Beating the elements
White Young Green Environmental reports on remediation strategies on a truly grand scale.
With any inner city regeneration programme it is always interesting to compare the development schemes of the present with the land use of the past.
The ambitious Edinburgh Waterfront development is a case in point where over 30 per cent of the site under development is the former home of Scotland’s largest single gas-producing unit, the Granton Gasworks.
When the new purpose-designed 10,000m2 office building by architects Foster & Partners is completed in the summer of 2003, the only subtle reminder of the site’s history will be in the name of the building’s occupiers. Scottish Gas will use the totally energy efficient building as its Scottish headquarters.
The substantial transformation during the last five years has taken place with more than £20 million of financial support from SecondSite (formerly Lattice Property Holdings) which manages the former property portfolio of British Gas. This funding has primarily been used towards the cost of a new site access road and to undertake the current programme of site works including the demolition of the site’s largest gasholder, and diversions of gas supply pipeworks to free up the site for redevelopment.
In its heyday, Granton Gasworks which opened in 1899 was producing over 35 million cubic feet of gas per day; providing town gas, chemical by-products and coke for Edinburgh and the surrounding area. Expansion and modernisation continued throughout the life of the works. However, the coal carbonisation plant ceased production in the 1960s and since the introduction of natural gas in the mid 1970s the site has been used simply as a gas distribution centre and service depot.
Until recently, the majority of the site was operated by Transco and included three gasholders, offices, a quarry pond and areas of remediated ground and surplus land.
The Edinburgh office of consulting engineers and project managers White Young Green has been involved at the 110-acre site since 1998. The civil and structural team has looked after the development and infrastructure at Granton whilst the environmental division, with its substantial experience of remediation techniques, was particularly well-placed to carry out investigation and remediation works.
Ground contamination investigations began with the excavation of 153 trial pits and three trial trenches. Phase 2 comprised 127 trial pits and six boreholes. Previous investigations on the site had identified that two groundwater tables existed beneath the site.
Surprisingly with such an intensive industrial past, site investigations revealed that the majority of the site was uncontaminated. However, significant soil and groundwater contamination by spent oxide, coal tars and hydrocarbons was identified in certain areas of the site.
The remediation strategy for Phase I works was devised so that target levels would be protective of groundwater and appropriate for the proposed mixed commercial / residential use with predominantly hard cover.
Phase I remediation works were undertaken between 2001/2002 and involved the demolition of some site structures, removal of redundant gas pipework and plant, excavation of contaminated soils and uncontaminated overburden soils to allow access to potentially contaminated areas.
Andy McCusker, associate at White Young Green Environmental and project manager for the remediation works, takes up the story:
“The biggest challenge overall was in agreeing the remediation strategy with the local authority whilst the statutory regime was still being developed and implemented. Discussions were ongoing at a time of emerging legislation under Part IIA of the Environmental Protection Act 1990 which was introducing the concept of risk-based assessment for remediation work along with the development of source-pathway-receptor conceptual site models.
“On the practical side, the greatest challenge was the sheer volume of material we had excavated and removed. At its peak we had in excess of 60 truck loads of material leaving the site for licensed tips.
“One of the key issues for us was the need to reclaim and reuse as much of the contaminated material as possible in line with our commitment to sustainable solutions and the need to minimise the volume of waste to landfill.
“To this end, approximately 4,500m3 of excavated diesel contaminated soils were placed in biopiles for ex-situ bioremediation. This process involves adding nutrients to stimulate naturally occurring bacteria in the soils to break down the hydrocarbon contamination.
“Screening of broken out and demolished site structures generated graded material for re-use as fill during the remediation and redevelopment works.
“Infilling of excavations was undertaken using site derived selected excavation materials.
“Moderately contaminated water encountered in remediation excavations was removed and treated on site before discharge to a foul sewer via an existing Transco effluent treatment plant.
“At the present time we have recently begun the second phase of remediation works which will address the outstanding areas of contamination from within the main former process areas of the gas works.”