The road linking the massive MediaCity in Manchester to the motorway network sits on a highly contaminated former dock site that had to be cleaned up fast if the development is to open on schedule next year. GE reports.
Manchester’s ambitious MediaCity scheme is taking shape at Salford Quays, with the first phase set to open next year.
The development, designed to provide a purpose-built home for creative and digital businesses, could eventually cover as much as 81ha of former dockland as it grows over the next decade.
Phase 1, due to be completed in 2011, includes 65,000m2 of office space across five buildings, 23,000m2 of studios, 7,000m2 of retail and leisure facilities, 378 apartments, a 218-bed hotel, a new tram link, a public piazza for up to 4,000 people, 300 cycle bays and a 2,200-space multi-storey car park.
It is set to become the home to Salford University and the BBC, which is relocating five departments - including BBC Sport - and 2,500 staff from London.
Management contractor Bovis Lend Lease began construction in summer 2007, but a planning condition of the £500M scheme is that the project’s developer, Peel Media, must build a link road between the development and the motorway network before Phase 1 can open.
The route of the new 1km road, called the Broadway Link, partly follows a rail line from a former fuel storage facility to the Manchester Ship Canal that required a serious clean-up before construction could start.
Initial investigation works showed that the link road junction with Pacific Way had been contaminated by fuel leakages over an area estimated at about 740m2.
In September 2008, remediation specialist Terra Vac was contracted to investigate the full extent of contamination and undertake remediation in time for the completion of the road and opening of Phase 1.
“Our first task was to delineate the exact extent of the contamination by installing a series of remediation specification wells. In doing so we established that the contamination was far more widespread than initial assessments suggested,” explains Terra Vac director Andy Fraser.
“The lateral extent was found to be more than 3,300m2 - four times the original estimates. In places, fuel oil layers were in excess of 4m thick.”
This delineation exercise resulted in Terra Vac installing more than 200 boreholes to extract the contaminant.
Soil conditions consisted of made ground underlain by very sandy, clayey silt up to 3.5m thick.
This was underlain by an aquitard - a bed of low permeability between aquifers - made up of sandy, very clayey silt varying in thickness from 1m to 4m which, in turn, was underlain by a minor aquifer within coarse sand and gravel.
The thickness of the aquitard varied from 1m to 1.5m in the eastern half of the contaminant plume and 3m to 4m thick in the western half.
These varying subsoil conditions resulted in the two halves of the plume yielding an average contaminant thickness in the east of 186mm, with a maximum recorded thickness of 1.403m, and an average thickness in the west of 465mm, with a maximum recorded thickness of 4.31mm.
“When we discovered the size of the challenge - a plume four times the area originally thought, coupled with very difficult soil conditions - I knew that this time frame would be a challenge using in situ techniques and we negotiated a completion date of January 2010,” says Fraser.
Terra Vac presented the client with a range of options for remediating the site.
“The lateral extent was found to be more than 3,300m2 - four times the original estimates.”
Simple excavation and disposal was discounted, along with thermal treatment, on the basis of cost.
Due to the significant quantities of free phase oils, insitu processes such as oxidation and enhanced bioremediation were considered inappropriate.
Odour nuisance, space constraints and limited time meant ex-situ bioremediation was not really an option, as the remediation works had to run concurrently with the road construction.
Instead, high vacuum free product recovery was chosen, with two Terra Vac systems - specified, designed and built at the company’s offices and workshops in Castleford - installed to remove the contamination within the increasingly tight time frame.
The system works by creating sub-surface air flows induced by vacuum applied to each extraction well.
The vacuum is generated by an electrically-powered rotary lobe blower unit, and airflows are then used to lift liquids and vapours from soil pore spaces within the designed radius of influence of the extraction wells.
These liquids and vapours are then transported under vacuum with air flow to a phase separator vessel, where the vapours are filtered and discharged into the atmosphere and the liquids are pumped to an oil/water separator.
Recovered oils are separated and removed periodically by licensed carriers for potential re-use or recycling, and water is continuously filtered and discharged to sewers in accordance with local water company constraints.
“We operated two bespoke systems independently on the east and west sectors, on a 24/7 basis, using more than 200 extraction wells which were installed to 5m below ground level in the east sector and 7m below ground level in the west sector,” says Fraser.
“Free phase oils were targeted independently at each extraction point, with an operational run time of 330 days in the east sector and 350 days in the west sector, due to the differing soil conditions.”
In all, 8,400 litres of oil were recovered in the east sector and 17,874 litres of free phase oil in the west sector before the two systems were shut down at the end of December last year.
“Various complexities led to more negotiations with the Environment Agency than anticipated and, at a crucial point near completion, came the big freeze of December 2009, which presented additional site challenges,” recalls Fraser.
“However, the technique we chose is particularly efficient where the contamination is spread over a large area, and it can be applied to most light, non-aqueous phase liquid contaminants in a variety of soil conditions, but is particularly effective in less cohesive soils with soil pore spaces which permit better air flows.”
With the contaminants now removed, the road construction can press on, ensuring the development is ready for its new occupants when they arrive next year.