An A-Z of remediation and geotechnical work is being used to match an equally diverse range of ground conditions for a seafront development in the Channel Islands.
Alegacy of contamination is requiring a whole host of remediation work on the seafront at St Peter Port, Guernsey. The 7.3ha site at Admiral Park, Le Bouet includes a former gasworks, petrol station, car workshop and power station.
Offices, a supermarket, DIY store and houses will comprise the redevelopment.
In 1998 consultant WSP Environmental undertook a contaminated land site investigation for landowner Comprop. This has been developed into a remediation design. The scope of work covers contamination assessment, geotechnical investigation and temporary works design, structural engineering services, infrastructure design, odour advice, waste disposal advice and remediation services.
Les Banques road separates the foreshore from the development, which is divided into two main areas, the Le Bouet site and the Blue Diamond site.
Ground conditions were generally fill over a sandy silt that overlies a silty clay and Gabbro bedrock. Investigations concluded there were low concentrations of heavy metals and phytotoxic metals in most of the fill. Tar and oil hydrocarbon contamination, particularly in the gasworks and fuel storage areas, were also found.
'Guernsey does not have any specific contaminated land legislation, and remedial target criteria were agreed with the States of Guernsey using a similar risk based approach to that in the UK, ' explains Martin Richell of WSP Environmental.
'Had UK mainland guidance been in force, the polyaromatic hydrocarbon (PAH) and total petroleum hydrocarbon (TPH) concentrations identified would most likely have classified the soils as 'special waste' [requiring disposal in an engineered landfill].'
Landfills on the island are not licensed to accept contaminated soil and agreements with the UK government restrict its export.
Disposal elsewhere in Europe would not have been commercially viable.
WSP's approach was to remediate and reuse the materials, a process which involves the on-site bioremediation of 10,000m3 of soils.
WSP worked with local engineering firm Miller & Baird, which provided plant and machinery. WSP was responsible for design, management, implementation and validation of remediation within the former power station, gasworks and service station.
A phased approach was adopted to allow areas to be released for construction and development over two to three years.
Infrastructure, including gasholder bases, underground tanks and structures were broken out and concrete crushed for reuse.
Remediation began with cleanup of hydrocarbon contamination in the former scrapyard and power station site.
A large area of hydrocarbon contamination in the former service station and car workshops was dealt with next, followed by the former gasworks.
'The objective was to remove and treat contamination posing a potential risk to future site occupiers and to minimise the potential ingress of contamination from adjoining sites, ' explains George Cowie, WSP Remediation resident engineer.
'Remediation targets were 1000mg/kg for TPH and 1500mg/kg for PAH.' Bioremediation involved screening the soil to remove oversize material, then mixing in nutrients and organic additives to optimise treatment conditions.
The soil was aerated using a screening bucket mounted on an excavator. Windrows were covered while not being turned and then 2% lime was added to stabilise the treated materials before using it as backfill.
WSP's laboratories in Nottingham carried out chemical tests for acceptability on proposed backfill materials and for validation on excavation formation depth after remediation was carried out on soils.
After treatment the soils in the windrows had concentrations of TPH and PAH well below the remediation criteria and were therefore suitable for reuse as backfill. TPH concentrations ranged from below detectable limits to 863mg/kg, and most of the PAH concentrations from below detectable limits to 200mg/kg.
The gasworks retort house has presented particular difficulties because of the structures and depth of contamination. The 0.7ha site is earmarked for a retail store.
'When work began in this part of the site in late 2001, parts of the retort house were still occupied and only the yard area could be remediated, ' says Cowie.
'After demolition, we found a broad layer of TPH and PAH contaminated soil under the yard area which went on for about 15m under the building.
Concentrations of TPH in this layer were over 40,000mg/kg and free tar was also discovered.' Other contaminants associated with gaswork sites, including heavy metals, cyanides, sulphur and sulphate, were not generally found in concentrations that would represent a risk to human health or the environment.
Remediation included the removal of contaminated soils to maximum depths of 6m.
Redundant services were sealed at the site boundary to mitigate potential preferential contaminant migration pathways.
A double layer of 1000 gauge Visqueen gas membrane was used at the site boundary to form an impermeable barrier preventing contamination of clean fill from adjacent areas that have not been remediated.
The excavations were backfilled using a combination of clean imported fill with treated site material placed below the top 500mm of crushed concrete.
As the site was near residential areas, the Environmental Health Department had expressed concern about odours from the excavation and bioremediation.
'To reduce the risk, an odour suppressing compound was used, ' says Cowie.
'Bioremediation was stopped when the wind could have caused odours to spread and windrows were covered when not being aerated.' A monitoring programme was also undertaken with the co-operation of local residents.
Water treatment and dewatering of excavations also proved a key part of the project.
Perched water was frequently encountered resting on the clay silt layer at the base of the excavations. It was removed using compressed air powered submersible pumps.
The water was stored in a holding tank before being pumped through a treatment plant. This comprised an oil/water separator and an activated charcoal filter, allowing water to be discharged to foul sewer.
WSP's involvement in the overall project is expected to finish with the construction of the last phase of the site next year.
Car park complexities
The basement car park for the Admiral Park development is two storeys deep and covers 1.3ha of the frontage.
It is estimated that about 90,000m3 will be excavated and removed from site. WSP is also designing the structure and proposed that the lower slab and the outer walls be reinforced concrete, built inside a temporary anchored/ propped sheet pile retaining wall. The lower slab will act as a raft foundation to transfer column loads from the structures to the ground.
'The sheet piles were driven to refusal by ground contractor Geomarine in the weathered gabbro, ' explains Richard Evans, associate director at WSP Geotechnics. Limited penetration meant the sheet pile retaining wall needed anchoring or propping to be stable during excavation.
Typically two or three rows of anchors have to penetrate into the weathered gabbro bedrock to get sufficient fixed anchor length.
Props will be used in sections of the retaining wall where adjacent services or buildings preclude the use of anchors.
Ascertaining the depth of the gabbro was vital to design, so a probe hole investigation using hollow stem auger equipment was carried out earlier this year. Most of the holes were around the site perimeter, close to the line of the proposed sheet pile wall.
Probe holes were taken to a depth where a significant increase in resistance was encountered. This was taken as the interface between the relatively soft overlying beach and lagoon deposits and the dense weathered gabbro strata.
From this, a plan was developed showing the likely founding geology across the site. It is anticipated that ground conditions will vary greatly from the softer beach deposits, weathered gabbro (sometimes seen as a gravel deposit) to the intact fresh gabbro.
The basement slab was therefore designed using estimated spring stiffnesses derived from laboratory tests and anticipated movements modelled under the changing tidal groundwater regime.
The level of the basement relative to the groundwater levels at the site will require measures to resist hydrostatic uplift forces.
Tie down anchors will be used to resist these forces both in the temporary and permanent conditions. Those in the permanent structure will have to be double corrosion protected from the salty groundwater.
Dewatering will be needed to allow construction of the lower basement slab, 7m down. High inflows of between 1m 3/s to 1.5m 3/s will be controlled by adopting a phased approach for the excavation works.
Groundwater treatment has consisted of desilting before disposal. But WSP is monitoring water quality and should hydrocarbon contamination be encountered the oil/water separator and activated charcoal filter treatment plant would be used.
Waste disposal criteria have been accepted by the States Board. Disposal of waste materials and waters from the excavation will be monitored by WSP.