permeability of peat: an engineering case study · ‐lower permeability with higher humification...
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ET Hanrahan Memorial Symposium University College Dublin, Belfield, Dublin 4, IrelandUniversity College Dublin, Belfield, Dublin 4, Ireland
11th September 2013
PERMEABILITY OF PEAT:
an engineering case study
Dr Paul Jennings and Turlough Johnston
Applied Ground Engineering Consultants (AGEC) Ltd.,Bagenalstown County CarlowBagenalstown, County Carlow
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Contents
• Introduction ‐ Peat in Engineering Works
‐ Examples of peat failures
• Case Study – Corrib Gas Onshore Pipeline
‐ Proposed construction – stone road
‐ Peat Conditions
‐ Typical Peat Engineering Properties
‐ Peat Permeabilityy
‐ Literature review
‐ Direct measurement ‐ laboratory testing
Direct measurement insitu testing‐ Direct measurement ‐ insitu testing
‐ Permeability Results and Analytical modelling
‐ Construction and mitigation measures
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Introduction ‐ Peat in Engineering Works
• Peat is generally not considered as a material for engineering works primarily due to:p y
‐ lack of consensus on engineering behaviour
‐ compressible nature
‐ low shear strength
• Engineering properties for intact peat (eg permeability) tend to be extremely variable spatiallybe extremely variable spatially
• Peat referred to as one of the ‘troublesome soils’ (Hanrahan, 1979)
• Notwithstanding peat (in the right circumstances) can be used as• Notwithstanding peat (in the right circumstances) can be used as an engineering material with appropriate controls
• But, many examples of peat failures in engineering works, y p p g g
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Settlement – high compressibility
200c.200mm
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Settlement – high compressibility
c.300mm
c.400mm
c.600mm
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Peat Slide ‐ low shear strength
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Bigger Peat Slide ‐ low shear strength
c.50m
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Bearing failure ‐ low shear strength
Ref: Raven, K & Assinder, P. (2008).Use of Geotextiles in Construction over Soft Ground. Thames Valley Geological Society/International Geosynthetic Society, Royal Holloway College
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Case Study – Corrib Gas Onshore Pipeline
• Corrib gas pipeline located in County Mayo (NW Ireland):
‐ 84km offshore
‐ 8.3km ‘onshore’, including
‐ 4.7km estuary/bay tunnel
‐ 3 6km mostly within peatland (Atlantic blanket bog)3.6km mostly within peatland (Atlantic blanket bog)
• Peatland habitat in/adjacent to works protected under EU Habitats Directive
• Construction in peatland using stone road (to avoid instability issues)
• Concern that in peatland areas vertical & lateral groundwater leakage as a result of works would damage protected habitats
• Construction methodology developed using reworked peat to d i d h biprotect designated habitats
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Glengad Headland (sandy soil)Onshore Section ‐ Gas Pipeline
Onshore estuary/bay tunnel
Peatland (blanket peat)
Gas Terminal (blanket peat)( p )
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Onshore Section ‐ Gas Pipeline
Onshore estuary/bay tunnel
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Onshore Section ‐ Gas Pipeline
Peatland – blanket peat
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Proposed Construction in Peatland– Stone Road
Stone road
Pipeline within trench
Intact peat
Mineral soil
P t ti l ti l d l t l t l k th h kPotential vertical and lateral water leakage through works
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General Peat Conditions• General
‐ Onshore route within forested and open peatland
‐ Peat thickness typically 3m with locally up to about 5mPeat thickness typically 3m with locally up to about 5m
‐ Peat organic content (>90%) with 2 distinct layers within vertical peat profile, namely
• Uppermost peat layer (acrotelm) contains actively growing plants• Uppermost peat layer (acrotelm) contains actively growing plants‐ Typically 0.3m to 0.7m thick
‐ Relatively strong
‐ Permeability: say 10‐1 to 10‐6 m/s
• Lower peat layer (catotelm) contains decaying plant matterMany metres deep‐ Many metres deep
‐ Plant matter in different degrees of humification
‐ Minimal strength (typically cu 2 to 15kPa)
‐ Permeability: say 10‐6 to less than10‐10 m/s
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In Situ Vane Undrained Shear Strength, cu vane (kPa) CPT Undrained Shear Strength, cu CPT (kPa)
Typical Peat Engineering Properties ‐ Peat Strength vs Depth
0.0
0.5
0 5 10 15 20 25 30 35 40 0 5 10 15 20 25 30 35 40g u ( )
1.0
1 51.5
2.0
th (m
bgl)
2.5
3.0
Dep
3.5
4.0
4.5
Upper fibrous and lower humified peat layers
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Typical Peat Engineering Properties ‐ General
0.0
0 500 1000 1500 2000 2500
Moisture Content (%)
0.60 0.80 1.00 1.20 1.40
Bulk Density (Mg/m3)
85 90 95 100
Organic Content (%)
0.0
0 1 2 3 4 5 6 7 8 9 10
von Post Humification (Hn)
100
0.5
1.0
1 5
0.5
1.0
1 5
0
50
100
1.5
2.0
2.5
Dep
th (m
bgl)
1.5
2.0
2.5
Dep
th (m
bgl)
0
50
50
100
3.0
3.5
D
3.0
3.5
0
50
50
100
4.0
4.5
4.0
4.5
0
Variation of moisture content, bulk density, organic content and von Post humification with depth for a Low‐level Blanket Bog
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Peat Permeability ‐ Determination• Require mitigation to protect designated habitats using principally
(reworked) peat to maintain the hydrological balance in peatland
• Reworked peat included in works as hydraulic impedance layers
• Determination of peat permeability by:
‐ Literature review
‐ Direct measurement of peat permeability (laboratory & insitu)
Direct measurement of mineral soil permeability‐ Direct measurement of mineral soil permeability
‐ Direct measurement at existing sites containing reworked peat
• Analytical seepage modelling used to determine extent/type of reworked peat impedance layers
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Literature Review ‐ Findings
• Typical insitu permeability from 10‐5 to 10‐7 m/s for fibrous peat decreasing to 10‐7 to 10‐12 m/s for more humified peat
• Significant decline in permeability with reduction in void ratio as a result of loading
• Decrease in permeability with reworking. Reworked peat with pre‐loading giving permeability in the range of 10‐8 to 10‐11 m/s (Dillon et al, 2004)
Peat Description Permeabilit (m/s) So rcePeat Description Permeability (m/s) SourcePeat 10‐6 Coley (1950)Fibrous Peat 4 x 10‐6 Hanrahan (1954)Amorphous and Fibrous Peat 10‐6 to 10‐7 Lee and Brawner (1963)Canadian Muskeg 10‐5 Adams (1965)Amorphous Peat 3 x 10‐8 to 10‐7 Galvin and Hanrahan (1967)Amorphous Peat 3 x 10 to 10 Galvin and Hanrahan (1967)Peat 10‐5 Weber (1969)Amorphous Peat 4 x 10‐7 Berry and Poskitt (1972)Fibrous Peat 8 x 10‐7 Berry and Poskitt (1972)Fibrous Peat 10‐5 to 10‐6 Berry and Vickers (1975)Amorphous to fibrous peat 10‐6 Dhowian and Edil (1980)p p ( )Fibrous Peat 10‐1 Ivanov (1981)Fibrous Peat 3 x 10‐7 Füstenberg et al (1983)Fibrous Peat 10‐5 to 10‐10 Lefebvre et al (1984)Blanket Peat 5.78x10‐8 to 1.7x10‐7 Mulqueen (1986)Sphagnum Peat 1.15x10‐7 to 2.3x10‐7 Mulqueen (1986)Peat 10‐5 ‐ 10‐7 Carlsten (1991)Fibrous Peat 6 x 10‐7 to 6 x 10‐8 Mesri et al (1997)
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Literature Review ‐ Peat Permeability vs Humification
1.00E‐04
1.00E‐03 Sphagnum Peat ‐Hobbs (1986)
Sedge & Brushwood Peat ‐ Hobbs (1986)
Sphagnum Cotton Sedge & Heather Peat Hobbs (1986)
1.00E‐06
1.00E‐05
Sphagnum, Cotton Sedge & Heather Peat ‐ Hobbs (1986)
Fibric Peat ‐ Ryden (1990) in Magnussen (1994)
Hemic Peat ‐ Ryden (1990)
1.00E‐08
1.00E‐07
Perm
eability (m
/s) Sapric Peat ‐ Ryden (1990)
1.00E‐10
1.00E‐09
1.00E‐12
1.00E‐11
0 1 2 3 4 5 6 7 8 9 10
Humification, von Post (Hn)
Relationship between humification and permeability for different types of peat
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Direct Measurement ‐ Laboratory Testing
Material General TestPeat Classification Testing Moisture Content
Index properties (Atterberg)Particle Size DistributionpHOrganic Content
Strength Testing Undrained Triaxial Strength TestPermeability Test
(H d f t 1 5 t 2 5
Constant Head Triaxial Permeability (remoulded)
C t t H d T i i l P bilit ( di t b d)(Head of water: 1.5 to 2.5m – simulate anticipated conditions)
Constant Head Triaxial Permeability (undisturbed)Constant Head Triaxial Permeability with geotextile (Terram 4000) (remoulded)Constant Head Triaxial Permeability (remoulded with 30% gravel) g )
Mineral Soil Classification Testing Moisture ContentIndex properties (Atterberg)Particle Size DistributionpHOrganic ContentOrganic Content
Strength Testing Undrained Triaxial Strength TestPermeability Test
(ditto)
Constant Head Triaxial Permeability (remoulded)
Constant Head Triaxial Permeability (undisturbed)
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Direct Measurement ‐ Insitu Testing
• Undisturbed (intact) peat
‐ Piezometer measurement
‐ 2 test areas in undisturbed peat adjacent/on route
‐ 12 nos. piezometers
‐ Large scale k‐testsLarge scale k tests
‐ 4 no. large diameter pipes tests (T1, T2, T3, & T4)
• Reworked (disturbed/remoulded) peat:
‐ Piezometer measurement
‐ 3 sites with placed reworked peat
‐ 21 nos piezometers‐ 21 nos. piezometers
‐ Tank test
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Direct Measurement ‐ Insitu Testing
50mm dia. standpipe with data‐logger
Water level
600mm dia. pipe 1 50m long 1 00m
Disturbed/placed PEAT
STONE FILL
Geotextile
1.50m long
0 50m
1.00m
1.50m
SAND
0.50m
0.30m
0.30m
Plastic tank
Tank Test
22
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Peat Permeability Results• Permeability results from laboratory tests:
‐ range 10‐10 to 10‐11 m/s
reworked peat has a lower permeability than undisturbed peat‐ reworked peat has a lower permeability than undisturbed peat
‐ results slightly lower than insitu testing (piezometer and large scale)
‐ lower permeability with higher humification – but no appreciable difference in permeability between H5 and H7 to H8between H5 and H7 to H8
‐ H5 or greater then permeability less than 10‐9 m/s
• Insitu permeability results in piezometers9 10 /‐ reworked peat permeability of 10‐9 to 10‐10 m/s
‐ undisturbed peat permeability of about 10‐9 m/s
• Permeability results from large scale field tests:
‐ ranged from 10‐8 to 10‐9 m/s
‐ results higher than those obtained from permeability tests in piezometers
• Analytical modelling showed suitably humified (+H5) and reworked peat wouldAnalytical modelling showed suitably humified (+H5) and reworked peat would provide an acceptable hydraulic barrier
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Construction & Mitigation Measures
• Construction and mitigation measures included to protect designated habitats using principally reworked peat to maintain g g p p y pthe hydrological balance
• Suitably humified and reworked peat used to provide acceptable hydraulic impedance layers
• Mitigation measures proposed:
‐ Basal peat layer constructed as part of stone road (stone/peat matrix)
‐ Peat plugs
‐ Turve reinstatement on regulation layer (designated areas)g y ( g )
‐ Drainage control – contoured raised turves (optional)
‐ Peat impedance layer over trench (designated areas)
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Stone road construction and basal peat layer (peat/stone matrix)
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Peat plugs
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Mitigation Measures ‐ Peat impedance layer
Impedance layer (1m thick) of selected peat (+H5) above
Geo‐composite above trench
p ( 5)pipeline
Intact peat
Mineral soil
Peat impedance layer placed over trench in designated areas where trench is within mineral soil
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Finish
Acknowledgements
Sh ll E&P I l d Li it dShell E&P Ireland Limited
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Types of Peatland (Ireland)• Peat coverage in Ireland
‐ 17 to 20% of national land area is peatland
‐ c.13,000 km2 of peatland
• Blanket Bog‐ Rainfall typically >1250mm/year
‐ Peat thickness up to 5m
‐ Includes both high level and low level (oceanic)g ( )
‐ West of Ireland and upland areas
• Raised BogR i f ll i ll 750 1000 /‐ Rainfall typically 750‐1000mm/year
‐ Peat thickness 3m to 12m
‐ Irish Midlands