design and construction of a reinforced soil embankment · pdf filedesign and construction of...

Design and construction of a reinforced soil embankment on soft soil

Russell Jones, Golder Associates

Gareth Swift, University of Salford

OverviewBackground

Site details

Design philosophy

Design issues

Construction issues

Conclusions

BackgroundIntroduction

Site is located south of Basildon in Essex

Bounded by:

• Wat Tyler country park on western boundary

• Pitsea Marshes (SSSI) along the northern boundary

• East Haven Creek along southern boundary (Thames beyond)

Background

Pitsea landfill site located south of Basildon in Essex, and covers an area of approx.284ha (>50ha currently operational)

Introduction

BackgroundIntroduction

Paper and presentation deals with design and construction issuesrelating to a large leachate lagoon to be constructed on soft, compressible soils

Design carried out in June 2002

Construction between July 2002 and January 2003

Lagoon filled to capacity March 2008

Final Certificate issued by the Panel Engineer July 2008

BackgroundClient brief:

Minimum leachate capacity of 150,000m3

Maximum cost of build £1.3mill

Maximum bund height 8mAOD (planning constraint)

……needs to be buildable!

BackgroundAdditional design constraints:

Ensure stability

Minimise soil imports

Maximise lagoon area (hence, minimise bund height)

Minimise excavation in to existing soils

BackgroundAdditionally…

Satisfy requirements of Reservoir Act 1975

the Act applies to Large Raised Reservoirs, defined as:

‘being designed to hold or capable of holding more than 25,000m3 of water as such above the natural level of any part of the land adjoining the reservoir (including the bed of any stream)’

OverviewBackground

Site details

Design philosophy

Design issues

Construction issues

Conclusions

Site DetailsGround conditions

The area is relatively even but groundwater levels close to, or at, the ground surface

Waste dating from the 1950’s underlies the site in a layer between 1m and 7m thick

Waste comprises ash, clinker, glass, cans in hydraulic continuity with the surrounding landfill

This overlies a generally soft stratum of alluvial clays and sands (mv 0.2 – 0.8m2/kN).


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.

OverviewBackground

Site details

Design philosophy

Design issues

Construction issues

Conclusions

Design PhilosophyOriginal design philosophy

Reducing the existing ground level of approximately 3 m AOD to a minimum formation level of 1.0 m AOD

Pumping and dewatering required (estimated quantity 20,000 to 75,000m3)

Forming embankment slopes of 1v:4h utilising the excavated waste

Using a single ethylene inter-polymer alloy geomembrane liner on both the base and embankment slopes

Liner manufactured to a specific prefabricated size and shape tosuit the design


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

Dewatering logistically difficult due to up-gradient landfill (with leachate)

Single geomembrane liner not the most effective barrier

Tenderers’ comments

Design PhilosophyProposed design

No excavation

• Reinforced basal platform 20 m wide by 700 mm thick constructed at the existing ground level

Steeper face angles

• Reinforced soil perimeter embankment, 5.5 m high

• External side slopes 1v:2h

• Internal side slopes 1v:1v


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)

Underdrainage geocomposite drainage layer to limit hydraulic pressures from the leachate and gas

Lining of the perimeter embankment with

• 2 mm thick LLDPE geomembrane

• Geocomposite drainage layer connected to a piped drainage system

OverviewBackground

Site details

Design philosophy

Design issues

Construction issues

Conclusions

Design IssuesProposed design

Since material was limited to on-site sources, the quality would be variable

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

Granular material used for foundation layer

Cohesive material used for most of embankment but granular material used for upper section

Design IssuesProposed design

Design IssuesReinforcement 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

Design IssuesSettlement

Final top of embankment = 7.5m AOD

Constructed to = 8m AOD due to anticipated settlement

Design Issues

mv = 0.2m2/MNk = 1 x 10-8m/s

0.2 to 0.8

25019Alluvium

Beneath northern embankment

0.325016Old waste

Beneath main slope

0.525016Old waste

-0.130018Foundation layer

-0.124020Embankment fill

Additional information

ruφ′(deg)

c′(kPa)

γb

(kN/m3)Material

Material parameters

Design IssuesValue Engineering

Time constraints

Design optimisation during construction

Supported by field trials where possible

ECC form of contract

Design IssuesValue Engineering

NEC/Engineering and Construction Contract

Six 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

Design IssuesContract

NEC/Engineering and Construction Contract

Six 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

Design IssuesStability

A key factor in the design of the perimeter embankments is theirstability

• Internal – Inside face and outside face

• External – Global failure and siding

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


Piezometers used on site to monitor pore water pressures

Construction of toe berms would improve short term stability

• Factor of safety >1.3 if ru <0.7

• Toe berms allow increase in ru to 0.8


700mm thick reinforced foundation layer:•300mm drainage layer •400mm stability layer


Typical output:

1.509

Top Layer of Fornit 20/20

3 Layers of Stablenka

Top Layer of 120RELower Layer of 160RE

10 kPa

Design IssuesGroundwater

Geotextile reinforcement allows dissipation of pore pressures inembankment fill

Geocomposite allows dissipation of pore pressures beneath the lagoon

75mm crushed concrete layer allows dissipation of pore pressures beneath the embankment

Design IssuesEnvironmental considerations

Leachate containment

• 2mm LLDPE geomembrane

• GCL

• UV protection geotextile

Underdrainage

• Complete system required to remove gases beneath lining system

• 12mm geocomposite

OverviewBackground

Site details

Design philosophy

Design issues

Construction issues

Conclusions

Construction IssuesConstruction works

Groundwater monitoring

Settlement monitoring


Poor ground conditions


Generally, a flatlying area in a floodplain


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

Existing leachate trench backfilled

Dewatering, where required

Installation of piezometers

Construction of cut-off trench


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 geogridreinforcement


Foundation layer, first layer of geogrid


Foundation layer, second layer of geogrid


Foundation layer


Reinforced embankment

• Compacted general fill material (Class 6I, 6J, 7B, 7C or 7D)

• Geotextile primary reinforcement

• Geogrid secondary reinforcement


Lining system

• Drainage geocomposite

• Geomembrane

• Perforated pipe

• Soil retention geocomposite


Installation of perforated pipe at toe of inside slope


Embankment starting to be constructed


Embankment works continuing


Embankment works continuing, inside face


Embankment works continuing, outside face


Embankment works, becoming difficult with cohesive fill


Trafficability concerns…


Embankment works, revised fill


Embankment works, revised fill, impact on geotextile reinforcement


Embankment works, revised fill, impact on geogrid reinforcement


Embankment works, working in the winter


Embankment works, geocomposite and geomembrane deployment


Embankment works, uv protection geotextile and tyre wave wall


Embankment works, soil retention geocomposite


Basal works, prior to geocomposite deployment


Basal works, deployment of geocomposite, GCL and geomembrane


Basal works, deployment of geocomposite


Basal works, deployed GCL


Basal works, deployment of geomembrane


Basal works, hydraulic bund


Perimeter leachate trench


Tested to capacity for Reservoir Act sign off, 2008


Completed lagoon, 2008

Construction IssuesGroundwater monitoring

piezometers installed at a number of locations to monitor PWP


leachate level in lagoon


ru typically between 0.2 and 0.3

Construction IssuesSettlement monitoring

Temporary vertical and horizontal stations at toe of embankment

Permanent stations at top of embankment


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

Lev

el (m

aO

D)

Leachate level in lagoon

Approximate elevation of top of bund –between 7.6m and 8.0mAOD

Elevation of foundation layer

Construction IssuesCQA

Quality assurance and conformance testing of all materials:

• Separator geotextiles

• Reinforcing elements

• Lining elements

• All soils

In accordance with the approved CQA Plan

Construction Issues

OverviewBackground

Site details

Design philosophy

Design issues

Construction issues

Conclusions

ConclusionsReview of the design issues relating to the construction of a leachate storage lagoon has been presented

Geotechnical and geoenvironmental applications of geosynthetics have been used to ensure the short and long term stability of the perimeter embankment of the lagoon and to maximize containment capacity

Design revised during construction as part of a Value Engineering approach

Lagoon now tested by filling to capacity

Conclusions

Soil retentionGeocomposite

Secondary component of composite linerGeosynthetic Clay Liner

Primary component of composite linerGeomembrane

DrainageGeopipes

ReinforcementGeogrid

DrainageGeocomposite

UV protectionGeotextile

Reinforcement and DrainageGeotextile

FunctionGeosynthetic Material

A range of geosynthetics was used in the project

This project could not have been constructed without the use of these materials

AcknowledgementsVeolia Environmental Services (formerly Cleanaway)

Steve Smith (now Golder Associates)

John Bowers

Nick Sinclair

Panel Engineers

Michael Kennard (retired)

George Hallowes (deceased)

Chris Hoskins

Conclusion slide

Thank you for your attention

[email protected] [email protected]

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