design and construction of a reinforced soil embankment on soft soil

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



Overview

¾ Background

¾ Site details

¾ Design philosophy

¾ Design issues

¾ Construction issues

¾ Conclusions



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)



Background

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

¾ Introduction



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



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!



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



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)’



Overview

¾ Background

¾ Site details


¾ Design issues


¾ Conclusions



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).



Site Details


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.



Site Details




Overview

¾ Background

¾ Site details


¾ Design issues


¾ Conclusions



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



Design Philosophy




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



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



Design Philosophy




Design Philosophy

¾ Proposed design



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



Overview

¾ Background

¾ Site details


¾ Design issues


¾ Conclusions



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



Design Issues

¾ Proposed design



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



Design Issues

¾ Settlement

¾ Final top of embankment = 7.5m AOD

¾Constructed to = 8m AOD due to anticipated settlement



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



Design Issues

¾ Value Engineering

¾ Time constraints

¾Design optimisation during construction

¾ Supported by field trials where possible

¾ ECC form of contract



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



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



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



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



Design Issues

¾ Stability



Design Issues

¾ Stability

700mm thick reinforced foundation layer:

•300mm drainage layer

•400mm stability layer



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



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



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



Overview

¾ Background

¾ Site details


¾ Design issues


¾ Conclusions

C t ti I



Construction Issues

¾ Construction works

¾ Groundwater monitoring

¾ Settlement monitoring

C t ti I



Construction Issues


Poor ground conditions

Constr ction Iss es



Construction Issues


Generally, a flat

lying area in a

floodplain

Construction Issues



Construction Issues


Construction Issues



Construction Issues


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



Construction Issues


Construction Issues



Construction Issues


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



Construction Issues


Foundation layer, first layer of geogrid

Construction Issues



Construction Issues


Foundation layer, second layer of geogrid

Construction Issues



Construction Issues


Foundation layer

Construction Issues



Construction Issues


¾ Reinforced embankment

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

• Geotextile primary reinforcement

• Geogrid secondary reinforcement

Construction Issues



Construction Issues


¾ Lining system

• Drainage geocomposite

• Geomembrane

• Perforated pipe

• Soil retention geocomposite

Construction Issues



Construction Issues


Construction Issues



Construction Issues


Installation of perforated pipe at toe of inside slope

Construction Issues



Construction Issues


Embankment starting to be constructed

Construction Issues




Embankment works continuing

Construction Issues




Embankment works continuing, inside face

Construction Issues




Embankment works continuing, outside face

Construction Issues




Embankment works, becoming difficult with cohesive fill

Construction Issues




Trafficability concerns…

Construction Issues




Embankment works, revised fill

Construction Issues




Embankment works, revised fill, impact on geotextile reinforcement

Construction Issues




Embankment works, revised fill, impact on geogrid reinforcement

Construction Issues




Embankment works, working in the winter

Construction Issues




Embankment works, geocomposite and geomembrane deployment

Construction Issues




Embankment works, uv protection geotextile and tyre wave wall

Construction Issues




Embankment works, soil retention geocomposite

Construction Issues




Basal works, prior to geocomposite deployment

Construction Issues




Basal works, deployment of geocomposite, GCL and geomembrane

Construction Issues




Basal works, deployment of geocomposite

Construction Issues




Basal works, deployed GCL

Construction Issues




Basal works, deployment of geomembrane

Construction Issues




Basal works, hydraulic bund

Construction Issues




Perimeter leachate trench

Construction Issues




Tested to capacity for Reservoir Act sign off, 2008

Construction Issues




Completed lagoon, 2008

Construction Issues




¾ piezometers installed at a number of locations to monitor PWP

Construction Issues




leachate level in lagoon

Construction Issues




ru typically between 0.2 and 0.3

Construction Issues




¾ Temporary vertical and horizontal stations at toe of embankment

¾ Permanent stations at top of embankment

Construction Issues




Construction Issues




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

Construction Issues



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

Construction Issues



Overview



¾ Background

¾ Site details

¾Design philosophy

¾ Design issues


¾ Conclusions

Conclusions

¾ R i f th d i i l ti t th t ti f



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



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 )



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



Conclusion slide

Thank you for your attention

[email protected] [email protected]

design and construction of a reinforced soil embankment on soft soil

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