design and construction of a reinforced soil embankment on soft soil
TRANSCRIPT
7/28/2019 Design and Construction of a Reinforced Soil Embankment on Soft Soil
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Design and construction of a reinforced
soil embankment on soft soil
Russell Jones, Golder Associates
Gareth Swift, University of Salford
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Overview
¾ Background
¾ Site details
¾ Design philosophy
¾ Design issues
¾ Construction issues
¾ Conclusions
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Background
¾ Introduction
¾ Site is located south of Basildon in Essex
¾ Bounded by:
• Wat Tyler country park on western boundary
• Pitsea Marshes (SSSI) along the northern boundary
• East Haven Creek along southern boundary (Thames beyond)
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Background
Pitsea landfill site located south of Basildonin Essex, and covers an area of approx.284ha (>50ha currently operational)
¾ Introduction
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Background
¾ Introduction
¾ Paper and presentation deals with design and construction issues
relating to a large leachate lagoon to be constructed on soft,
compressible soils
¾ Design carried out in June 2002
¾Construction between July 2002 and January 2003
¾ Lagoon filled to capacity March 2008
¾ Final Certificate issued by the Panel Engineer July 2008
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Background
¾ Client brief:
¾ Minimum leachate capacity of 150,000m3
¾ Maximum cost of build £1.3mill
¾ Maximum bund height 8mAOD (planning constraint)
¾ ……needs to be buildable!
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Background
¾ Additional design constraints:
¾ Ensure stability
¾ Minimise soil imports
¾ Maximise lagoon area (hence, minimise bund height)
¾ Minimise excavation in to existing soils
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Background
¾ Additionally…
¾ Satisfy requirements of Reservoir Act 1975
¾ the Act applies to Large Raised Reservoirs, defined as:
‘being designed to hold or capable of holding more than25,000m3 of water as such above the natural level of any
part of the land adjoining the reservoir (including the bed of
any stream)’
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Overview
¾ Background
¾ Site details
¾ Design philosophy
¾ Design issues
¾ Construction issues
¾ Conclusions
7/28/2019 Design and Construction of a Reinforced Soil Embankment on Soft Soil
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Site Details
¾ Ground conditions
¾ The area is relatively even but groundwater levels close to, or at,
the ground surface
¾ Waste dating from the 1950’s underlies the site in a layer between
1m and 7m thick
¾ Waste comprises ash, clinker, glass, cans in hydraulic continuity
with the surrounding landfill
¾ This overlies a generally soft stratum of alluvial clays and sands
(mv 0.2 – 0.8m2/kN).
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Site Details
¾ Ground conditions
¾ A similar, but earlier, lagoon encountered significant difficulties
during construction associated with the high groundwater levels
and the trafficability of the waste and the soft alluvial material.
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Site Details
¾ Ground conditions
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Overview
¾ Background
¾ Site details
¾ Design philosophy
¾ Design issues
¾ Construction issues
¾ Conclusions
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Design Philosophy
¾ Original design philosophy
¾ Reducing the existing ground level of approximately 3 m AOD to a
minimum formation level of 1.0 m AOD
¾ Pumping and dewatering required (estimated quantity 20,000 to
75,000m3)
¾ Forming embankment slopes of 1v:4h utilising the excavated
waste
¾ Using a single ethylene inter-polymer alloy geomembrane liner on
both the base and embankment slopes
¾ Liner manufactured to a specific prefabricated size and shape to
suit the design
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Design Philosophy
¾ Original design philosophy
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Design Philosophy
¾ Original design philosophy
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Design Philosophy
¾ Original design philosophy
¾ Ground conditions are poor at best:
• Groundwater/leachate levels at or near to the ground surface; and
• Material to be excavated to produce a formation level comprised
1950’s waste of questionable engineering integrity
¾ Dewatering logistically difficult due to up-gradient landfill (with
leachate)
¾ Single geomembrane liner not the most effective barrier
¾Tenderers’ comments
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Design Philosophy
¾ Proposed design
¾ No excavation
• Reinforced basal platform 20 m wide by 700 mm thick constructed atthe existing ground level
¾ Steeper face angles
• Reinforced soil perimeter embankment, 5.5 m high
• External side slopes 1v:2h
• Internal side slopes 1v:1v
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Design Philosophy
¾ Original design philosophy
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Design Philosophy
¾ Proposed design
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Design Philosophy
¾ Proposed design
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Design Philosophy
¾ Proposed design
¾ Base area of approximately 32,000 m2 lined with a composite
lining system
• 2 mm thick Linear Low Density Polyethylene (LLDPE) sheet
• Geosynthetic clay liner (GCL)
¾
Underdrainage geocomposite drainage layer to limit hydraulicpressures from the leachate and gas
¾ Lining of the perimeter embankment with
• 2 mm thick LLDPE geomembrane
• Geocomposite drainage layer connected to a piped drainage system
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Overview
¾ Background
¾ Site details
¾ Design philosophy
¾ Design issues
¾ Construction issues
¾ Conclusions
7/28/2019 Design and Construction of a Reinforced Soil Embankment on Soft Soil
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Design Issues
¾ Proposed design
¾ Since material was limited to on-site sources, the quality would be
variable
¾ Side slopes would be steep in order to achieve capacity
• Geosynthetic reinforcement, geotextile rather than geogrid to aid
dissipation of pore pressures
• Primary geotextile reinforcement
• Secondary geogrid reinforcement
¾ Granular material used for foundation layer
¾ Cohesive material used for most of embankment but granular
material used for upper section
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Design Issues
¾ Proposed design
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Design Issues
¾ Proposed design
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Design Issues
¾ Reinforcement details
4Fornit 20/20
3Stabilenka 100/50Class 6I, 6J, 7B, 7C or 7D
material
Embankment
1Tensar 120RECrushed concrete finesBasal layer 2
1Tensar 160RE75 mm crushed concrete (min
50 kN 10 % fines)
Basal layer 1
No. of
Layers
ReinforcementMaterialLayer
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Design Issues
¾ Settlement
¾ Final top of embankment = 7.5m AOD
¾Constructed to = 8m AOD due to anticipated settlement
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Design Issues
mv = 0.2m2/MN
k = 1 x 10-8m/s
0.2 to
0.8
25019 Alluvium
Beneath northern
embankment
0.325016Old waste
Beneath main
slope
0.525016Old waste
-0.130018Foundationlayer
-0.124020Embankment
fill
Additional
information
r u
φ′
(deg)
c′
(kPa)
γb
(kN/m3)
Material
¾ Material parameters
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Design Issues
¾ Value Engineering
¾ Time constraints
¾Design optimisation during construction
¾ Supported by field trials where possible
¾ ECC form of contract
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Design Issues
¾ Value Engineering
¾ NEC/Engineering and Construction Contract
¾Six Main Options• Option A - Priced with Activity Schedule
• Option B - Priced with BQ
• Option C - Target with Activity Schedule
• Option D - Target with BQ
• Option E - Cost Reimbursement
• Option F - Management Contract
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Design Issues
¾ Contract
¾ NEC/Engineering and Construction Contract
¾Six Main Options• Option A - Priced with Activity Schedule
• Option B - Priced with BQ
• Option C - Target with Activity Schedule
• Option D - Target with BQ
• Option E - Cost Reimbursement
• Option F - Management Contract
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Design Issues
¾ Stability
¾ A key factor in the design of the perimeter embankments is their
stability
• Internal – Inside face and outside face
• External – Global failure and siding
¾ Major issue is the development of pore water pressure in the sub-
grade due to embankment construction
• Slope/w and Seep/w used to examine the effects of pore water
pressure on stability
• Results indicate that the rate of build up and dissipation was critical to
stability
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Design Issues
¾ Stability
¾ Piezometers used on site to monitor pore water pressures
¾Construction of toe berms would improve short term stability• Factor of safety >1.3 if r u <0.7
• Toe berms allow increase in r u to 0.8
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Design Issues
¾ Stability
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Design Issues
¾ Stability
700mm thick reinforced foundation layer:
•300mm drainage layer
•400mm stability layer
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Design Issues
¾ Stability
¾ Typical output:
1.509
Top Layer of Fornit 20/20
3 Layers of Stablenka
Top Layer of 120RELower Layer of 160RE
10 kPa
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Design Issues
¾ Groundwater
¾ Geotextile reinforcement allows dissipation of pore pressures in
embankment fill
¾ Geocomposite allows dissipation of pore pressures beneath the
lagoon
¾ 75mm crushed concrete layer allows dissipation of pore
pressures beneath the embankment
D i I
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Design Issues
¾ Environmental considerations
¾ Leachate containment
• 2mm LLDPE geomembrane
• GCL
• UV protection geotextile
¾Underdrainage• Complete system required to remove gases beneath lining system
• 12mm geocomposite
O i
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Overview
¾ Background
¾ Site details
¾ Design philosophy
¾ Design issues
¾ Construction issues
¾ Conclusions
C t ti I
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Construction Issues
¾ Construction works
¾ Groundwater monitoring
¾ Settlement monitoring
C t ti I
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Construction Issues
¾ Construction works
Poor ground conditions
Constr ction Iss es
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Construction Issues
¾ Construction works
Generally, a flat
lying area in a
floodplain
Construction Issues
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Construction Issues
¾ Construction works
Construction Issues
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Construction Issues
¾ Construction works
¾ Relatively flat formation level (3.0mAOD) required:
• Removal of bushes/trees
• Re-profiling of hummocky areas
• Elevated ground in NE was excavated
• Low area in south backfilled
¾ Existing leachate trench backfilled
¾ Dewatering, where required
¾ Installation of piezometers
¾ Construction of cut-off trench
Construction Issues
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Construction Issues
¾ Construction works
Construction Issues
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Construction Issues
¾ Construction works
¾ Basal layer
• Separator geotextile on existing ground
• 9 mm Drainage geocomposite
• 300 mm thick drainage layer
• Separator geotextile on drainage layer
• 400 mm thick stability layer including two layers of geogrid
reinforcement
Construction Issues
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Construction Issues
¾ Construction works
Foundation layer, first layer of geogrid
Construction Issues
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Construction Issues
¾ Construction works
Foundation layer, second layer of geogrid
Construction Issues
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Construction Issues
¾ Construction works
Foundation layer
Construction Issues
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Construction Issues
¾ Construction works
¾ Reinforced embankment
• Compacted general fill material (Class 6I, 6J, 7B, 7C or 7D)
• Geotextile primary reinforcement
• Geogrid secondary reinforcement
Construction Issues
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Construction Issues
¾ Construction works
¾ Lining system
• Drainage geocomposite
• Geomembrane
• Perforated pipe
• Soil retention geocomposite
Construction Issues
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Construction Issues
¾ Construction works
Construction Issues
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Construction Issues
¾ Construction works
Installation of perforated pipe at toe of inside slope
Construction Issues
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Construction Issues
¾ Construction works
Embankment starting to be constructed
Construction Issues
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¾ Construction works
Embankment works continuing
Construction Issues
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¾ Construction works
Embankment works continuing, inside face
Construction Issues
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¾ Construction works
Embankment works continuing, outside face
Construction Issues
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¾ Construction works
Embankment works, becoming difficult with cohesive fill
Construction Issues
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¾ Construction works
Trafficability concerns…
Construction Issues
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¾ Construction works
Embankment works, revised fill
Construction Issues
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¾ Construction works
Embankment works, revised fill, impact on geotextile reinforcement
Construction Issues
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¾ Construction works
Embankment works, revised fill, impact on geogrid reinforcement
Construction Issues
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¾ Construction works
Embankment works, working in the winter
Construction Issues
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¾ Construction works
Embankment works, geocomposite and geomembrane deployment
Construction Issues
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¾ Construction works
Embankment works, uv protection geotextile and tyre wave wall
Construction Issues
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¾ Construction works
Embankment works, soil retention geocomposite
Construction Issues
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¾ Construction works
Basal works, prior to geocomposite deployment
Construction Issues
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¾ Construction works
Basal works, deployment of geocomposite, GCL and geomembrane
Construction Issues
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¾ Construction works
Basal works, deployment of geocomposite
Construction Issues
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¾ Construction works
Basal works, deployed GCL
Construction Issues
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¾ Construction works
Basal works, deployment of geomembrane
Construction Issues
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¾ Construction works
Basal works, hydraulic bund
Construction Issues
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¾ Construction works
Perimeter leachate trench
Construction Issues
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¾ Construction works
Tested to capacity for Reservoir Act sign off, 2008
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¾ Construction works
Completed lagoon, 2008
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¾ Groundwater monitoring
¾ piezometers installed at a number of locations to monitor PWP
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¾ Groundwater monitoring
leachate level in lagoon
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¾ Groundwater monitoring
ru typically between 0.2 and 0.3
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¾ Settlement monitoring
¾ Temporary vertical and horizontal stations at toe of embankment
¾ Permanent stations at top of embankment
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¾ Settlement monitoring
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¾ Settlement monitoring
3
4
5
6
7
8
9
1-Jan-03 1-Apr-03 1-Jul-03 1-Oct -03 1-Jan-04 1-Apr-04 1-Jul-04 1-Oct-04 1-Jan-05
L e v e l ( m a
O D )
Leachate level in lagoon
Approximate elevation of top of bund –between 7.6m and 8.0mAOD
Elevation of foundation layer
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¾ CQA
¾ Quality assurance and conformance testing of all materials:
• Separator geotextiles
• Reinforcing elements
• Lining elements
• All soils
¾ In accordance with the approved CQA Plan
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Overview
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¾ Background
¾ Site details
¾Design philosophy
¾ Design issues
¾ Construction issues
¾ Conclusions
Conclusions
¾ R i f th d i i l ti t th t ti f
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¾ Review of the design issues relating to the construction of aleachate storage lagoon has been presented
¾ Geotechnical and geoenvironmental applications of
geosynthetics have been used to ensure the short and long termstability of the perimeter embankment of the lagoon and to
maximize containment capacity
¾ Design revised during construction as part of a ValueEngineering approach
¾ Lagoon now tested by filling to capacity
Conclusions
¾ A f th ti d i th j t
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Soil retentionGeocomposite
Secondary component of composite liner Geosynthetic Clay Liner
Primary component of composite liner Geomembrane
DrainageGeopipes
ReinforcementGeogrid
DrainageGeocomposite
UV protectionGeotextile
Reinforcement and DrainageGeotextile
FunctionGeosynthetic Material
¾ A range of geosynthetics was used in the project
¾ This project could not have been constructed without the use of
these materials
Acknowledgements
¾ V li E i t l S i (f l Cl )
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¾ Veolia Environmental Services (formerly Cleanaway)
¾ Steve Smith (now Golder Associates)
¾ John Bowers
¾ Nick Sinclair
¾ Panel Engineers
¾ Michael Kennard (retired)
¾ George Hallowes (deceased)
¾ Chris Hoskins
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Conclusion slide
Thank you for your attention