ET Hanrahan Memorial Symposium University College Dublin, Belfield, Dublin 4, Ireland
University 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 Carlow
Bagenalstown, County Carlow
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 ‐
Direct measurement insitu testing
insitu testing
‐ Permeability Results and Analytical modelling
‐ Construction and mitigation measures
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 spatially
be extremely variable spatially • Peat referred to as one of the ‘troublesome soils’ (Hanrahan, 1979)
• Notwithstanding
Notwithstanding peat (in the right circumstances) can be used as peat (in the right circumstances) can be used as
an engineering material with appropriate controls
,
y
p
p
g
g
• But, many examples of peat failures in engineering works
Settlement – high compressibility
c.200mm
200
Settlement – high compressibility
c.600mm
c.400mm
c.300mm
Peat Slide ‐ low shear strength
Bigger Peat Slide ‐ low shear strength
c.50m
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
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 protect designated habitats
d i
d h bi
Glengad Headland (sandy soil)
Onshore Section ‐ Gas Pipeline
Onshore estuary/bay tunnel
Peatland (blanket peat)
Gas Terminal (blanket peat)
(
p )
Onshore Section ‐ Gas Pipeline
Onshore estuary/bay tunnel
Onshore Section ‐ Gas Pipeline
Peatland – blanket peat
Proposed Construction in Peatland– Stone Road
Stone road Pipeline within trench
Intact peat
Mineral soil
P t ti l
Potential vertical and lateral water leakage through works
ti l d l t l t l k
th
h
k
General Peat Conditions
• General ‐ Onshore route within forested and open peatland
‐ Peat thickness typically 3m with locally up to about 5m
Peat 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 matter
‐
‐
‐
‐
Many 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
Typical Peat Engineering Properties ‐ Peat Strength vs Depth In Situ Vane Undrained Shear Strength, cu vane (kPa)
0
5
10
15
20
25
30
35
40
0
CPT Undrained Shear Strength, c
g
( )
u CPT (kPa)
u 5
10
15
20
25
30
0.0
0.5
1.0
Depth (m bgl)
15
1.5
2.0
2.5
3.0
3.5
4.0
4.5
Upper fibrous and lower humified peat layers 35
40
Typical Peat Engineering Properties ‐ General Bulk Density (Mg/m3)
Moisture Content (%)
0
500 1000 1500 2000 2500
0.60
0.80
1.00
1.20
Organic Content (%)
1.40
85
90
95
von Post Humification (Hn)
100
0 1 2 3 4 5 6 7 8 9 10
0.0
0.0
100
50
1.0
Depth (m bgl)
D
15
1.5
2.0
2.5
3.0
0
100
50
0
100
50
0
100
3.5
50
4.0
0
4.5
0.5
1.0
15
1.5
2.0
2.5
3.0
3.5
4.0
4.5
Variation of moisture content, bulk density, organic content and von Post humification with depth for a Low‐level Blanket Bog
Depth (m bgl)
0.5
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 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
Peat Description Peat
Fibrous Peat
Amorphous and Fibrous Peat
Canadian Muskeg
Amorphous Peat
Peat
Amorphous Peat
Fibrous Peat
Fibrous Peat
Amorphous
p
to fibrous p
peat
Fibrous Peat
Fibrous Peat
Fibrous Peat
Blanket Peat
Sphagnum Peat
Peat
Fibrous Peat
Permeability (m/s)
Permeabilit
(m/s)
10‐6
4 x 10‐6
10‐6 to 10‐7
10‐5
3 x 10‐8 to 10‐7
3 x 10
10‐5
4 x 10‐7
8 x 10‐7
10‐5 to 10‐6
10‐6
10‐1
3 x 10‐7
10‐5 to 10‐10
5.78x10‐8 to 1.7x10‐7
1.15x10‐7 to 2.3x10‐7
10‐5 ‐ 10‐7
6 x 10‐7 to 6 x 10‐8
So rce
Source
Coley (1950)
Hanrahan (1954)
Lee and Brawner (1963)
Adams (1965)
Galvin and Hanrahan (1967)
Weber (1969)
Berry and Poskitt (1972)
Berry and Poskitt (1972)
Berry and Vickers (1975)
Dhowian and Edil ((1980))
Ivanov (1981)
Füstenberg et al (1983)
Lefebvre et al (1984)
Mulqueen (1986)
Mulqueen (1986)
Carlsten (1991)
Mesri et al (1997)
Literature Review ‐ Peat Permeability vs Humification
1.00E‐03
Sphagnum Peat ‐Hobbs (1986)
Sedge & Brushwood Peat ‐ Hobbs (1986)
1.00E‐04
Sphagnum Cotton Sedge & Heather Peat Hobbs (1986)
Sphagnum, Cotton Sedge & Heather Peat ‐
Hobbs (1986)
1.00E‐05
Fibric Peat ‐ Ryden (1990) in Magnussen (1994)
Hemic Peat ‐ Ryden (1990)
Permeability (m/s)
1.00E‐06
Sapric Peat ‐ Ryden (1990)
1.00E‐07
1.00E‐08
1.00E‐09
1.00E‐10
1.00E‐11
1.00E‐12
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
Direct Measurement ‐ Laboratory Testing
Material
Peat
General
Classification Testing
Strength Testing
Permeability Test
(Head of water: 1.5 to 2.5m (H
d f t 15t 25
– simulate anticipated conditions)
Mineral Soil
Classification Testing
Strength Testing
Permeability Test
(ditto)
Test
Moisture Content
Index properties (Atterberg)
Particle Size Distribution
pH
Organic Content
Undrained Triaxial Strength Test
Constant Head Triaxial Permeability (remoulded) C t tH dTi i lP
Constant Head Triaxial Permeability (undisturbed)
bilit ( di t b d)
Constant Head Triaxial Permeability with geotextile (Terram 4000) (remoulded)
Constant Head Triaxial Permeability (remoulded with 30% g
gravel) )
Moisture Content
Index properties (Atterberg)
Particle Size Distribution
pH
Organic Content
Organic Content
Undrained Triaxial Strength Test
Constant Head Triaxial Permeability (remoulded) Constant Head Triaxial Permeability (undisturbed)
Direct Measurement ‐ Insitu Testing
• Undisturbed (intact) peat ‐ Piezometer measurement
‐ 2 test areas in undisturbed peat adjacent/on route
‐ 12 nos. piezometers
‐ Large scale k
Large scale k‐tests
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
Direct Measurement ‐ Insitu Testing
Water level
50mm dia. standpipe with data‐logger
600mm dia. pipe 1 50m long
1.50m long
1.50m
1 00m
1.00m
STONE FILL Disturbed/placed PEAT
Geotextile
0 50m
0.50m
0.30m
SAND
Plastic tank
0.30m
Tank Test
22
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 H
between H
and H7 to H
to H8
‐ H5 or greater then permeability less than 10‐9 m/s ‐
‐
‐
‐
• Insitu permeability results in piezometers
10 m/s ‐ reworked peat permeability of 10‐99 to 10‐10
/
‐ 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
Analytical modelling showed suitably humified (+H
modelling showed suitably humified (+H5) and reworked peat would ) and reworked peat would
provide an acceptable hydraulic barrier
Construction & Mitigation Measures
• Construction and mitigation measures included to protect designated habitats using principally reworked peat to maintain g
gp
p y
p
the 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)
Stone road construction and basal peat layer (peat/stone matrix)
Peat plugs
Mitigation Measures ‐ Peat impedance layer
Impedance layer (1m thick) of selected peat (+H
p
( 5)) above pipeline
Geo‐composite above trench
Intact peat
Mineral soil
Peat impedance layer placed over trench in designated areas where trench is within mineral soil
Finish
Acknowledgements
Sh ll E&P I l d Li it d
Shell E&P Ireland Limited
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 Bog
‐ Rainfall typically 750‐1000mm/year
R i f ll
i ll 750 1000
/
‐ Peat thickness 3m to 12m
‐ Irish Midlands