Limite Liquido en Deslizamientos

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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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Landslides 3 • (2006) 205



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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Landslides 3 • (2006)206



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)




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

Original article




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




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

Original article




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




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

Original article




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

Original article




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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Limite Liquido en Deslizamientos

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