Effective stress strength testing of peat

Environmental GeotechnicsVolume 2 Issue EG1

Effective stress strength testing of peatO’Kelly

Environmental Geotechnics February 2015 Issue EG1Pages 34–44 http://dx.doi.org/10.1680/envgeo.13.00112Paper 13.00112Received 11/10/2013; accepted 10/03/2014Published online 01/04/2014Keywords: codes of practice & standards/failures/strength and testing of materials

ICE Publishing: All rights reserved

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It has been approximately 80 years since the first effective stress strength testing of peat was performed using triaxial

apparatus. In light of recent unexpected failures of embankments, dykes, foundations and slopes in peat deposits, it is

timely to review current laboratory practice and also provoke discussion about the best way forward for understanding

and determining the effective stress strength properties of peat. Compared with fine-grained mineral soils, significant

fabric and structural differences (including the porous, compressible nature of the organic solids themselves) make the

direct application of classical soil mechanics strength models doubtful for peat. Uncertainties and difficulties of effective

stress testing of peat using standard strength apparatus are discussed. Compared with triaxial compression, direct

simple shear testing appears to provide conservative estimates of the strength parameter values, and the specimen

deformation (failure) mode more closely represents that occurring in translational planar slides. It is recommended

that the geotechnical profession investigates alternative theoretical frameworks to explain and predict peat strength

(and compression) behaviour along with the development of new apparatus and methodologies for determining

appropriate parameter values.

Brendan C. O’Kelly PhD, FTCD, CEng CEnv MICEAssociate Professor, Department of Civil, Structural and Environmental Engineering, Trinity College Dublin, Dublin, Ireland

Effective stress strength testing of peat

Notationc¢ apparent cohesionFC fibre contentm scaling factor (0 £ m £ 1)z deptha orientation of the major principal stress to the vertical

directionb slope angleea axial strainf¢ effective angle of shearing resistanceg bulk unit weightgw unit weight of waterσ ¢v effective vertical stress

hσ ¢ effective horizontal stress (confining pressure)1σ ¢ effective major principal stress 3σ ¢ effective minor principal stress (cell pressure in triaxial

apparatus)t shear stresstmax maximum shear stress

IntroductionPeat deposits are formed by the gradual accumulation of the remains

of dead plant vegetation under waterlogged conditions. The physical

nature and origins of peat deposits are explained from an engineering

perspective by Hobbs (1986). Morphological differences arise

from the circumstances surrounding the peatland formation and its

constituent plant types. The differences extend to structure, fabric,

humification and proportion of non-organic constituents, factors

that have a considerable impact on engineering behaviour (Hobbs,

1986). For instance, raised bogs get their nutrients from rainwater,

resulting in a particular type of vegetation growth, and are made

up almost entirely of organic material. Other bogs are fed from

groundwater that can contain a greater proportion of inorganic

particles, generally referred to as mineral particles in this context.

The nature of the peat can also be affected by other external factors,

for example, atmospheric dust or grazing and burning of a bog over

a period of time. Organic content, generally determined by loss on

ignition of the oven-dried material, can be used as a measure of the

purity of the peat. For geotechnical purposes, Landva et al. (1983)

defined peat as soil with an ash content of <20%.

The overall fabric of peat comprises an assemblage of decaying

plant cellular structures (these are the elementary structures) that

are interconnected by frequent fibres (plant leaf stalks, stems and

root(lets)) in a less decayed state. The reader is referred to the

paper of Landva and Pheeney (1980), which presents a series of

electron microscope photographs of Sphagnum and sedge peats in

various stages of decomposition. The fibres, comprising a mixture

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of coarse and fine parts, also have an open cellular structure (e.g.

see Figure 1). In the case of fibrous peat, between one-third and

two-thirds of the water content of this material is contained within

the fibres themselves (Landva and Pheeney, 1980; O’Kelly and

Zhang, 2013). Hence, peat material generally has extremely high

water content, ranging from a few hundred per cent to greater

than 2000% on a dry mass basis. Unlike pure frictional contacts in

uncemented mineral soils, connectivity between fibres is provided

by cellular connections and fibre entanglement. The cellular

structure also deforms when transferring interparticle forces.

Initially, decomposition of the cellulose structure affects the plant

leaves, with the soft inner cell walls being attacked first, but this

process can develop to the stems and roots of the dead plants,

eventually leading to complete decomposition, which produces

an amorphous material consisting of mainly gelatinous organic

acids that have a sponge-like structure (Hobbs, 1986; Landva and

Pheeney, 1980; O’Kelly and Pichan, 2013). However, complete

breakdown of the plant remains is rare in colder climatic regions.

The change in the peat fabric as humification increases is not

uniform, with considerable variations in geotechnical and hydraulic

properties expected over short distances (Hobbs, 1986; O’Kelly and

Pichan, 2014; O’Kelly and Sivakumar, 2014). This is explained by

considering the growth patterns of the plant vegetation, with plants

of different character living in communities, and the subsequent

non-uniform decomposition rate occurring throughout the deposit.

Peat is a difficult and unconventional geomaterial, exhibiting low

virgin shear strength and very high compressibility, which presents

significant challenges in ground engineering. Engineering problems

with peats have tended to be dominated by issues associated with

undrained shearing resistance and settlement, with stability studies

usually driven by consideration of the undrained shear strength

of the peat. Secondary compression often accounts for the major

portion of the total deformation response under sustained loading.

In recent years, unexpected failures of peat embankments (den Haan

and Feddema, 2013) and containment dykes (McInerney et al., 2006;

van Baars, 2005) and natural peat slopes (Long and Jennings, 2006)

have focused greater attention on understanding the properties and

mechanical behaviour of peat. Excessive settlement that necessitates

increased amounts of maintenance is not uncommon for peat deposits

acting as pavement subgrades or foundations for roads (Osorio et

al., 2008) and railroads (Hendry et al., 2012). In The Netherlands,

many dykes are constructed of peat and can also be founded on peat

or other highly organic soil deposits (den Haan and Feddema, 2013;

van Baars, 2005). Ongoing monitoring and stability assessment of

these structures are of significant importance. Catastrophic peat

landslides have increased awareness of this geohazard. For instance,

Figure 2 shows major landslide events that occurred in upland

blanket bog at Pollatomish, County Mayo (Long and Jennings,

2006) and Derrybrien, County Galway (Lindsay and Bragg, 2005),

Ireland, during the autumn of 2003. The Pollatomish event (Figure

2(a)) comprised about 50 separate translational planar-type slides of

peat and weathered rock, with individual slides involving between

15 and 20 000 m3 of material. These slides caused considerable

damage to roads, bridges and property and led to the evacuation

of over 40 families. The Derrybrien event originated as a slide but

degraded into a debris flow as it moved downslope, involving 450

000 m3 of peat material. Its track passed through mature forestry

(Figure 2(b)), continuing along a natural drainage line to a stream-

head, following the course of the stream, until its toe initially came

to rest 2·45 km away from its upper limit. The width of the failure

scar ranged from ~45 m at the head to a maximum of ~270 m some

750 m downslope. Re-activation of the bog slide about 10 days after

the initial event led to a rapid flow of peat (Figure 2(c)) down to

the Owendalulleegh River and onward for a further 20 km to enter

Lough Cutra.

In the wake of such peat slide incidents and the increasing number

of onshore wind farms being developed in peatland sites, there is a

greater focus on peat hazard risk assessment (Scottish Executive,

2006), including a requirement for effective stress slope stability

analysis. For instance, the relatively simple infinite-slope stability

analysis (see Equation 1 after Haefli (1948) and subsequently

Skempton and De Lory (1957)) is suited to determining the factor of

safety (FoS) against translational planar slides occurring at a blanket

bog site, assuming steady seepage occurs in the direction parallel

to the slope. As the failures in this case may be due to higher than

normal water pressures, any stability analysis of potential slides

must be undertaken using effective stress strength parameters. Some

case studies that adopted this approach include Carling (1986),

Warburton et al. (2003) and Long and Jennings (2006)

1. w

( ) tanFoS

sin cos tan

c m

z

γ γ φγ β β γ β

-¢ ¢= +

Cellular connections

SEM HV: 20·00 kVSEM MAG: 193×Date(m/d/y): 08/31/11

WD: 24·6420 mmDet: BE DetectorCMA

500 µmDigital Microscopy Imaging

MIRA\\ TESCAN

Finefibre

Fine fibre

Coarse fibre

Open cellular structure

Figure 1. Moderately humified fibres taken from an Irish Sphagnum peat (adopted from O’Kelly BC and Zhang L, Consolidated-drained compression testing of peat, Geotechnical Testing Journal, 36, (3), 2013)

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where b is the slope angle measured relative to the horizontal; z

is the depth of the sliding mass; g and g w are the bulk unit weight

of soil (peat) and water, respectively, with the groundwater table

running parallel to the slope and located at a height of mz (0 ≤ m ≤ 1)

above the failure plane.

In The Netherlands, effective stress strength parameters have mostly

been used to analyse embankments and dykes on organic soils (den

Haan and Feddema, 2013). The effective stress approach is also used

in finite-element (FE) modelling; for example, analysis of highway

construction in peat bog using the Soft Soil Creep model in PLAXIS

FE software (Tan, 2008) and analysis of the measured response of

peat foundations underlying railway embankments to heavy axle

loads using GeoSlope software Sigma/W (Hendry, 2011).

However, the fibrous nature of peat and its extremely high water

content, which make it very difficult to sample and extremely

compressible when loaded, have challenged the geotechnical

profession in seeking to determine representative values of pertinent

strength parameters from in situ tests or, in particular, from tests

on specimens in the laboratory. Apparatus and testing methods

employed in the laboratory to determine the effective stress strength

properties of peat have, in principle, generally remained unchanged

and are, in the main, the same as used for fine-grained mineral soils,

namely, direct shear (DS) (i.e. shear box), triaxial, ring shear (RS)

and direct simple shear (DSS) (Figure 3).

These standard strength apparatus and testing methodologies do not

give special consideration to the fibre content, high compressibility

or relatively high initial permeability and gas content of fibrous

peats (Farrell, 2012). For instance, Landva et al. (1986) reported

that shear box testing was not particularly useful for fibrous

peat on account of the uncertain stress distribution and mode of

specimen deformation. Another drawback is that axial compression

September 2003; (b) Slieve Aughty Mountains, Derrybrien, County Galway: 16–18th October 2003; (c) debris flow, Derrybrien, County Galway: 28th October 2003

Figure 2. Major landslide events in upland blanket bogs on the west coast of Ireland (all images courtesy of Dr. Eric Farrell): (a) Dooncarton Mountain, Pollatomish, County Mayo: 19th

Minor roadsHouses

Sea

(a)

(b)

(c)

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of the specimen on account of significant secondary consolidation

occurring during the drained shearing stage means that the specimen

shear plane must try and continuously realign upward with respect

to the mid-height (i.e. predetermined shear plane) of the shear box.

A major technical hurdle in evaluating peat strength is that, due to

the low bulk unit weight of peat of typically 9·5–11·5 kN/m3, the

effective stresses occurring in the field are lower than the lower

bound stress-level capabilities of most testing equipment. Bulk unit

weights are low on account of the low specific gravity of solids

and the buoyancy effect caused by trapped biogas. Hence, in situ

effective stresses are generally very low or negligible for a horizon

only constituted of peat. It is usually not possible to simulate and

measure these low levels of stress with accuracy in standard strength

apparatus. The most accurate pressure-controlling devices in state-

of-the-art triaxial setups are usually only capable of resolving

to about ±2 kPa. Rather than, or in addition to, controlling or

measuring absolute pressures, a differential pressure transducer can

be used to more accurately determine, for instance, the differential

pressure acting between cell and back-pressure controlling devices

or discrete measurement points (e.g. see O’Kelly and Naughton,

2009). Accordingly, one is usually compelled to test at stress

levels greater than the in situ values and then extrapolate the

results downward to the in situ levels. For this reason, much of

the knowledge gained about peat strength has been for effective

stress levels significantly greater than those encountered in situ.

Pore pressure measurements may be influenced by the presence of

biogas trapped within the peat test-specimens.

From a review of the literature, no comprehensive study has been

performed concerning specimen scale effects in the strength

measurement of peat or definitive conclusions drawn in this regard. In

an intact state, the peat fibres themselves have relatively high tensile

stiffness and strength and they also provide conduits for preferential

flow of water (Hobbs, 1986; O’Kelly, 2007). The orientation of the

fibres in undisturbed peat is in a predominantly horizontal direction

on account of the manner in which the dead plant vegetation was

laid down and subsequently consolidated to large strains under the

overburden and any applied loading (Hendry et al., 2012; Yamaguchi

et al., 1985). This results in undisturbed peat effectively behaving as

a cross-anisotropic material.

Although peat is a decaying mass of organic material having

extremely high water content, as described earlier in this paper,

geotechnical calculations routinely involve effective stress strength

analysis, with the general consensus that the principle of effective

stress and routinely used soil mechanics strength models correlate

with the mechanical behaviour of peat to a sufficiently high degree.

However, Zhang and O’Kelly (2014) pointed out that no complete

experimental or in situ proof or rational theoretical explanations

have been put forward to validate the appropriateness of effective

stress theories for peat. Unlike shearing at frictional contacts in

uncemented mineral soils, the strength of peat is derived from the

connectivity between elementary structures; that is, failure mostly

involves tearing of elementary structures, entangled fibres and

cellular connections (Boylan et al., 2008; Landva et al., 1986).

This led Boylan et al. (2008) to postulate that the direct application

of classical soil mechanics strength models may be doubtful for

peat. Furthermore, in common with other unsaturated soils, if the

peat test specimens are not fully saturated on account of trapped

biogas generated by ongoing decomposition, then the Mohr–

Coulomb failure envelop will have a concave curvature, becoming

less steep for higher confining pressure (O’Kelly, 2013). Hanrahan

(1954) reported that the gas content in Irish Sphagnum peats may

be considerably greater than 5%, and for this degree of saturation,

most of the gas occurs as occluded bubbles.

Hence, it is timely to review the current practice for effective stress

strength testing of peat material in standard triaxial, RS and DSS

apparatus. The aim is to provoke discussion about the best way

forward for understanding and determining the effective stress

strength properties of peat. This discussion is also pertinent to other

highly organic and (or) fibrous soils. Drained shear box testing is

not included in this discussion since, as outlined earlier in the paper,

this approach is not particularly useful for effective stress strength

determination (Landva et al., 1986). Some central questions include

the following:

(a) Are the effective stress strength parameters c¢ and f¢ (apparent cohesion and effective angle of shearing resistance

respectively) appropriate for peat and are their values stress-

level dependent?

(b) Can pertinent effective stress strength and stiffness parameter

values be obtained from element tests?

Apparatus Orientation of major principle stress

Triaxial

α = 0º 0º < α < 90º α = 90º

σv’ σ1’

σ3’σ3’σh’σh’

σ3’

σ1’ σ1’

Triaxial compression Triaxial extension

σv’ σh’σv’ σh’

N/A

N/A

N/A

N/A N/A

N/A N/A

Ring shear

Direct simple shear

σv’ σv’

σn’ σn’

τ

ττ

τ

Direct shear (shear box)

Figure 3. Specimen stress, deformation and boundary conditions for some strength apparatus (N/A: not applicable)


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(c) Considering the constituent peat fibres and high secondary

compression rates, are the values of c¢ and f¢ significantly

affected by scale effects (e.g. what is the significance of fibre

tensioning in relation to the nominal specimen dimension and/

or aspect ratio) and (or) strain rate effects?

Standard triaxial strength testingThe first triaxial testing device was developed by von Kármán

(1910, 1911) and was used to test brittle rocks. Westerberg (1921)

adapted the method for soft clays. Ehrenberg (Ehrenberg and

Tiedemann, 1928), working at the Department of Foundation

Engineering, Prussian Laboratory of Hydraulics and Shipbuilding,

Berlin, developed a ‘cell device’ embodying the essential features

of current triaxial apparatus, introducing the test for soil mechanics

generally. Subsequent devices for the testing of soils evolved

simultaneously in Germany (Seifert and Körner, 1933); the

Technical University Delft, The Netherlands (Buisman, 1934); the

USA (Housel, 1936; Jürgenson, 1934; Stanton and Hveem, 1934);

and von Terzaghiʼs Soil Mechanics Laboratory in Vienna, Austria

(Rendulic, 1935, 1936). Arthur Casagrande appears to be at the root

of triaxial testing in the USA, having developed triaxial apparatus,

starting in 1930 (de Boer, 2005) at the Massachusetts Institute

of Technology and subsequently when he moved to Harvard

University. Of these early triaxial studies, the paper by Buisman

(1934) reports the first triaxial testing of peat (veen), which was

performed using the Dutch Cell test apparatus. This differs from

conventional triaxial apparatus mainly by the cross section of the

loading piston being equal to that of the test specimen (see den

Haan and Feddema (2013)). Reviews of more contemporary triaxial

investigations of peat, beginning with Hanrahan (1954), have been

presented by Landva and La Rochelle (1983), Long (2005) and

O’Kelly and Zhang (2013).

For the standard triaxial apparatus, basically two methodologies can

be employed to determine the effective stress strength properties

(refer to BS1377: Part 8 (BSI, 1990)). Consolidated undrained (CU)

testing can be performed in which test specimens under different

confining pressures are allowed to consolidate to target levels of

effective stress, followed by a displacement (strain)-controlled

shearing stage, in either triaxial compression or extension, with

measurement of the specimensʼ deviatoric stress and pore water

pressure (pwp) responses against axial strain. The applied axial

strain rate is sufficiently slow for substantive equilibration of

excess pwp to occur throughout the test specimen.

Alternatively, after consolidation to different target levels of

effective stress, the specimens can be sheared in a drained

condition at a sufficiently slow rate for substantive dissipation of

excess pwp generated during the shearing stage, thereby allowing

direct measurement of the effective stress shear response, that is,

consolidated drained (CD) testing. Since, for a given material,

the shearing stage of CD triaxial tests takes significantly longer

to perform, CU triaxial testing along with pwp measurement is

generally performed instead in practice. A minimum of three triaxial

tests performed under different effective confining pressures are

necessary in order to locate the Mohr–Coulomb failure line (curve)

and hence determine the values of the effective stress parameters

c¢ and f¢ for the soil under consideration. Triaxial testing provides

close control of the specimen stress and boundary conditions,

although relatively large corrections may have to be applied to

the measured deviatoric stress values in order to account for the

restraining effect of the rubber membrane and also the reinforcing

effect of filter paper drains, if fitted around the side of the test

specimen.

Undrained triaxial compression testingDifficulties arise in performing standard triaxial testing on peat and in

the interpretation of the experimental data. During the consolidation

stage, gross changes occur in the shape and volume of the test

specimen (Edil and Wang, 2000; Long, 2005). During undrained

triaxial compression, induced pwp can build up and approximately

equal the applied cell pressure for axial strains ea > 5%−10%

(Boulanger et al., 1998; Farrell and Hebib, 1998; Yamaguchi et

al., 1985). Cola and Cortellazzo (2005) demonstrated that as the

effective minor principal stress ( 3σ ¢) approaches zero, the specimen

membrane could easily expand and the pore water move toward the

lateral boundaries of the test specimen. With the maximum strength

mobilised for ea >> 15%−20% and a small 3σ ¢ value (Hanrahan, 1954;

Marachi et al., 1983), interpretation of the experimental data from a

suite of three or more CU triaxial compression tests for the purpose

of determining the values of c¢ and f¢ is difficult and likely to result

in surprisingly high f¢ values in the range of 40°–60° (Farrell and

Hebib, 1998; Landva and La Rochelle, 1983; Long, 2005; Mesri

and Ajlouni, 2007). This compares with the measured f¢ values of

generally less than 35° for soft clay and silt compositions.

The reported f¢ values are generally greater for higher organic

content (den Haan and Feddema, 2013). It is possible that during

undrained triaxial compression, higher creep rates expected for

higher organic content soils would develop higher excess pwp and

(a)

σ1’

φ’

σ3’ » 0

(b)

For c’ = 0,

WL

ß = φ’

ß

Figure 4. Interpretations for f¢ = 90o (WL, water level): (a) drained unconfined compression; (b) submerged semi-infinite slope with face angle b at limiting equilibrium

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hence lower (approximately zero) values of 3σ ¢ at specimen failure,

that is, analogous to unconfined compression. In this scenario, the

best-fit Mohr–Coulomb failure line would produce higher f¢. Values

of up to 90° have been reported from CU triaxial compression testing

of peat (den Haan and Feddema, 2013), although this implies one-

dimensional (1D) compression (Figure 4(a)) rather than shearing in

the conventional sense. Intuitively, it would seem that such high f¢ (i.e. angles of repose of up to 90°) cannot be mobilised, considering

for instance a semi-infinite submerged peat cutting in the field

(Figure 4(b)). Another case, for example, would be a smooth

vertical wall retaining saturated fibrous peat for which Rankine

active and passive wedge failure planes acting at 90o (i.e. 45 ± f¢/2)

have no physical meaning. It is acknowledged in reality that such

high f¢ values cannot be mobilised for peat at larger scale in the

field. For instance, den Haan and Feddema (2013) reported that

empirical methods are used to obtain acceptable, reduced values of

f¢ from triaxial tests for use in stability analysis; for example, f¢ is

determined for e a = 2%–5%, which is well below specimen failure.

Drained triaxial compression testingFor fibrous peats in CD triaxial compression, shearing in the

conventional sense usually does not occur, with the test specimen

essentially undergoing 1D consolidation. Unlike fine-grained

mineral soils, the deviatoric stress 1 3( )σ σ-¢ ¢ and principal stress

ratio ( 1 3/σ σ¢ ¢) responses continue to increase approximately linearly,

without reaching a peak value (Farrell, 2012; Hollingshead and

Raymond, 1972; O’Kelly and Zhang, 2013), that is, the f¢ value

is strain-level dependent. For the purposes of analysis, 20% axial

strain is often taken as an arbitrary failure criterion, although it is

unclear how this value relates to the field condition. There may be

scope for using relatively taller triaxial test specimens (i.e. greater

than the standard height to diameter ratio of 2:1), which along with

necessary modifications to the triaxial apparatus and instrumentation

would allow for the application of e a >> 20% while still maintaining

a reasonable specimen aspect ratio the possibility of shear failure

to occur.

Inconsistencies between results of CU and CD triaxial testing for peatPhysically identical test specimens of fine-grained mineral soil

consolidated under the same drainage conditions and effective

confining pressure would be expected to produce similar effective

stress strength responses and hence similar effective Mohr–

Coulomb failure envelopes for undrained (with pwp measurement)

and drained triaxial compression. However, in the case of fibrous

peat, significant differences may be expected to occur on account of

fibre effects, creep and the extremely high water content.

In terms of the effective stress strength response, compression of

undisturbed fibrous peat in the vertical direction mobilises internal

tensile reinforcement provided by the predominantly horizontally

orientated fibres. For the drained condition, the high frictional

resistance mobilised between the fibres and with the elementary

structures under the effective normal stress causes the specimen

to respond by undergoing approximately 1D compression instead

of general ductile bulging normally associated with fine-grained

mineral soils. As described earlier in this paper, during undrained

compression, high creep rates cause pwp values to rapidly build

up, approaching the applied cell pressure for e a > 5%–10%, thereby

mobilising low (approximately zero) values of 3σ ¢ at specimen

failure. Fibre reinforcement effects diminish under reducing

effective normal stress, with the frictional resistance between the

fibres and elementary structures only partly mobilised or not at all,

depending on the magnitude of the excess pwp developed. Hence,

these specimens deform by more general ductile bulging, with the

0

20

40

60

80

100

Axial strain: %

Dev

iato

ric s

tres

s: k

Pa

R

R

R

PPU

U

0 5 10 15

(a)

20 25 30 35

1 cm

(b)

Figure 5. Drained triaxial compression of pseudo-fibrous Sphagnum peat specimens for σ ¢3 = 30 kPa (O’Kelly and Zhang, 2013): (a) deviatoric stress against axial strain for three undisturbed (U), five reconstituted (R) and three pureed (P) peat specimens (note: only a selection of the 11 data plots shown have been labelled); (b) condition of reconstituted test-specimens at end of compression stage (e a = 20%)


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peak deviatoric stress mobilised for e a ≈ 15%–25%. For instance,

the reported effective Poisson’s ratio values of 0·10 (Zwanenberg,

2005), 0·11 (Rowe et al., 1984) and 0·15 (Rowe and Mylleville,

1996; Tan, 2008) for undrained triaxial compression are considerably

greater than the ranges of 0·02–0·03 and 0·04–0·05 deduced from

drained triaxial compression of undisturbed and remoulded fibrous

peat respectively (O’Kelly and Zhang, 2013). The internal restraint

provided by the fibres is demonstrated by considering that the

pureed (blended) peat material had a reported effective Poisson’s

ratio range of 0·13–0·16 in drained triaxial compression. For

CU triaxial compression of fibrous peat, the usual 1 3/σ σ¢ ¢ range

encountered is 10–100, with such high values primarily arising on

account of the low 3σ ¢ values mobilised. On the other hand, the 1 3/σ σ¢ ¢ ratio is normally in single figures for drained triaxial compression;

for example, O’Kelly and Zhang (2013) found that for pseudo-

fibrous Sphagnum peat tested under 3σ ¢ = 30 kPa, this ratio increased

approximately linearly in value from unity to 3·7 for e a = 0%–30%.

Intuitively, one would also anticipate higher f¢ values for more

structured/fibrous material. However, O’Kelly and Zhang (2013)

found that testing of normally consolidated pseudo-fibrous peat

specimens of the same morphology in drained triaxial compression

produced approximately similar deviatoric stress–strain responses

for undisturbed, remoulded (FC = 64%, where FC is the fibre

content (ASTM, 2008)) and pureed (FC = 17%) materials (see

Figure 5). Similar findings have been reported for fibrous and

amorphous peats by Edil and Wang (2000).

The major difficulties of build-up of excess pwp causing 3σ ¢ to

approach zero in undrained compression, the internal restraint

provided by fibres preventing specimen shearing in the conventional

sense in drained triaxial compression and the gross changes in

specimen shape and volume occurring during triaxial testing of fibrous

peat have led many researchers, including Landva et al. (1986) and

Zhang and O’Kelly (2014), to conclude that neither standard CU nor

CD triaxial testing is of particular value in determining the effective

stress strength properties of (fibrous) peats. In an age where FE

methods and models are in common use, it is not only shear strength

that we are interested in; rather, the stress–strain behaviour in general

needs to be known. In light of inconsistencies between the effective

stress strength responses for undrained (with pwp measurement) and

drained triaxial compression highlighted earlier in this paper, it is

probable that triaxial testing of (fibrous) peat is also of limited value in

the determination of effective Young’s modulus and Poisson’s ratio

values. Further research is necessary in this regard.

Another consideration is the relative change in physical state of the

organic solids themselves occurring during undrained and drained

compression. Compared with undrained compression, further

significant reductions in volume of saturated test specimens occur

during drained compression on account of the high secondary

consolidation taking place and longer stage duration (slower rate of

shearing) required to prevent build up of excess pwp. This additional

volume change occurring during drained triaxial compression alters

the physical state of the organic solids on account of the flexible

and compressible nature of their cellular structure, that is, further

compression of the organic solids themselves occurs.

Other strength measurement approachesOther laboratory strength test-methods have been used to determine

the effective stress strength properties of fibrous peats, including

drained DS, undrained DSS with pwp measurement and drained RS

(see Figure 3). As explained earlier in this paper, drained DS (shear

box) testing is not particularly useful for fibrous peat (Landva

et al., 1986) and therefore is not discussed further. RS testing is

used to study peat at large shear strains for which fibre effects

are eliminated (Landva and La Rochelle, 1983). In the author’s

experience, it is rather difficult to prepare annular test specimens of

undisturbed fibrous peat for RS testing, with commercial apparatus

typically accommodating 100-mm outer diameter by 70-mm inner

diameter by 20-mm high specimens (McInerney et al., 2006).

DSS testing shows much promise, particularly in the determination

of the effective strength parameter values for translational planar

slides involving peat. Rather than using a split shear box, the side

walls of the simple shear apparatus are usually formed of either

plates that can rotate or a series of laminations that can move

relative to each other or are flexible. In theory, such arrangements

allow rotation of the sidewalls of the test specimen, which imposes

a pure state of simple shear. For fully saturated test specimens, the

change in vertical stress that occurs during shearing in constant

volume DSS tests is assumed to equal the change in pwp that

would have occurred had the test been performed truly undrained.

Although confirmed by Dyvik et al. (1987) for saturated normally

consolidated (Drammen) clay below 10% shear strain, according

to Boylan and Long (2009), this assumption has not been verified

experimentally for DSS testing of peat which often involves

applying significantly greater shear strains. Nevertheless, constant

volume DSS testing of peat (shearing stage maintains a constant

specimen height) with pwp measurement is routinely performed in

the same manner as for clays (den Haan and Kruse, 2007; Farrell and

Hebib, 1998) in order to determine undrained and effective stress

strength properties. Due to the strong anisotropy of undisturbed

fibrous peat, f¢ values of typically 30°–35° are obtained from

DSS testing (shearing occurs in the horizontal direction along

the predominantly horizontal-orientated fibres), compared with

40°–60° generally measured in CU triaxial compression (Farrell

and Hebib, 1998; Long, 2005). The reported DSS values were

deduced from the peak horizontal shear stress mobilised during

shearing or alternatively the shear stress measured at 15% shear

strain, whichever occurred first (Boylan and Long, 2014). Farrell

and Hebib (1998) reported that clear peak values may not be

achieved, even for shear strains > 20%. Another concern is the

non-uniform shear strain developed throughout the test specimen.

Localisation(s) (ruptures) may develop above 15% shear strain,

and hence the simple shear condition would no longer exist. A

central question therefore is how the f¢ values determined for these

‘failure’ criteria in DSS testing relate to field conditions involving,

for instance, translational planar slides. Another issue concerns

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the interpretation of the experimental data; for example, whether

to take the strength at the peak shear stress t max or the maximum

max v/τ σ ¢ ratio (Farrell, 2012), where vσ ¢ is the corresponding

effective vertical stress.

As explained earlier in this paper, a major technical hurdle in

evaluating peat strength is that the effective stresses in the field

(often <5 kPa) are usually lower than the lower-bound stress level

capabilities of commercial DSS equipment. Consolidation of peat

specimens under these low vertical effective stresses demands much

greater accuracy in measurement and control systems. Shearing

also becomes more complicated because slippage at the interface

between the top (or bottom) cap and the specimen is more likely to

occur and is further facilitated by the materialʼs high water content

(Grognet, 2011). Slippage increases stress nonuniformity in the

test specimen and should be prevented from occurring. Corrections

related to apparatus compliance and friction can form a large

percentage of the measured strength. The device needs to have the

lowest friction possible in its carriage and measurement gauges and

also the lowest resistance possible for the boundary (i.e. membrane,

rings) enclosing the test specimen (Grognet, 2011). DSS prototypes

designed to test peat at low effective stresses have been reported by

Boylan and Long (2009) and Grognet (2011). The sidewalls of the

assemblies used to house the test specimens in these apparatus are

transparent and make possible a visual assessment of the specimen

deformation and insight into the evolving failure mechanism

during shearing using particle image velocimetry image analysis

techniques. Further research on a wide range of peats using such

devices is required to identify the criteria for assigning failure with

evolving non-uniform specimen deformation. Larger specimens

than for usual testing and also investigations for specimens having

larger height to diameter ratio are desirable to investigate the

influence of the peat structure (fibres) on the test results (Boylan

and Long, 2009; den Haan and Kruse, 2007).

Other observations

In relation to the porous and compressible nature of the organic solidsFor most mineral soils, the constituent solids are incompressible over

the stress range of engineering interest. Hence, a pertinent question

relates to the often extremely compressible nature of the organic

solids in peat. Following the two-level structure assumption of micro-

and macropores (Berry and Poskitt, 1972; Dhowian and Edil, 1980),

individual peat fibres will exist in significantly different physical

conditions for different strain levels on account of the associated flow

of micropore water (i.e. contained within the porous cellular structure

of the fibres) into the void space between the solids. As reported

earlier in this paper, between one- and two-thirds of the initial water

content of fibrous peat is contained within micropores. Given that the

solids in peat can undergo significant physical changes, especially

for the slower strain rates applied in drained triaxial compression,

the author postulates that deduced effective stress strength parameter

values may depend on the stress level during the compression stage.

Since the creep rate is a function of effective stress, with significant

differences expected between effective stress levels above and below

the apparent pre-consolidation pressure (O’Kelly, 2006), the author

postulates that f¢ is likely to be strain-rate dependent, possibly

slightly lower in value for effective stresses above the apparent pre-

consolidation pressure. Further research is warranted in this regard.

In relation to shearing rate (time to failure)It has been well documented that for triaxial consolidation, the

‘end of primary’ condition was more reliably established from

analysis of pwp dissipation against time data rather than from

volumetric strain against time (Edil and den Haan, 1994; O’Kelly

and Zhang, 2013). For peat, ‘end of primary’ refers to the soil state

corresponding to the substantive dissipation of excess pwp. In

other words, axial strain rates for the triaxial compression stage

deduced from measured volumetric strain against consolidation

time responses were unnecessarily slow (conservative). For

instance, O’Kelly and Zhang (2013) found that for pseudo-

fibrous Sphagnum peat tested under drained triaxial compression

with 3σ ¢ = 30 kPa, the estimated axial strain rates deduced from

volumetric strain data were about an order of magnitude slower

than the permissible values based on analysis of pwp dissipation

against time data. Also, as described earlier in this paper, it was

not uncommon for the resistance of fibrous peat in drained triaxial

compression to continue increasing for e a > 30% without reaching

a peak (O’Kelly and Zhang, 2013). For mainly pragmatic reasons

(e.g. limiting travel of specimen loading piston and (or) time

constraints for completing the tests), the triaxial compression

tests were often terminated at 20% axial strain, with this arbitrary

strain value selected as representing some notional specimen

failure. In other words, a conservative assumption had to be

made regarding the strain at failure before calculations could be

performed to estimate the axial strain rate subsequently applied

during the compression stage. The actual strain rate applied was

a compromise between the higher strain rates allowable with

filter paper drains fitted around the sidewall of the test specimen

(although significant corrections to the measured deviatoric stress

response were then required) or slower strain rates that would

allow greater secondary consolidation and hence a greater change

in the physical state of the organic solids to occur.

In relation to apparent cohesion c ¢Values of c¢ = 0 have been reported for normally consolidated

fibrous peat by many researchers, including Marachi et al. (1983)

and Farrell and Hebib (1998). In the case of waterlogged peat

slopes, the effective stress can be very small or negligible. Hence,

the resisting force largely depends on the apparent cohesion, if this

is present. From Equation 1, it is clear that for a typical blanket

bog site (with the bulk unit weight of the peat similar to that of

water and the groundwater table high (i.e. m ≈ 1)), the FoS value

against translational planar slides is much more sensitive to the

value of c¢ than f¢. Given that c¢ is established by projecting the

Mohr–Coulomb failure line (curve) back to the shear stress axis, its

value is intimately dependent on the interpretation of f¢. However,

as described earlier in this paper, the definition of failure for fibrous


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peat specimens in different strength apparatus and its relation

to field conditions are not clearly understood. Furthermore, at

microscale, shearing of (fibrous) peat involves tearing of entangled

fibres and cellular connections. Hence, it is questionable whether

c¢ = 0 is a realistic value for fibrous peat, particularly at the (very)

low effective stress values typically encountered for these deposits.

ConclusionsCompared with fine-grained mineral soils, significant fabric and

structural differences may make the direct application of classical

soil mechanics strength models doubtful for peat. Fundamental

research on peat strength and how it is derived needs to be

performed. For example, it is not clearly understood how laboratory

f¢ values determined for ʻfailureʼ criteria based on (semi) arbitrary

strain levels in different strength apparatus relate at a larger scale

in the field. Detailed investigations of the full stress–strain–time/

stress/strain rate behaviour are warranted and ideally need to be

performed at the material’s natural water content (appropriate

effective stress levels), noting the possible effects of the soil

structure and compression of the organic solids themselves. Factors

including changes in volume and pwp produced by rotation of

the stress tensor during specimen shearing (refer to Figure 3) and

anisotropy of creep rates merit investigation.

Further development of existing and new specific material models

is recommended in order to simulate more accurately the soft and

viscous nature of peat, its general cross-anisotropic fabric and the

reinforcement provided by fibres in peats with low humification.

This is likely to require a full spectrum of testing, including

element tests and physical modelling at different scales, specialist

laboratory tests and instrumented field trials. In practice, the

shearing mechanism in the field must be carefully considered in

choosing a suitable strength testing apparatus. For example, of the

current strength apparatus, DSS testing best represents the condition

occurring in translational planar slides, providing an appropriate

(and conservative) f¢ value for stability analysis.

AcknowledgementsThe images shown in Figure 2 are reproduced with the kind

permission of Dr. Eric Farrell. The author would like to thank Dr.

Evert den Haan for assistance is compiling the historical review

of triaxial testing and Ms Lin Zhang for help in preparing Figures

3 and 5(a). The author would also like to thank the reviewers for

many helpful comments.

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WHAT DO YOU THINK?

To discuss this paper, please submit up to 500 words to the editor at [email protected]. Your contribution will be forwarded to the author(s) for a reply and, if considered appropriate by the editorial panel, will be published as a discussion in a future issue of the journal.

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Effective stress strength testing of peat

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