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CASE HISTORIES: PEAT KOSTAS SYMEONIDIS

MSc Engineering geology for ground models 2011 6/3/2012

1.  Peat

Peat is an accumulation of plant remains that has

undergone some degree of decomposition. Inorganic

soil material may occur as secondary constituents in

peat.

2.  Classifications

Principal types of peat forming ecosystems: Fens, Bogs,

Swamps, Marshes. 

Main peat formations: Raised bog peat, Fen peat

Peat flow, Blanket bog peat, Hill peat, Basin peat

3.  Accumulation and development

Accumulation depends on factors like: climate,

topography, superfluity of water, geological conditions

in terms of nutrient source availability.

Peat accumulation takes place when the rate of addition

of matter exceeds the rate of decay

4.  Engineering geology of peat

Multiple dep. environments and sensitivity to changes

(like climatic changes, sea level changes, glacial advanceor retreat) leads to complexity and variability in nature

and extend of peat formations.

Peat and organic soils can be hazardous to engineering

works, due to their nature: highly compressible and

subject to severe long term creep, low unit weight,

methane gas may be present, cause very large

settlements, slope stability problems. Can also be

hazardous to engineering works, also due to their

complexity as geological formations

Field identification: Fibrous peat, Pseudo-fibrous/hemic

peat, Amorphous/sapric peat.5.  Engineering properties of peat

Difficult to obtain undisturbed samples: Presence of gas,

Loss of moisture during extrusion and preparation,

Disturbance of sample when forces in the retainer

Cone penetration testing (standard CPT and piezocone

CPTu) may be used to obtain detailed site stratigraphy

information.

Physical properties

The shear strength of a peat deposit depends on its

moisture content, degree of humification and mineral

content. Relatively lightweight (i.e., low dry density),

saturated, no significant stress history, thus their

strengths are usually very low.

Primary settlement will occur over a relatively short timewhile the majority of the total settlements will result

from the long-term secondary compression.

6.  Engineering in peat

Construction methods include Load adjustment, Soil

replacement, Stage construction/Soil improvement,

geotextiles and embankment piling.

7.  Case: Wilnis peat dyke breach, Netherland

Location: Wilnis is a village in the Dutch province of 

Utrecht, about 30km south of Amsterdam.

Failure: At 26-Aug-2003 1:30am a peat dyke failed alongthe ring canal near the village center. About 60m of dyke

translated horizontally ~15m towards the north, leaving

two breaches at the sides.

Consequences: 600 houses under 0.5m of water, 2000

residents evacuated but returned to their house at the

evening.

Site investigation: No prior available data about the dyke,

or the soil profile at the site. Investigation conducted

after the failure and included borings, CPTs and

piezometer, near and in the dyke. Lab tests including

triaxial tests (CU) and Direct shear test (more propriate).

Failure mechanism: The prolonged dry and hot season

caused the drying of the crest peat, thus further

reducing the unit weight of the already lightweight peat

and consequently the passive shear resistance enough

to be: SF < 1.

Possible precaution: Pressure berm calculation

Pressure berm: 0.9m height and 3.75m length, plus

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CASE HISTORIES: PEAT KOSTAS SYMEONIDIS

MSc Engineering geology for ground models 2011 6/3/2012

FS horizontal = 105.5/94.35 = 1.19

8.  Case: A5 Llyn Ogwen peatslide, North WalesLocation: hillside above the A5 London to Holyhead

trunk-road in the Llyn Ogwen area.

Failure: occurred on Tuesday at 10:00am 8-Nov-2005

during a period of intense rainfall. The peatslide was a

shallow translational type.

Consequences: four workers injured, damages to a

temporary building, delay to a nearby construction

project and blockage of the A5 road by debris.

Lab and field tests: No boreholes, from two peat

samples collected, Atterberg limits LL between 573%

and 720%, and moisture content w between 732% and

828%. Non-plastic peat samples, with ash content of 

2.2%. From in-situ application of hand Vane Test at the

head scour the vane shear strength range from 10 to

15kPa.Peat field description: Soft to very soft, dark brown,

Sphagnum Eriophorum peat, between 0.2m and 0.8m

thick. Degree of humification varies between H4 and H7

(according to van Post classification), indicating

moderate to strongly decomposed peat. Fibers are

readily identifiable, both coarse and fine fibers are

present.

Failure mechanism: during the period of intense rainfall

water percolated to the base of the peat along cracksand the subsurface pipe network. Thus the increased

pore pressure at the base of the peat reduced the

effective stress and the resistance to sliding.

Back analysis

Su is back-calculated to be 2.78 kPa

Remediation: removal of blocks of peat left behind on

the upper part of the rupture surface, the use of 

intercepting ditches, natural vegetation recovery

REFERENCESASTM 2974, D2974-07a Standard Test Methods for

Moisture, Ash, and Organic Matter of Peat and Other

Organic Soils

Boylan, N., Jennings, P., and Long, M., 2008, Peat slope

failure in Ireland, Quarterly Journal of Engineering

Geology and Hydrogeology, v. 41; p. 93-108

Hartlen J. and Woski, W., ed., Developments in

Geotechnical Engineering, 80, Elsevier

Dykes and Warburton 2007, Discussion of ‘A5 Llyn

Ogwen peatslide, Capel Curig, North Wales’ by D. Nichol,

G.K. Doherty & M.J. Scott, Quarterly Journal of 

Engineering Geology and Hydrogeology, 40, 293 –299

Hobbs, N.B, 1984, Mires and Peats, The engineering

group of the geological society, London

Jarrett, P.M. editor Testing of Peats & Organic Soils,

ASTM STP 820

Nichol, D, Doherty, G.K., and, Scott, M.J., 2007, A5 Llyn

Ogwen peatslide, Capel Curig, North Wales, Quarterly

Journal of Engineering Geology and Hydrogeology, 40,

293 –299

Van Baars, 2005, The horizontal failure mechanism of 

the Wilnis peat dyke, TECHNICAL NOTE, Geotechnique

55, No. 4, 319 –323Van Baars, 2008, Dutch Peat Dyke

Failure During The Dry Summer Of 2003, SEC2008, Paris,

France