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Landslides (2006) 3:205–216DOI 10.1007/s10346-006-0038-zReceived: 23 May 2005Accepted: 13 March 2006Published online: 3 June 2006© Springer-Verlag 2006
J. Yang . A. P. Dykes
The liquid limit of peat and its applicationto 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 bepresent 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-μ m fraction for testing
without incorporating artificially fragmented particles of fibres.
The sampled peat was classified as H8–H10 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.
Keywords Bogflows . Blanket peat . Mass movements . Liquid
limit
Introduction
Reports of bogflows and bog bursts indicate a widespread
occurrence in Ireland (e.g. Sollas et al. 1897; Delap et al. 1932;
Mitchell 1935; Colhoun et al. 1965; Alexander et al. 1986; Wilson et
al. 1996) with few examples known from elsewhere in the world.
However, detailed understanding of the phenomenon is still
lacking. Failures of raised bogs, usually referred to as ‘bog bursts’,
are rare events and unlikely to recur, but ‘bogflows’—failures of
blanket bog in which semi-liquid basal peat appears to have
escaped from beneath the less humified surface peat—have
continued to occur in Ireland up to the present time (Dykes and
Kirk 2006). With over 16% of the combined land area of Northern
Ireland and the Republic of Ireland covered with peat (Fig. 1), these
failures have the potential to be very damaging to property and
infrastructure. Furthermore, the environmental/ecological value of
blanket bog was recognised internationally during the 1980s,
and the British Isles (including Ireland) contain around 10% of
the global blanket peat deposits (Tallis 1997). Any disturbance
of the remaining peat must be carefully planned so as to
minimise the possibility of a failure occurring and, thus,
damaging blanket bog and associated watercourses (McCahon
et al. 1987; Wilson et al. 1996). This, and any assessment of thepotential hazard from peat failures, poses a significant challenge
to engineers and geologists/geomorphologists, respectively.
The development of different types of peat deposits is
explained as background information for ground engineers by
Hobbs (1986) and Bell (2000); the following is based on relevant
parts of those accounts. As is the case elsewhere in the world,
most Irish bogs originated in poorly drained hollows or lowlands
subjected to continuous inundation by runoff water and
sediments. The minerotrophic waterlogged conditions caused
dead plant material to accumulate to form fen peat, the plant
species being influenced by the nutrient-rich runoff waters.
Permanent waterlogging reduces the oxygen supply so that only
anaerobic organisms with relatively slow metabolic activity are
available to break down the cellular structure of the plant
remains. Therefore, partially decomposed plant remains accu-
mulate to form the peat. Further accumulation involved different
botanical assemblages as nutrient-poor rainwater provided an
increasing proportion of the water inputs, giving rise to
mesotrophic (intermediate nutrient status) conditions and
transitional peat characteristics; the local water table rose within
the peat deposit to remain always at or near the surface. Raised
bog developed as accumulation continued more rapidly in the
centre, with the oligotrophic (nutrient poor) rainwater input
dominating the hydrology and resulting in slower decomposi-
tion. The very low permeability caused the water table to rise,
thus maintaining a self-perpetuating positive feedback (Fig. 2).
The latter ombrotrophic (rainwater-fed) stage of raised bog
development is also the mechanism of blanket bog development.
Small, shallow, waterlogged depressions with impermeable
substrates enabled bog peat to accumulate locally and spread
laterally to join with other local sources to produce a continuous
blanket. Blanket bogs can also extend upslope, by locally
damming rainwater at the upper margin, and downslope, as
water draining from the lower margins maintains waterlogging.
The depth of the blanket bog is related to the local topography
(e.g. Fig. 2), being thicker at the sites of the original depressions
and very thin if present on slopes exceeding around 20–25°. The
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limiting conditions for blanket bog development in the British
Isles (and Ireland) are climatic, generally requiring at least
1,250 mm of annual rainfall on at least 225 days/year, with cool
summers and high humidity (Hammond 1979) so that rainfall
always exceeds evapotranspiration.
Most of the reported bogflows appear to have been triggered by
heavy or prolonged rainfall, but the exact mechanisms of failureare not yet known. This makes it difficult to assess the possibility of
more frequent bogflows, resulting primarily from the increasing
frequency of high-magnitude and high-intensity rainfall events
predicted for parts of the British Isles by current climate change
scenarios (Ekström et al. 2005). However, the properties of the peat
are thought primarily to account for its instability, the key
properties perhaps being the degree of humification (the process
of decay or decomposition of the plant remains by means of
biochemical oxidation in anaerobic waterlogged conditions;
Hobbs 1986), shearing and tensile strength, water content and
permeability, and the bulk physical properties and structure of
the peat mass. All of these properties are strongly determined by
the botanical composition of the constituent material (Hobbs
1986; Dykes and Kirk 2006). Extrinsic causes of the occurrence of
failure (i.e. given a rainfall ‘trigger’) include human disturbance
of the original peat mass by peat cutting/extraction (Alexander et
al. 1985), pre-forestry ploughing (Hendrick 1990), and cutting
boundary or drainage ditches (Tomlinson 1981). These may be
the most important factors in terms of future instability hazard
assessment.
A two-phase system is often recognised in peat deposits: the
acrotelm is the upper surface layer of the bog, 0.1–
0.6 m thick (Hobbs 1986), defined by the range of fluctuation of the water table
(Ingram 1978); and the catotelm is the lower, permanently
saturated and often largely amorphous part of the peat profile.
Strong but largely circumstantial field evidence obtained shortly
after bogflows indicates failure involving fluidisation of the
catotelm in situ beneath the acrotelm, the latter often remaining
within the source area (e.g. Mitchell 1935; Wilson et al. 1996; Kirk
2001). Furthermore, although understanding of the engineering
properties and behaviour of peat is reasonably well-developed in
some respects (Carlsten 1993), in others it is not. In part, this is
because few data have been obtained and published. For example,
the relationships between index properties of peat discussed by
Hobbs (1986) and Bell (2000) are dominated by data from Britishfen peats, with a few results from bog peats of low humification
and/or water content and some results from peats elsewhere in the
world (predominantly Canada). In the context of this paper, there
are no known published data from ‘pure’ (i.e. very low ash
content), highly humified, amorphous bog peats with high natural
water contents. More importantly, the influence of fundamental
factors such as the botanical composition and environmental
history of the development of the peat deposit on the engineering
properties are largely unknown. The use of index properties may
therefore provide a means of understanding and assessing peat
instability before the establishment (or not) of any relationships
between the engineering and eco-historical factors. The wide range
of very high water contents of peat, which dominantly comprises
colloidal organic remains, combined with the known propensity of
peat to apparently fluidise, led to consideration of the liquid limit
(w L) as a potentially useful indicator of the susceptibility of peat to
failure. Its usefulness as an index property of mineral soils relies on
a standard procedure being used to measure it, but the w L of peat
has rarely been determined, and a fully standardised method has
never been used (Hobbs 1986).
The difficulties of reliably determining some engineering
properties of peat are well-known (Hobbs 1986). These particu-
larly relate to the nature of the organic particles and the
associated water-holding capacities, the fibrous nature of some
peats and the different ways in which the fibres affect the bulk
properties of the peat mass, the structural heterogeneity of peat
Fig. 1 Distribution of peatlands in Ireland (after Hammond 1979). The rectangle
indicates the 70-km wide area shown in Fig. 3
Fig. 2 Diagrammatic representation
of raised bog and blanket bog
(after Hobbs 1986)
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deposits, and—in terms of testing procedures—the changes in
the properties and behaviour of peat that would result from
drying and associated oxidation. The British Standard for
classifying mineral soils (BSI 1998a) provides an acceptable
procedure for determining the w L of organic soils (including
peats) (Skempton and Petley 1970) but does not constitute a
standardised procedure. Removal of large fibres and fragments is
unlikely to be consistent between different workers, and there is
no stated method that prescribes how a homogeneous paste canbe obtained from the remaining peat (for example, domestic food
blenders have been used in some instances; Hobbs 1986). The
chemistry of peat water has been shown previously to affect the
value of the w L (Hanrahan et al. 1967), although it is difficult to
judge the nature and magnitude of this influence from so little
published data. A fully replicable, standardised method, which
provides a theoretical basis for the w L as an index property that
corresponds with that for mineral soils whilst taking into account
the factors highlighted above, should be available. This will
necessarily include the assumption that the plasticity/liquidity
behaviour of peat depends only on the interaction between water
and the <425-μ m size fraction of the constituent particles, as is
the case for mineral soils. This assumption is reasonable becauseany fibres or woody fragments that remain within peat do so
because of their resistance to decomposition, therefore retaining
their original plant structures and not yet contributing to the fine
colloidal matter that is associated with plasticity –water content
relationships. Furthermore, eliminating the fibrous material
completely from the w L determination would leave the test
material only one step removed from the original peat as
sampled, rather than two steps when artificially disrupted
fragments of fibres are also included (Hobbs 1986).
In this paper, we present values for the w L of peat from three
sites in Ireland where small failures of blanket bogs occurred in
recent years. These values were obtained using five different test
procedures. The aims are (1) to provide the first systematic
comparison of how the procedures affect the w L values for peat,
and to suggest an improvement to the existing standard procedure
based on these results and the principles behind their determina-
tion; and (2) to use the results to assess the likelihood that in situ
fluidisation of the peat may have been the main failure mechanism
at all three study sites, notwithstanding the previous observation
that natural field water contents of bog peats tend to exceed the w Lof those peats (Hobbs 1986).
Study sites
Three relatively recent blanket bog failures in northwest Ireland(Figs. 3 and 4) were chosen to provide material for laboratory
testing. All were surveyed and sampled during the period 12–14
April 2004. The first, subsequently referred to as ‘Geevagh’,
involved around 20,000 m3 of peat (Figs. 4a and 5) and occurred
within a few months before May 1991, on the southern side of the
plateau-ridge top of Carrane Hill in Straduff Townland near
Geevagh, County Sligo (54°7.3′N, 8°13.0′W). It is located at an
altitude of 400 m, around 500 m northwest of the large bogflow of
1984 (Alexander et al. 1986). Between these two is the smaller scar
of a much older bogflow, and others have occurred on the same
ridge since 1831 (Alexander et al. 1986). Like the 1984 bogflow, this
one appears to have resulted from initial failure of the deep (up to
ca. 3 m) bank of peat that acted as a natural dam along the edge of adistinct escarpment, holding back the bog on the plateau.
Morphological evidence is consistent with previously published
accounts of bogflows: the initial breach released a flow of (semi-)
liquid catotelm peat from the bog behind. This outflow dragged the
fibrous acrotelm with it, the latter breaking up into blocks and
strips that were carried from the central part of the source area or
that subsided into the source area around the margins, as
retrogressive development of catotelm flow reduced and then
stopped. The flow of catotelm peat with acrotelm blocks left a trail
of peat slurry and debris up to 50–60 m wide down the face of the
escarpment for around 250 m then through a planted conifer forest
for 100 m before crossing a minor road. Despite felling some small
trees on entry to the forest and leaving blocks of peat up to 1 m
wide resting against the upslope side of some larger trees, the flow
still exerted sufficient force to destroy the fences on either side of
Fig. 3 Locations of the bog failures
discussed in this paper (black crosses
with red labels), mountain summits
(black triangles with brown labels), land
above 300 m altitude ( grey shading),
major lakes (Loughs), coastal waters
(blue outlines and hatching) and the
international border ( yellow line)
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the road below the forest. The extent of the flow beyond this point
could not be determined as visible evidence had been lost during
the 7–8 years since the event.
Examination of an accessible profile of the in situ peat at the
margin (Fig. 6) revealed a fibrous woody layer at 0.99–1.19 m depth
(von Post: H5–6) that may have controlled the development of the
failure to some extent. The remainder of the profile (except 0.52–
0.72 m that was H8), including the upper 0.30 m, was H9 or H10. The
present depth of the profile away from the escarpment edge is
around 1.50 m, though post-failure shrinkage of the exposed peat
face may account for at least half a metre of depth reduction. The
present layer between 1.19 and 1.44 m depth was found to be
particularly weak, containing localised pockets of muddy water
with small lumps of peat and grey bands of liquid slurry with water
contents >1,000%, randomly arranged throughout a mostly firm
medium (red-)brown coloured peat. These variations can be seen
in Fig. 6. Samples were obtained from the lower part of this
horizon, which was assumed to be representative of the peat that
failed initially. This material was classified in the field as H9–10 B4 F2
R1 W0, using von Post’s (1922) classification as presented by
Landva and Pheeney (1980) and Hobbs (1986) (see Table 3).
The second bogflow (‘Stony River’), involving around 22,000 m3
of peat, occurred within a few months before November 1998 on a
small north-facing plateau at 440 m altitude (Figs. 4b and 7) within
the Stony River catchment on the north side of Slieve Anierin,
County Leitrim (54°6.3′N, 7°58.7′W). An aerial survey in November
1998 did not reveal any evidence of other peat failures on the Slieve
Anierin–Bencroy mountains. Morphological evidence indicates a
flow-type failure similar to other bogflows but with different
patterns of tearing and shearing of the acrotelm round the margins
of the source area. Like the Geevagh bogflow, this one appears to
have involved a naturally occurring failure of the peat margin at
the edge of an escarpment (visible at the right of Fig. 7). However,
whilst the western side (the upper side as seen in Fig. 7) and most
of the central part of the source area appear to have involved an
outflow of (semi-)liquid catotelm that dragged the acrotelm with it,
the latter breaking up and largely being transported from the
source area, the catotelm is intact in the head zone and along the
eastern side. The movement of the acrotelm over flowing catotelm
dragged adjacent acrotelm over in situ catotelm, indicating that the
tensile strength of the acrotelm was sufficiently high to overcome
the shearing resistance provided by its contact with the upper
catotelm. A narrow trail of peat slurry and debris led from the
escarpment breach down the slope for about 400 m before entering
the Stony River. The nature and extent of any downstream impacts
are unknown.
An exposed peat profile at the margin revealed almost 2 m of
peat, suggesting a depth of nearer 2.5 m at the time of failure. The
humification increased with depth from H3–4 at 0–0.3 m to H9 at
1.1–1.6 m, the latter horizon containing some dense woody fibres.
Below 160 cm, a much higher content of woody fibres within a stiff
peat of H8 contained pockets or sheets of grey H10 peat slurry
similar to those encountered at Geevagh. The peat at Stony River
was otherwise generally firmer throughout. Samples were obtained
Fig. 4 Outline plans of the bog
failures, at the same scale and orien-
tation, based on field GPS surveys.a Geevagh, showing gradients of the
failed slope, concentric tears in the
peat around the head, tension cracks
along the eastern margin, and ex-
panding flow track beyond the source
area off the edge of the escarpment.
b Stony River, showing the same
features as Geevagh with selected peat
blocks but no tension cracks. c East
Cuilcagh, indicating the large zone of
accumulated displaced peat, thrust
beneath intact peat around the toe
margin. In all cases, X marks the
sampling site
Fig. 5 Geevagh. View upslope into the
twin source areas, April 2004
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from 1.7 to 1.9 m depth where the peat was recorded as H8 B3 F2 R2
W3. The very thin smear of peat slurry covering the mineral
substrate over the lower part of the source area indicated that
failure occurred within, or at least involved, this lower layer.The third, smaller failure (‘East Cuilcagh’, ca. 10,500 m3)
occurred probably during 1997 (estimated) on the eastern mid-
slopes plateau of Cuilcagh Mountain (Figs. 4c and 8), County
Cavan, at an altitude of 360 m (54°11.8′N, 7°46.4′W). It was adjacent
to (and overlapping with) an earlier failure (within a few years
before 1989) and 1.2 km south of a larger and potentially damaging
bog slide that occurred in August 1992 (Walker and Gunn 1993).
The northern and eastern slopes of Cuilcagh Mountain display the
scars of over 40 other peat failures of various types (Kirk 2001;
Dykes and Kirk 2006). Although originally identified as a bogflow,
a re-appraisal of its morphological characteristics, in the light of
such evidence from other failures, has indicated that this failure
might actually have been a bog slide (Dykes and Kirk 2006), in
which the lower catotelm failed by shearing with the semi-liquid
peat debris resulting from remoulding of the base of sliding peat
blocks (Bishopp and Mitchell 1946). This failure involved move-
ment of peat from a steeper slope (6°, cf. 4° at Stony River and 2–5°
at Geevagh) into a small, peat-filled tributary valley. Much of the
peat blanket appears to have slid intact (Fig. 8), although the
degree of disruption of the catotelm beneath this zone cannot bedetermined. The extensive toe lobe is defined by thrust features
around its margin; unusually, there was no loss of peat from this
failure site.
An accessible peat profile at the margin revealed highly
humified peat throughout, increasing from H7 at 0–0.18 m to H10
at 1.04–1.15 m depth. When first examined in early 1998, the peat
was up to 1.6 m deep around the margins (Fig. 9) and 2.1 m deep a
few metres upslope from the failure scar (Kirk 2001), indicating
rapid post-failure shrinkage of the exposed peat due to drying and
oxidation. Samples were obtained from the lowest 0.2 m of the
profile, described in the field as H10 B3 F2 R1 W0, where again the
grey slurry was also encountered.
Materials and methods
At each field site, the peat profile selected for inspection and
sampling was determined by the availability of an intact part of the
source area margin unimpeded by peat debris in front and
unaffected by tension cracks or other visible disturbance to the in
situ peat behind. The part of each peat profile, from which samples
were obtained, is identified in the previous section. All humifica-
tion values were determined in the field (and laboratory) using the
von Post test, in which peat is squeezed in the hand and the result
of this is matched to the nearest descriptive category (Table 1). This
is the accepted method in peat engineering (Hobbs 1986; Carlsten
1993; Bell 2000) in the absence of a quantitative alternative.
From all three field sites, eight small intact core samples 50 mm
long × 50 mm diameter were obtained, without disturbing the
original peat structure, for determination of the basic physical
characteristics of the peat at each site (water content, ash content,
bulk density and saturated hydraulic conductivity). Additional
blocks of peat were cut from the field peat profiles for carrying out
w L analyses using other methods described in this paper. Water
contents of samples were determined by oven-drying for 24 h at
105°C, and ash contents (loss on ignition) were measured after
ignition at 550°C for 3 h (Skempton and Petley 1970; Hobbs 1986). A
‘constant head’ method was used to measure the saturated
hydraulic conductivity.
Fig. 6 Geevagh. The peat profile described in the text, April 2004 (trowel for
scale)
Fig. 7 Stony River. View from a lightaircraft, November 1998
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The standard drop-cone penetrometer method was used to
determine thew L of the samples of peat from each site. This method
is described in detail in the Appendix . It has superceded the use of
the Casagrande apparatus as the preferred standard method
because it is easier to perform; it provides more reproducible
and, therefore, reliable results; and it requires less subjective
judgement from the investigator (BSI 1998a). In using this method
to determine thew L of peat, it was expected that the results obtained
would be influenced by the procedure used to prepare the samplematerial for testing. It is the nature and implications of the
preparation-dependent variability of w L values that are of concern
here. Five different preparations were applied to peat samples from
each bogflow, described below and summarised for comparison in
Table 2, and the corresponding w L values were then obtained using
the penetrometer. Many more water contents and corresponding
penetrations were obtained than specified by the standard method,
to verify the reliability of using 20-mm penetration as the w Ldeterminant given the general difficulties of determining engineer-
ing properties of peat and, thus, prior uncertainty of its likely
behaviour. Logistical constraints prevented us from replicating any
of the 15 w L tests, but we consider the standard penetrometer test
procedure, combined with the carefully specified sample prepara-tions, to be sufficiently rigorous to provide a good indication of the
true material characteristics. In any case, as the w L is an index
property, it should not be greatly influenced by natural variations in
the source material from each study site: the effect of one gross
variation in a source material property can be clearly identified and
explained (see ‘Results’).
Preparation for mineral soils (SPM)
The purpose of using this preparation method was to demonstrate
the effect of drying and oxidation on the determination of peat
properties and behaviour. Although test samples should not be
allowed to dry before w L determination, the British Standard does
allow air-drying ‘if required’ (BSI 1998a). One block sample of peat
(approximately 500 g) from each failure site was allowed to air-dry
at room temperature. Some coarse fibres and/or wood fragments
were picked out by hand before the peat was dry-sieved using a
425-μ m mesh (the British Standard specifies wet sieving; see ‘WSD’
below). The fine fragments that passed through the sieve were then
mixed to a paste with de-ionised water for w L determination.
Preparation for organic soils (SPO)
This is the method of preparation described by Skempton and
Petley (1970) and Hobbs (1986), as there are no other known
references to methods of w L determination for peat. It is used in
this study to provide reference w L values against which to compare
the others. Around 500 g of peat from each failure site was mixed
thoroughly in a large bowl using a broad-bladed knife to produce
the homogeneous paste for w L determination and coarse fibres
and/or wood fragments were removed using tweezers (as allowed
by the British Standard), with the remainder broken down by the
mixing process.
Preparation for organic soils by wet-sieving using de-ionised water(WSD)
This method was tried as a possible improvement to the SPO
method by standardising the nature and origin of the remoulded
‘fines’ (<425 μ m) for testing by wet-sieving the original peat.
Keeping the peat saturated will reduce the possibility of oxidation
effects influencing the results. Around 500 g of peat from each
failure site was placed in de-ionised water and stirred to a slurry,
then washed through a 425-μ m sieve using additional de-ionised
water (entirely as specified for soil in BSI 1998a). Large quantities
of water (e.g. >10 l) were needed to wash sufficient peat particles
through the sieve, requiring considerable time and energy to obtain
Fig. 8 East Cuilcagh. General view
from beyond the toe margin, May 2000
Fig. 9 East Cuilcagh. The upper part of the source area in the vicinity of the
sampling site, November 1998
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the peat sediment from the resulting suspension by evaporating off
most of the water. Further drying of the slurry-like sediment was
needed before the w L could be determined. An electric fan heater
was used for this; it was positioned far enough from the peat slurry
so that direct heat should not damage any of the particles.
Preparation for organic soils by wet-sieving using tap water (WST)
The purpose of using this method was to verify the observation of
Hanrahan et al. (1967) concerning the influence of water chemistry
on w L values. Details are the same as for WSD (above), but UK tap
water was used throughout instead of de-ionised water.
No preparation, undisturbed peat (NPU)
Although this method will clearly not provide w L values as such, in
it the small intact core samples of peat from each failure site were
subjected to the penetrometer test to indicate the influence of fibres and intact peat structures, even in H8–H10 peat in which
decomposition to an amorphous mass is most advanced, com-
pared with w L values determined using the other methods. Each
undisturbed sample was placed inside the standard brass contain-
er. A 2-mm gap between the sides of the sample and the inside of
the container was not seen to reduce due to sample deformation in
response to penetration of the drop-cone. Two measurements of
cone penetration were made from each core sample, roughly
20 mm apart on either side of the centre, and the corresponding
water content determined from 10 g subsamples from the
respective sides of the sample, giving 16 data points for each
location. Standardised replication of each penetration, as normally
required for each different water content (Appendix ), was not
appropriate for this method.
Two large undisturbed core samples of peat from the
Carntogher bog failure (Sperrin Mountains Northern Ireland) of
September 1993 (Wilson et al. 1996) were also available to us. These
were collected on 12 September 2004 with other samples for a
separate study. We used these for a supplementary test of the
‘sensitivity ’ of highly humified lower peat with some fibres, i.e. the
degree of loss of strength on remoulding. The 100 mm long ×
100 mm diameter cores were obtained from 1.25–1.35 m depth on
the southeastern margin of the failure scar where the current
maximum peat depth is around 1.4 m. An approximate indication
of the undrained strength of the undisturbed peat was obtained in
the laboratory using a field shear vane with the vane component
(comprising four blades each 32 mm high and 8 mm wide) attached
directly to the torque handle. A large core sample was securedupright and the vane was inserted to different depths at each of
four random horizontal positions in the sample. The peat was then
emptied from its core ring into a mixing bowl and broken up/
stirred to slurry. The shear vane was inserted into this slurry at six
random positions to obtain a reasonable estimate of the undrained
strength of the remoulded peat. The second core sample was then
used to obtain a replicate set of measurements.
Results
The general physical characteristics of the peat at each sample site
are shown in Table 3. The very strong similarity between these
peats must be emphasised. The potentially most important variant
Table 2 Summary of the different methods of sample preparation used in this study for liquid limit determination using the drop-cone penetrometer
Method(see text)
SPM SPO WSD WST NPU
Startingmaterial
500 g of field-wet peat 500 g of field-wet peat 500 g of field-wet peat 500 g of field-wet peat Intact peat cores 50 mm long× 50 mm diameter
Step 1 Air-dry at roomtemperature
Stir to slurry Stir to slurry inde-ionised water
Stir to slurry in tap water N/a
Step 2 Dry-sieve-pass 425 μmmesh
Remove fibres by hand usingtweezers
Wet-sieve-pass 425 μmmesh
Wet-sieve-pass 425 μmmesh
N/a
Step 3 Mix with de-ionised waterto form paste
Mix to paste (with addedde-ionised water if necessary)
Evaporate and collectsediment as paste
Evaporate and collectsediment as paste
N/a
Table 1 Humification of peat (after von Post 1922; Landva and Pheeney 1980; Hobbs 1986)
Degree of humification
Decomposition Plant structure Amorphousmaterial
Material passing between fingers whensqueezed
Nature of residue
H1 None Easily identified None Clear, colourless water
H2 Insignificant Easily identified None Yellowish water
H3 Very slight Still identifiable Slight Brown, muddy water; no peat Not pasty
H4 Slight Not easily identified Some Dark brown, muddy water; no peat Somewhat pasty
H5 Moderate Recognisable, but vague Considerable Muddy water and some peat Strongly pasty
H6 Moderatelystrong
Indistinct (more distinct aftersqueezing)
Considerable About one-third of peat squeezed out;water dark brown
H7 Strong Faintly recognisable High About half of peat squeezed out; anywater very dark brown
Fibres and roots moreresistantto decomposition
H8 Very strong Very indistinct High About two-thirds of peat squeezed out;also some pasty water
H9 Nearlycomplete
Almost unrecognisable Nearly all the peat squeezed out as afairly uniform paste
H10 Complete Not discernible All the peat passes between the fingers;no free water visible
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is the slightly lower humification with much higher content of
wood/shrub remnants at the Stony River bogflow. The much higher
hydraulic conductivity of the East Cuilcagh peat reflects the in situ
structure of the peat mass rather than the nature of the constituent
material, so it is unlikely to have any influence on the w L. Table 4
presents the ash contents of the peat used for the 15 w L
determinations. The similarity between all the samples (with oneexception) should again be noted, as should the extremely high
‘purity ’ of the peat (only previously represented in the literature by
an unidentified ‘Welsh bog ’ in Fig. 17 of Hobbs 1986). The
anomalously high ash content of one Stony River block sample
arises from the sample having been obtained from very close to a
highly irregular peat–substrate interface; the sample clearly
included some mineral substrate material.
The fact that the peat at all three sites appears to be largely the
same leads to the expectation that the w L of the peat might be
broadly similar at each site. The penetrometer test results for all the
w L determinations are presented in Fig. 10, and thesummary plot of
theliquidlimits obtained is shown in Fig. 11 (with the corresponding
data provided in Table 5). The similarity of most of the results
between the three sites is readily apparent, particularly with respect
to the NPU (yellow in Fig. 10), SPM (red) and WST (blue) methods,
and two of the WSD (green) methods. The WSD plot for the Stony
River peat is much lower than for the other two sites, but this canbe
explained by the high content of mineral matter that this sample
was found to contain (Table 4). This latter result is consistent with
published accounts that highlight a general inverse relationship
between w L and ash content (Skempton and Petley 1970; Hobbs
1986). But for this effect, it is reasonable to assume that the Stony
River WSD plot should be almost the same as the WST plot,as is the
case for the other sites. The least consistent results are those
obtained using the SPO method (black in Fig. 10), i.e. the method
used in the few previous determinations of the w L of peat
(Skempton and Petley 1970; Hobbs 1986).
The difficulty of controlling water content variations of peat to
satisfy the specified penetrometer test procedure (Appendix ) is
clear from the spread of points on each w L determination in Fig. 10.
Restricting the measured range of penetration depths to 15–25 mm
for each test would have produced some unreliable or unusableresults. Even with the extended range of penetration depths, the
wider scatter of points from the SPO test is apparent for each site.
The Stony River peat was generally less consistently amorphous in
character (Table 3), and this is reflected in the penetration–water
content relationships for the samples that were not wet-sieved.
Indeed, if the 15–25 mm penetration range was adhered to, no
result would have been obtained from the SPM or SPO methods.
The results from the new WST and WSD methods seem to
provide a more reliable basis for determining the w L of highly
humified blanket peat than the existing ‘standard’ method. The
results for each site are much closer together (except WSD-Stony
River, as discussed above), well within the spread of values
produced by the SPO method, and the order of values is the same:
thepeatfrom Geevagh has the highestw L in each case andthat from
Stony River has the lowest. These results appear to contradict
Hobbs’s (1986) conclusion that w L varies inversely with humifica-
tion, although that conclusion was based on results from fen peat.
In all cases, the WST and WSD w L results are exceeded by the field
water content of some of the peat at the time of sampling, following
a short spell of cool dry weather. Hanrahan et al. (1967) found that
using tap water instead of distilled water reduced their w L from 720
to 570%, but in our case the tap water produced slightly higher
values. The only comments we can make are that Hanrahan et al. ’s
results were obtained from unspecified remoulded peat with H4
humification (cf. H9–H10 in our case) and that there are insufficient
Table 3 Indicative values of the physical properties of the peat from the sample sites
Site Saturated watercontenta (vol.)
Dry bulk densitya
(g cm−3 )Saturated bulk densitya (g cm−3 )
Saturated hydraulicconductivitya,b (m s−1 )
Degree of humification(von Post) (Table 2)
Peat classification(von Post)
Geevagh 0.93 0.13 1.06 <10−11 H9–10 B4 F2 R1 W0
StonyRiver
0.90 0.15 1.05 <10−11 H8 B3 F2 R2 W3
EastCuilcagh
0.92 0.13 1.04 1×10−6 H10 B3 F2 R1 W0
Carntogher 0.95 0.12 1.00 2×10
−7
H10 B2 F2 R1 W0
B water content [1 (dry) to 5 (very high)], F fine fibres <1 mm diameter or width [0 (nil) to 3 (high content)], R coarse fibres >1 mm diameter or width [0 (nil) to 3 (high content)],
W wood and/or shrub content [0 (nil) to 3 (high content)] (Landva and Pheeney 1980; Hobbs 1986)aMean, n=6bVertical
Table 4 Mean ash content (%) of the peat samples that provided the results in Table 5 (n≥8)
Site NPU SPM SPO WSD WST Sensitivity tests
Geevagh 1.47 1.92 2.21 1.18 1.47
Stony River 1.60 1.47a 1.82 19.47b 2.27
East Cuilcagh 1.87 2.11 1.82 2.16 2.67c
Carntogher 3.10a
Using the relationship between organic matter content (100-percentage ash content) and specific gravity (Eq. 7 in Hobbs 1986), the ash contents correspond with specific
gravity=1.42an=6 for these valuesb Specific gravity is 1.55cSpecific gravity is 1.43
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The values of w L obtained from the basal catotelm of the Irish
blanket bogs (708–785%) are at the very lowest extreme for bog
peat values as identified by Hobbs (1986), which is consistent with
the very highly humified state of the test samples. Furthermore,
these values, combined with the ignition losses of >97% for almost
all samples, appear to correspond only with the published
properties of some Canadian peats (MacFarlane 1969). This is a
reflection of the paucity of data available to Hobbs (1986) and,
subsequently, Bell (2000) for their comprehensive reviews.
Our results indicate that under what were probably fairly typical
wetness conditions given recent weather before sampling, several
of the field water contents determined from the small intact cores
exceed the corresponding w L. The results were obtained fromblanket bog that had failed, so attention must be focussed on how
and why the high water content was able to mobilise the peat mass
into a flow (at Geevagh and Stony River if not East Cuilcagh). Small
amounts of additional water produce very large increases in the
water content and, therefore, large changes in the physical state of
the amorphous (colloidal) peat. Extreme rainfall events have been
implicated in some bogflow occurrences (e.g. Tomlinson 1981;
Wilson et al. 1996), although others followed wet periods
comprising several days of moderate or even low daily rainfalls
(Alexander et al. 1986; Hendrick 1990). Therefore, the nature of
peat must be considered further, particularly its retention of water
within the cells and the cell walls of the plant remains as well as in
the interparticle pores (Wilson 1978, cited in Bell 2000). Bell (2000)indicated that if stress is applied to peat, water will initially be
forced out of the pore spaces. However, if the saturated hydraulic
conductivity is as low as our results indicate (Table 3), then the
pore water pressure will instead increase in situ, possibly to the
point at which cellular water starts to be expelled into the pore
spaces (Bell 2000). With the overall water content already in excess
of the w L, rapid softening of the peat and associated deformation
and failure may then be expected (Wilson 1978, cited in Bell 2000);
the strength sensitivity of the Carntogher peat samples demon-
strates this loss of strength due to deformation and remoulding.
The susceptibility of Irish upland blanket bogs to large-scale flow
failure is thus explained, although the initiation of natural failures
in undisturbed bogs needs further research.Our findings also have implications for engineering projects on
upland blanket bogs. In the UK and Irish contexts, this relates
primarily to the construction of ‘wind farms’ that may comprise
several tens of turbines 100 m or more tall, together with access
roads capable of supporting the construction plant and turbine
delivery transporters. Unfortunately, the detailed reviews of peat
engineering properties by Hobbs (1986) and Bell (2000) reveal the
near-absence of relevant data. In particular, they highlight a
distinction between highly humified amorphous (granular) peat
and less humified fibrous peat. Most of the peat in Irish bogflow
scars comprises very highly humified amorphous material with
some or many fibres. This may mean that upland blanket peats
might exhibit responses to stresses that do not conform with the
expected generalisations of Hobbs and Bell. However, we can
accept the idea of pore water pressures within the voids of catotelm
peat being raised due to applied stress forcing water out of plant
cells, with failure by rapid creep or spreading mechanisms
associated with rapid softening being expected if the natural
water content exceeded the w L before the stress being applied.
Two further considerations apply. Firstly, excessive loads on
wind farm access roads would be transient, perhaps a few hours at
most in any place, so the critical sites would be those where long-term stress is created, for example where spoil material is dumped
onto in situ peat, with or without measures to spread the load over
a larger area. Secondly, Bell (2000) observed that even moderate
loads can induce serious shearing stresses in peat as settlement
occurs in association with creep or lateral spread as the peat
deforms towards adjacent unloaded/unconfined peat. If the load is
applied over a large area of peat, settlement of the peat beneath the
central part of the loaded area will not be accompanied by volume
change due to lateral deformations. The increase in pore water
pressures could result in rotational shear combined with rapid
softening if the water content >w L. This scenario probably gave rise
to the Canadian peat flow of 1983 (Hungr and Evans 1985) and a
similar failure of valley floor fen-transitional peat in the PeruvianAndes in early 2003 (Dykes and Kirk 2006). Other more recent
examples are known from Ireland, including two at the Derrybrien
wind farm, Co. Galway, in October 2003.
Conclusions
As with any other index test of material properties, determination
of the liquid limit of peat should be undertaken using a
standardised method. Given the distinctive nature of peat, this
standard method must take full and exclusive account of the
properties of the fine, chemically active, colloidal particles without
the test results being influenced by variable, researcher-dependent
quantities of fragments of residual fibres or woody material. Wet-
sieving the peat to obtain these fine particles would appear toprovide a more consistent—and therefore probably more reliable
—standard procedure than the existing, accepted method. Further
investigations are required to determine the influence of water
chemistry on the liquid limit of peat; results obtained using water
from the peat at the field site may be different again from those
obtained using tap water or distilled/de-ionised water.
The highly humified (H8 to H10) but very fibrous lower catotelm
peat from three blanket bogs in northwest Ireland had liquid limits
at the lower end of the range of values expected for bog peats and
Table 5 Liquid limit test results and field water contents (% dry mass) for peat samples obtained from the three failure sites
Site NPU SPM SPO WSD WST Field water content (n=16)
Geevagh 918 288 980 750 785 620–859
Stony River – 255 633 475 708 594–741
East Cuilcagh 1,023 327 786 719 767 655–990
Carntogher 621–798 (n=6)
Field water contents of the Carntogher peat are included for comparison
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had field water contents higher than their respective liquid limits.
All three bogs had failed, although one of these failures was
probably a bog slide rather than a bogflow. Given the high
sensitivity of these bog peats and the fact that water content is
often expected to exceed the liquid limit in the lower layers of all
blanket bogs, it appears that any initial disturbance of such peat
can give rise to widespread failure by strength loss on remoulding,
as cellular water is expelled into pore spaces by rapidly changing
local stress conditions. However, the nature of the‘initialdisturbance’ in naturally occurring bogflows is yet unknown.
In addition to the possible hazard arising from natural bogflows,
the occurrence of several failures associated with inappropriate land
management (e.g. excavating ditches, extracting peat) or engineering
works associated particularly with wind farm construction demon-
strates the need for a greatly improved understanding of the
geotechnical properties and behaviour of peat systems.
Acknowledgements
This research was funded by the University of Huddersfield. We
thank Edward Bromhead for constructive comments on an earlier
draft of this paper and the anonymous reviewer for suggesting
further improvements, and Nick Scarle for drawing Figs. 2, 3 and 4.
Appendix
Determination of the liquid limit using the drop-cone penetrom-
eter method (after BSI 1998a).
Details of this method can be found in standard textbooks such
as Head (1980) or Vickers (1983). Remoulded ‘soil’ (or other sample
material), with particles >425 μ m removed, is mixed to a paste with
distilled (or de-ionised; BSI 1998b) water, then carefully packed
(without trapping air) into a standard metal (usually brass) cup
with a flat bottom of 55 mm diameter and verticalsides 40 mm high.
The upper surface of the sample is leveled with the top of the cup.
The tip of the cone of a drop-cone penetrometer is positioned so
that it just touches the surface of the sample at its centre. The
standard 35-mm long conecomprises 80 g of polished stainless steel
forming a sharp point with an angle of 30°. The vertical position of
the cone is recorded to the nearest 0.1 mm from the scale fixedto the
penetrometer. The cone is then released to fall into the sample for
5 s, locking into its new position after this time. The new vertical
position of the cone is recorded to the nearest 0.1 mm and the
difference is recorded as the penetration in millimetres. The sample
paste is then re-mixed, re-packed into the cup and the test is
repeated. If both penetrations differ by ≤0.5 mm, the average is
recorded and the water content (% dry mass) of ca.10 g subsample
of the paste is determined by oven-drying to constant mass. If the
difference between the two penetrations is >0.5 but≤1.0 mm, then a
third test is done. If all three penetrations fall within a range of
≤1.0 mm, the average is recorded and the water content of ca.10 g
subsample is determined. If this criterion is not met, the entire test
for this water content is void and must be restarted.
This procedure is repeated for different moisture contents of
the original sample paste, with more distilled (or de-ionised) water
being added to the paste each time. A range of penetration values
should be obtained with at least two between 15 and 20 mm and
two between 20 and 25 mm, evenly spaced, so that a graph of water
content against penetration can be plotted. The liquid limit w L is
the water content that corresponds with a penetration of 20 mm on
the best-fit straight line on the graph.
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J. Yang
Geography, School of Environment and Development, University of Manchester,
Oxford Road,
Manchester M13 9PL, UK
A. P. DykesLimestone Research Group, University of Huddersfield,
Queensgate, Huddersfield HD1 3DH, UK
A. P. Dykes ())62 Lowerhouses Lane,
Huddersfield HD5 8JW, UKe-mail: [email protected]
Tel.: +44-1484-352714
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