

Hydrological controls of surficial mass movements in peat

Peat deposits occur in areas where water logging is common. Large areas of blanket mire occur throughout the temperate and cold regions of the Northern and Southern Hemispheres and are particularly extensive in the UK Uplands. Mass movements of peat, reported usually as peat slides and bog bursts, have been well documented for over 150 years. However, the fundamental controls of this form of shallow instability are still poorly understood. The aim of this paper is to review critically the available evidence linking peat hydrology and slope instability. Then, using data from the North Pennine uplands in Northern England, we examine the importance of rainfall, macroscale drainage conditions and peat hydrological processes in initiating slope instability. Shear failure by loading, buoyancy effects, basal liquefaction and surface or marginal rupturing have all been proposed as potential failure mechanisms. Data from Northern England demonstrate that hydrological processes are fundamental in determining the spatial and temporal occurrence of peat slides. Most instabilities are associated with convective summer thunderstorms , although instances of rapid snowmelt and intense winter rainfall have also triggered these events. All instabilities occur in association with distinct drainage features. Slides are initiated along natural drainage lines or in association with artificial drainage often brought about by mining activity or agricultural practices. At the scale of the soil profile the special hydrological properties of peat, in particular shallow water tables and low soil hydraulic conductivity , offer important clues to failure mechanisms. Pore water pressures are generally low and vary little in the peat profile and throughout the year. It is clear from observations of subsurface flow that significant volumes of runoff reach considerable depths in the soil. Macropores (flow occurring in pores greater than 1 mm in diameter) are important in delivering surface runoff to deeper parts of the profile and in some instances high pipe water pressures may develop. These findings define several important hydrological controls for surficial mass movements in peat but the prospect of predicting the location and timing of such events is still a long way off.

Anatomy of a Pennine peat slide, Northern England

AbstractThis paper describes and analyses the structure and deposits of a large UK peat slide, located at Hart Hope in the North Pennines, northern England. This particular failure is unusual in that it occurred in the winter (February, 1995) and shows excellent preservation of the sedimentary structures and morphology, both at the failure scar and downstream. The slide was triggered by heavy rain and rapid snowmelt along the line of an active peatland stream flush. Detailed mapping of the slide area and downstream deposits demonstrate that the slide was initiated as a blocky mass that degenerated into a debris flow. The slide pattern was complex, with areas of extending and compressive movement. A wave‐like motion may have been set up in the failure. Within the slide site there was relatively little variability in block size (b axis); however, downstream the block sizes decrease rapidly. Stability analysis suggests the area at the head of the scar is most susceptible to failure. A ‘secondary’ slide area is thought to have only been initiated once the main failure had occurred. Estimates of the velocity of the flowing peat mass as it entered the main stream channel indicate a flow velocity of approximately 10 m s−1, which rapidly decreases downstream. A sediment budget for the peat slide estimates the failed peat mass to be 30 800 t. However, sediment delivery to the stream channel was relatively low. About 37% of the failed mass entered the stream channel and, despite moving initially as debris flow, the amount of deposition along the stream course and on the downstream fan is small (only about 1%). The efficiency of fluvial systems in transporting the eroded peat is therefore high. Copyright © 2003 John Wiley & Sons, Ltd.

Initiation of a multiple peat slide on Cuilcagh Mountain, Northern Ireland

AbstractIn October 1998 a multiple peat slide occurred on the northern slopes of Cuilcagh Mountain, Co. Fermanagh, in response to a high‐magnitude rainfall event. Few peat slides have been recorded in Ireland, and a detailed field survey and investigation of the failure was undertaken within four weeks of the event. The morphological evidence indicated a distinct sequence of events which appeared to begin with the failure of a small segment of slope above a degraded transverse drainage ditch which was cut less than ten years previously. This segment of slope was no more than 42 m wide and 25 m long, with 0·7 m of peat overlying up to 0·5 m of a pale coloured clay, the latter containing small pipes and resting on the surface of a darker coloured loamy material. The failure surface was located at or near the base of the pale clay layer. Finite element software was used to model hydrological conditions within the upper segment of slope and to calculate factors of safety for different slope configurations including the presence or absence of a drain or a subsurface pipe. Using the peak shear strength of the pale clay, as determined in the laboratory, both the drain and subsurface pipes were required to obtain a factor of safety of 1·0 or less. Allowing for the uncertainties associated with the hydrological modelling of the pipes, it is suggested that the cutting of the drain and the hydrological impacts of its subsequent degradation are ultimately responsible for the failure. Copyright © 2001 John Wiley & Sons, Ltd.