Sullart Street, Cockermouth

DESIGN

The financial viability to the whole site was finding a way of importing 40,000 ton of material and being able to place housing on top of this fill. Imported clay with lime stabilisation was the preferred method after other scenarios were costed up, such as piling.

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SITE HISTORY

Historic records showed that the site was once a Gas Works and until the purchase of the site from Cumbria County Council it was a local Highways maintenance depot.

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GROUND CONDITIONS AND TOPOGRAPHY

Generally, the old surface was an existing yard was a mixture of concrete and tarmac. The whole area was split into two levels with a 6m high retaining wall running through the middle of the site. Ground Investigations identified the following issues:

• Historical pond/tar pit
• Underground diesel storage tanks & interceptor which contained contaminated water
• Hazardous hotspots
• Foundations from previous buildings
• No asbestos was present

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CHALLENGES

• As the site was split level it was decided to make the site one gradient. As reduced level dig was not an option due to the adjacent levels off site the only other option was to mass fill the lower level. This would require filling areas which were 6m in depth and importing 20,000 m3 of fill material. This then led onto the next problem, which was how could we build homes on fill without future settlement. Piling was discounted as it was making the site commercially unviable. The solution we settled upon was importing clay (which came from our other sites and was essentially free material) and then the imported clay was stabilised with lime. The new homes were built on this lime stabilised imported fill.
• Identifying hazardous hotspots and removing safely from site.
• Removing the minimum of contaminated material from site as this was a major cost component to the budget.
• Strategy for removal of invasive weeds such as Japanese Knotweed
• Pollution risk control measures required to protect the adjacent watercourse as the existing site surface water drainage emptied into the river Derwent which is a SSSI
• Nuisance to adjacent housing and elderly care home had to be minimised with respect to dust ,noise and vehicular movements

WORKS INCLUDED

• Removal of the Hydrocarbon hotspots around the site. Soils were excavated and stockpiled based on visual and olfactory evidence of hydrocarbon contamination.  The excavations were extended until all significant hydrocarbon contamination was removed.  A Photo Ionization Detector (PID) was used to confirm that the sides and bases of the completed excavations were free from contamination. All contaminated soil was transported off site as hazardous waste in 30 tonne wagons for treatment at the Augean Waste Treatment site at Port Clarence, Middlesbrough.
• Breaking up the existing concrete and tarmac surface, crushing & screening on site to create 10,000 tons of hardcore which was to be later used in the construction phase
• Contaminated water was transported off site by GED Environmental Services as hazardous waste by road tanker to a licensed water treatment facility
• Decommissioned and removal of the underground storage tanks in accordance with the Environmental Agency guidance on removal of underground storage tanks document. The Petroleum Licensing Office in Cumbria County Council was also informed of the remedial works. The three tanks and interceptor were cleaned, excavated, and removed and transported to a licensed waste recycling facility.
• Importing clay which was then Lime stabilised. 20,000m3 of clay was imported, graded to levels in 150mm thick layers. Each layer was then stabilised with lime and rolled to achieve the CBR value.
• Full time laboratory testing was required on all materials excavated within the site
• Removal of previous foundations and industrial workings

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LESSON LEARNT DURING STAGE 1 DESIGN

• Different design options need to be costed as the design progresses to value engineer the site

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SUCCESSFUL OUTCOMES

• All remediation was completed in line with the agreed strategy and to the stipulations set out by the Local Authority as part of the Planning Conditions.
• The remediation works were supervised and validated by a qualified and experienced Senior Environmental Specialist Consultant. Following the completion of the works a report was compiled by the Environmental Consultant detailing all site enabling works that were undertaken, waste consignment notes and all site investigations, laboratory test certificates and validation testing that was undertaken
• The project was delivered with zero reportable Health & Safety incidents.
• The project was delivered with zero environmental notices issued.
• No customer or third-party complaints
• Early contractor involvement was essential for a successful outcome of the scheme for buildability and budget constraints