piling & contaminated land - elqfelqf.org/assets/de - 2013 e mids contam land.pdf ·...
TRANSCRIPT
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Piling & Contaminated Land
Dr Derek Egan
Technical Director Keller Ltd
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The FPS began in 1963 with 5 companies.
Now we have 18 members accounting for circa £500M of
specialist geotechnical contracting per annum.
Membership by rigorous registration.
Three sub committees
Health, Safety & Training
Plant
Commercial
Technical
FEDERATION OF PILING SPECIALISTS
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Aims
The FPS...... • Promoting Best Practice in safety, innovation, technical
excellence, value engineering & quality management;
• Independently audited membership.
Past initiatives include:-
•safety of temporary working platforms,
•working adjacent to railway lines,
•ICE Piling & Embedded Retaining Wall Specification,
•publication of a range of guidance notes relevant to piling
www.fps.org.uk
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What governs the choice of foundation solution?
Structure Size/weight
Form
Geology &
Groundwater
Economy Time, Cost
Sustainability
Environment Noise
Vibration
Logistics
Access
Weight Limits
Performance Load-settlement
Durability
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Considerations when encountering contaminated land
Durability Ground
&
Ground water
Flow Paths Temporary
Permanent
Transport Pushing contaminated
Soil down/sideways
Health Chemical
Particulate
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Considerations when encountering contaminated land
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Replacement
• Continuous Flight
• Auger (CFA)
• Rotary Bored
Displacement
• Driven PC Piles
• Driven Cast In-situ
• Steel Tubes
• Sheet piles
Piling
• High capacity
• Low noise & vib.
• In-situ concrete
• Creates spoil
• Moderate capacity
• No spoil
• Noise & vib.
(but less so these days)
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Cast In-situ Displacement Piling
Driven cast in-situ piling
Olympic Stadium
Old car plant
Daganham
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Cast In-situ Displacement Piling
Vibro concrete columns
Olympic Stadium
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Other Cast Insitu Piling
Rotary Displacement
Continuous Flight
Auger
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Pre-formed Displacement Piles
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Replacement
• Continuous Flight
• Auger (CFA)
• Rotary Bored
Displacement
• Driven PC Piles
• Driven Cast In-situ
• Steel Tubes
• Sheet piles
• Soil Mixing
• In-situ/ex-situ
• Dynamic Compaction
• Vibro Stone Columns
• Vibro Compaction
Piling Ground
Treatment
• High capacity
• Low noise & vib.
• In-situ concrete
• Creates spoil
• Moderate capacity
• No spoil
• Noise & vib.
(but less so these days)
• Lower capacity
• No spoil
• Potential for pathways
Cast in-situ Pre-formed
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Ground Improvement Techniques include – vibro stone columns, vibro compaction, dynamic
compaction, deep soil mixing (wet & dry), grouting (jet grouting,
compaction grouting, in-fill grouting etc)
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Process
Decide if
site/groundwater
is contaminated EC7 Desk study & GI
No more action
required
No
Consider the need/do
a Foundation Works Risk
Assessment Report
Consider potential adverse
impacts using
Source-Pathway-Receptor approach
Yes
Mitigate risk
Consider 6 Scenarios
(Westcott et al)
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The 6 Scenarios
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The Six Scenarios Pollution Scenario 1
Creation of preferential Pathways through an aquitard to allow potential contamination of an aquifer.
Contaminated Layer
Aquitard
Aquifer
Driven pile, VCC,
Stone Column
Contamination
of Aquifer
Source
Pathway
Receptor
• Driven piles – no open void
- cast in-situ enhanced
sealing with the ground
• CFA – OK but opportunity for
temporary pathway
• VSC – open pores – unless
special measures taken
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Use of QC/QA to ensure installation is correctly recorded
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Pollution Scenario 2
Creation of preferential Pathways through a low permeability surface layer, allowing migration of landfill gas, soil gas or contaminant vapours to the surface.
Low Permeability Cap
Gassing material/soil
Founding stratum
Pile, VCC, Stone Column
Gas
Source
Pathway
Receptor
• Driven piles – no open void
- cast in-situ enhanced
sealing with the ground
• CFA – OK but opportunity for
temporary pathway
• VSC – open pores – unless
special measures taken
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Contaminated Layer
Aquitard
Aquifer
Contamination
of Aquifer
Source
Pathway
Receptor
The dilemma:-
• VSCs need to be ‘flexible’ to interact with the ground upon loading
• VSCs may need sealing to prevent a pathway
• Cementitious grout/concrete does not fulfil the flexibility criterion
Innovation - ESCs
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Innovation - ESCs
The solution - development of a flexible non-setting grout:-
• low permeability (k = 10-8 m/s)
• does not ‘set’ leading to hard spot
• allows standard column construction
Dartford, Powdermill Road
Strip Footing
qw=125kPa
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
0 20 40 60 80 100 120 140 160 180 200
Bearing Pressure (kPa)
Set
tlem
ent
(mm
)
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Pollution Scenario 3
Direct contact of site workers and others with contaminated soil arisings that have been brought to the surface.
Isolation Layer/Cap
Waste/contaminated material
Founding stratum
Replacement
Pile
Receptor
(Poisoned worker)
Contaminated
Spoil
Source
Pathway
• ‘Rotating’ replacement
piles CFA, rotary bored (lesser extend rotary
displacement)
• Displacement systems – no
spoil
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Pollution Scenario 4
Direct contact with contaminated soil or leachate causing degradation of materials - durability
Contaminated/aggressive
Soil
Founding stratum
Pile, VCC, Stone Column
Attack Attack Source Pathway
Receptor
• Mainly concerns concrete
• Use BRE SD1 (2005)
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Pollution Scenario 5
The pushing of solid contaminants down into an aquifer during pile driving.
Contaminated Layer
Aquitard
Aquifer
Pile, VCC,
Stone Column
Contaminated Soil
• Effects displacement piles
only - but not that critical.
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• Solid piles – drag down max. 3d – pointed shoes help – but
not really necessary?
• H piles – trapped soil risk
• Tubular piles – soil plugging
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Pollution Scenario 6
Contamination of groundwater and subsequently, surface waters by wet concrete, cement paste or grout
Fractured/permeable soil
Pile/VCC
Groundwater
contamination
• Natural soil + very open structure
& flowing water
• Services/drainage/voids –
greater risk?
• Only affects cast in-situ
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Foundation Works Risk Assessment Report
1. Geology/SI
2. Selection of preferred piling
method – considering geology,
structural, noise, vibration etc.
3. Adverse impacts due to
preferred method
4. Site specific assessment of
magnitude & consequence of
risk
5. Changes/special measures
required
6. QA/QC measures
7. Final selected method –
considering geotechnical,
financial, environmental
considerations.
To be undertaken by the scheme designer.
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A Note on Ground Investigation
Poor quality geotechnical investigations still rife.
FPS Survey 2006
• ~20% of all contracts in the UK in a 3 month period (of all sizes)
• 30% of projects had Poor SI
• 55% thought between geotechnical & environmental elements wrong
NHBC Survey 2010
•60% of respondents – insufficient SI undertaken for low rise developments;
• most strongly felt by foundation designers compared to house builders and developers (but are they at the front line of taking the risk?).
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• On 16% of projects no BH location plan
• On 73% of projects BHs not levelled (83% not
coordinated)
• In 59% of cases inadequate topographical
information
• Boreholes too shallow (at least 5m below pile
toe, may need to be more for large rafts)?;
• Insufficient insitu and/or laboratory geotechnical
testing (e.g. SPTs every 1m to 2m).
Some common failings
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Summary
Contaminated land:-
• presents an added, but surmountable, additional complexity;
• the 6 scenario approach provides a systematic assessment
framework;
• knowledge of different piling & ground improvement options informs
the choice;
• mitigation don’t be afraid to demand high levels of QA/QC from
contractors.
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Thank you for listening.