Piling & Contaminated Land. Dr Derek Egan Technical Director Keller Ltd

Transcription

1 Piling & Contaminated Land Dr Derek Egan Technical Director Keller Ltd

2 FEDERATION OF PILING SPECIALISTS 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

3 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 Aims

4 What governs the choice of foundation solution? Structure Size/weight Form Geology & Groundwater Performance Load-settlement Durability Environment Noise Vibration Economy Time, Cost Sustainability Logistics Access Weight Limits

5 Considerations when encountering contaminated land Transport Pushing contaminated Soil down/sideways Durability Ground & Ground water Health Chemical Particulate Flow Paths Temporary Permanent

6 Considerations when encountering contaminated land

7 Piling Replacement Continuous Flight Auger (CFA) Rotary Bored Displacement Driven PC Piles Driven Cast In-situ Steel Tubes Sheet piles High capacity Low noise & vib. In-situ concrete Creates spoil Moderate capacity No spoil Noise & vib. (but less so these days)

8 Cast In-situ Displacement Piling Driven cast in-situ piling Olympic Stadium Old car plant Daganham

9 Cast In-situ Displacement Piling Vibro concrete columns Olympic Stadium

10 Other Cast Insitu Piling Continuous Flight Auger Rotary Displacement

11 Pre-formed Displacement Piles

12 Piling Ground Treatment 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 High capacity Low noise & vib. In-situ concrete Creates spoil Cast in-situ Moderate capacity No spoil Noise & vib. (but less so these days) Pre-formed Lower capacity No spoil Potential for pathways

13 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)

14 Process Decide if site/groundwater is contaminated EC7 Desk study & GI No No more action required Yes Consider potential adverse impacts using Source-Pathway-Receptor approach Mitigate risk Consider 6 Scenarios (Westcott et al) Consider the need/do a Foundation Works Risk Assessment Report

15 The 6 Scenarios

16 The Six Scenarios Pollution Scenario 1 Creation of preferential Pathways through an aquitard to allow potential contamination of an aquifer. Driven pile, VCC, Stone Column 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 Source Contaminated Layer Pathway Receptor Contamination of Aquifer Aquitard Aquifer

17 Use of QC/QA to ensure installation is correctly recorded

18 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. Pile, VCC, Stone Column Receptor Driven piles no open void - cast in-situ enhanced sealing with the ground CFA OK but opportunity for temporary pathway Gas VSC open pores unless special measures taken Pathway Low Permeability Cap Source Gassing material/soil Founding stratum

19 Innovation - ESCs 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 Source Pathway Receptor Contamination of Aquifer Contaminated Layer Aquitard Aquifer

20 Settlement (mm 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 Bearing Pressure (kpa)

21 Pollution Scenario 3 Direct contact of site workers and others with contaminated soil arisings that have been brought to the surface. Replacement Pile Receptor (Poisoned worker) Contaminated Spoil Rotating replacement piles CFA, rotary bored (lesser extend rotary displacement) Displacement systems no spoil Pathway Isolation Layer/Cap Source Waste/contaminated material Founding stratum

22 Pollution Scenario 4 Direct contact with contaminated soil or leachate causing degradation of materials - durability Mainly concerns concrete Use BRE SD1 (2005) Pile, VCC, Stone Column Source Pathway Receptor Attack Attack Contaminated/aggressive Soil Founding stratum

23 Pollution Scenario 5 The pushing of solid contaminants down into an aquifer during pile driving. Pile, VCC, Stone Column Effects displacement piles only - but not that critical. Contaminated Layer Contaminated Soil Aquitard Aquifer

24 Solid piles drag down max. 3d pointed shoes help but not really necessary? H piles trapped soil risk Tubular piles soil plugging

25 Pollution Scenario 6 Contamination of groundwater and subsequently, surface waters by wet concrete, cement paste or grout Pile/VCC Natural soil + very open structure & flowing water Services/drainage/voids greater risk? Only affects cast in-situ Groundwater contamination Fractured/permeable soil

26 Foundation Works Risk Assessment Report To be undertaken by the scheme designer. 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.

27 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 % 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?).

28 Some common failings 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).

29 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.

30 Thank you for listening.