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Discussion |
1 School of Architecture, Landscape and Civil Engineering, University College Dublin , , Newstead Building, Belfield, Dublin 4, Ireland
2 Present address: Center for Offshore Foundation Systems , , MO53, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia
3 Applied Ground Engineering Consultants Ltd (AGEC) , , Bagenalstown, Carlow, Co. Carlow, Ireland
* Corresponding author (e-mail: boylan{at}civil.uwa.edu.au)
Noel Boylan, Paul Jennings & Michael Long reply: The authors thank the discusser for his interest in their paper and would like to respond to his observations. The discusser suggests that we "appear to introduce information that may be interpreted as contradictory to some previous and very precise accounts of peat engineering and instability that aimed to provide coherent and consistent frameworks for future researchers and practitioners (e.g. Hobbs 1986)". The discusser does not substantiate this claim and raises issues that are mostly beyond the stated scope of our paper, which was to ‘summarize the issues surrounding peat slope failure in Ireland that would be of interest to an engineer or engineering geologist assessing this geohazard’. The additional points raised by the discusser relate more to geomorphological interpretations of these events rather than the assessment of this geohazard. We clarify the points raised as follows.
Peatlands and their properties
The authors agree that fen peat deposits are an additional peatland type found in Ireland; however, their significance nowadays, especially with respect to peat failures, did not merit their inclusion in the paper as a separate entity. In Ireland, fen peatlands have been widely developed for agricultural use and only 22 180 ha remain that are in a relatively intact condition (Foss 2007). This constitutes only 2.3% of the total peatland resource of 0.95 Mha in the Republic of Ireland (Connolly et al. 2007). In addition to this, there is no available evidence as far as the authors are aware of previous instability associated with this type of deposit in Ireland. The authors clearly differentiated between the terms ‘upland’ and ‘lowland’ blanket bog. The inclusion of the term ‘oceanic’ in parentheses is to provide clarification, as this term is sometimes used to refer to lowland blanket bogs in Ireland (Feehan and O'Donovan 1996) although by strict definition it can describe all the blanket bogs.
Types of peat failure
Although we highlighted the presence of two different classification systems for peat failures (Dykes and Warburton 2007; Hutchinson 1988), we did not adopt the scheme co-authored by the discusser. This scheme is largely based on their interpretation of descriptions of failures, by a large number of different people, from a period of over 200 years. This is also supplemented with field surveys conducted in recent years. The reliance on the descriptions from a large number of different people, who in most cases did not observe the actual failure taking place and in some cases did not make contemporaneous investigations of the failures, and recent field investigations of landscapes that could have significantly changed over time through erosion and creep of the remaining peat, can create a misleading picture that is open to interpretation. This scheme seeks to classify these events, whose mechanisms are still not understood, into a strict framework based on these interpretations.
The discusser disagrees with our statement that ‘in many bog flows that there was probably an initial shear failure on a discrete sliding surface prior to the failed mass breaking down into a slurry’. Although we agree that bog flows associated with raised bogs (defined as ‘bog bursts’ by Dykes & Warburton 2007) appear to usually occur as a result of a build-up in hydrostatic pressure and the release of the weak lower amorphous peat, we do not believe that this is the mechanism for all bog flows. In the case of the bog flow in Co. Kerry described by Sollas et al. (1897), a 3 m high turf cutting gave way after a heavy downpour of rain. The fact that turf was being cut in the first instance and that a 3 m high turf cutting was stable for any amount of time suggests that the peat in this raised bog was not in a near-liquid state prior to failure. For this failure to occur there would have been no need for a huge build-up in water pressure to create the burst and subsequent flow. It is more likely that the effect of rainfall ingress into the peat would have disturbed the delicate equilibrium created by the turf cutting, resulting in failure that initially involved displacement of large peat blocks, followed by subsequent breakdown and disaggregation of intact peat blocks into a liquid flow material, as well as releasing near-liquid peat in the core of the raised bog. The presence of detached large rafts of intact peat at the edges of the failure scar would tend to corroborate that shear failure along discrete planes (zones) was the dominant mode of failure. Indeed, the statement by Sollas et al. (1897, p. 476) that this bog flow ‘was clearly not instantaneous’, which was based on the evidence of the closest observer, further supports the proposition that the failure was progressive and evolved from a slide to a flow.
When one looks to the evidence of other bog flows associated with blanket bogs, there is also evidence of possible failure planes and the initial failure mechanism being due to sliding on a plane at or near the base of the peat. In the bog flow that occurred in Co. Clare in 1934, Mitchell (1935) described the lowest layer of the source area as being a ‘greasy surface which was most treacherous to walk upon’. The peat above this greasy layer and beneath the surface mat is believed to have moved downslope under the force of gravity, applying pressure on the escarpment bank and causing its subsequent failure. It is likely that this movement of peat downslope led to remoulding and the creation of peat in a close to fluid state trapped beneath the intact surface. The properties of this greasy layer are crucial to understanding how the overall failure mechanism evolved. Indeed, the use of the phrase ‘greasy surface’ is also highly suggestive of a shear surface. Similarly, for the bog flow that occurred at Glendun in 1963, Colhoun et al. (1965) suggested that it is possible that a similar mechanism of peat moving downslope beneath the surface could have been involved before the torrential rainfall event triggered the overall failure.
We therefore disagree with the discusser's proposition that the balance of evidence suggests that bogflows ‘occur by catastrophic in situ strength loss and fluidization’. The authors do not consider that every bog flow failed by the same mechanism, and particular conditions present at a failure (i.e. rainfall, cutting and ditches, slope morphology, profile of peat decomposition, etc.) are likely to play a dominant role in how the failure mechanism evolves. Descriptions provided in the literature are mostly of the material after the mechanism has taken place and it is not possible to base firm conclusions on them. The interaction of peat with different subsoils is crucial to understanding the origin and properties of the basal peat layers and the ‘greasy’ layers identified at some failures (Long and Jennings 2006; Mitchell 1935). Until we understand the behaviour of all the materials involved we cannot make reliable interpretations of the failure mechanisms taking place in different types of peat failures.
Review of previous peat failures
The data presented in this section are based in part on information from historical publications. In addition to this, field surveys have been conducted by the authors and analysis of aerial photography has been used to obtain unreported information. The authors have deliberately not subdivided the data into ‘bog slides’ and ‘bog bursts’ given the uncertainty about the exact failure mechanism in many failures. The authors cannot really comment upon the point raised by the discusser with respect to the peat slide described by Nichol et al. (2007). We assume that a high-resolution global positioning system was used and that the discusser's measurements are thus not subject to the relatively large errors (plus or minus a few metres) that afflict handheld consumer units. However, the discusser appears not to distinguish between the 40 m3 that he measured as having been lost from the ‘source area’ and the 250 m3 reported by Nichol et al. (2007) as having been deposited at road level; the latter would, of course, include entrained material, which is often a major part of the material deposited from such events, and certainly Nichol et al.'s figure 2 appears to support the idea of significant erosion below the source area.
Peat strength properties
The authors in the paper use undrained strength properties (su) over drained strength properties (c', 
) for assessing the stability of peat deposits. At a fundamental level, the structure of peat is not that of a frictional material such as sand or clay, and to simply assume that peat behaves in this manner is, in our view, incorrect. Indeed, recent research into the failure of organic clays at a microscopic level has shown them to fail by microcracking and buckling of microstructures, suggesting that the concept of Coulomb (i.e. friction of soil particles) does not apply to these soils (Barends 2005). Similar studies of peat at a microscopic level are required to provide insight into the failure mechanism that takes place in this material. In any regard, the effective stresses (
') at the base of a typical blanket bog with a high water table are likely to be nominal, and a stability analysis using drained parameters is dominated by the value of cohesion (c') (see Boylan et al. 2008, fig. 15). Cohesion values are usually defined using an extrapolation of a line to the zero normal stress axis, from yield points that are highly subjective, on tests often conducted at stress levels far greater than the operational stresses of the situation under investigation. The reliability of operational drained strength parameters (essentially c') derived from this approach is highly questionable.
To provide strength parameters for stability analyses of peat deposits, the authors suggest that a possible approach is the use of undrained shear strength (su), measured at the appropriate operational stress level. The advantage of this approach is that it is measuring a controlling parameter of the stability directly rather than using a dubious extrapolation. This approach is not without its difficulties and these are discussed in Boylan et al. (2008), but the level of uncertainty is far less than with drained strength parameters.
Although research on peat strength has been under way since the 1940s, the level of knowledge available about the strength properties of this material is low compared with other geotechnical materials. Indeed, Barends (2005) remarked in his Terzaghi oration that ‘its mechanical properties are hardly known’. Large gaps in research still have to be filled to understand the interaction between hydrology and peat strength, the change in strength properties under various loading and drainage conditions, and the nature of yielding of peat deposits before reliable analysis of the risk of peat failures, and the back analysis of failures that have already occurred, can take place.
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Sollas, W.J., Praeger, R.L., Dixon, A.F. & Delap, A. 1897. Report of the Committee appointed by the Royal Dublin Society to investigate the recent bog-flow in Kerry. Scientific Proceeding of the Royal Dublin Society. 8, 475–508.
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