An old South Yorkshire colliery and coke-works is getting a facelift. Damon Schünmann examines the nip and tuck.
Not so very long ago, redundant coke-works were buried underground and forgotten about. Now in an era of comparative enlightenment, these former industrial sites are being excavated, scrubbed up and returned to their former glory.
One such beneficiary is the former Dinnington colliery and coke-works in South Yorkshire where 700,000m 3of made ground is being dug out and cleaned where necessary, ready for re-use.
Main contractor Birse Civils appointed Scott Wilson to carry out the design and validation elements of the £10M project that will reclaim 0.85km 2of land, 0.23km 2of which is earmarked for redevelopment while a major part on the site's southern side will remain as countryside park for local residents. Work includes roads, paths, landscaping, and ecological and traffic assessments.
Works started on site in April 2003. Client Renaissance South Yorkshire retained Encia Consulting to project manage the scheme following its involvement at the initial site investigation stage.
Contamination is concentrated at the north end of the site where the coke-works and colliery were situated, and it is here that earthworks subcontactor Cheetham Hill Construction is digging out the made ground - up to 4m deep - covering the old works, so that it can be categorised and remediated.
The ground comprises colliery spoil containing coke-works wastes, and sits on sandy clay overlying a sequence of coal measures strata of sandstone and mudstone.
Scott Wilson head of land contamination David Cragg says: 'We needed tight control of the information on the location of the materials and test results both at the point of excavation, and where re-deposited in the finished works, for the potential purchasers and users of the site.'
A network of sump tanks and connecting pipework had leaked a heady brew of waste.
This included polyaromatic hydrocarbons, oils, ammoniacal compounds and a range of inorganic compounds such as heavy metals.
Birse Civils site agent Lee Tinkler says: 'Such was the varied material in the ground, getting to grips with it was one of the biggest challenges.'
Scott Wilson resident engineer James Briscoe adds: 'You could be digging out plain colliery shale that could be re-used, and then 2m further the bucket would come out dripping tar as it excavated inside a coke-works tank.'
The degree of contamination has required an extensive multi-phased site investigation. As well as the preliminary investigation conducted in 1997, additional trial pits have been carried out on a 25m by 25m grid in the coke-works area, which site workers have nicknamed 'Battenberg grids'.
Samples from these at 1m depth intervals confirm contaminant concentrations to allow 'sentencing' of materials for either re-use in the development or remediation prior to re-use. On top of this, the Scott Wilson design specification requires a point of excavation suite of chemical tests for every 5,000m 3of material.As expected, site workers are finding that the heavier contaminants like liquid tar have sunk to the bottom of the made ground.
Subcontractor WSP Remediation (WSP) is responsible for work to reduce hydrocarbon concentrations using a combination of biopiles and windrows. In both methods, indigenous bacteria in the soil are encouraged to break down the pollutants in the process of respiration, the breakdown products being carbon dioxide, water, inorganic compounds and cell protein.
This aerobic degradation generates heat as a by-product and WSP is using fleeces to cover the windrows and biopiles to regulate the temperature and moisture content. The fleeces also control the odours from excavated material to protect local residents.
WSP adds shredded green waste, phosphates and nitrates to accelerate the bioremediation process. It is also injecting air into the static biopiles via a network of perforated pipework to promote aerobic conditions and growth in the bacteria. The windrows are turned with excavators negating the need for air injection to achieve this.
But there is still the potential for pollutants to sink back into the ground while these processes take place. Briscoe explains: 'Any leachate from the contaminated material on the remediation area is collected in a perimeter drainage trench and pumped into water treatment tanks. Primary settlement tanks remove solids and a flotation tank separates floating matter, which is skimmed. After this, it goes through carbon filters.' The remainder is fit for sewer discharge.
However, heavy contamination has not spread throughout the region. Of the 600,000m 3removed and processed from the coke-works area, only 30,000m 3to 35,000m 3has required bio-remediation.
To make the site attractive to a developer, there are stringent requirements for the reinstated made ground, which must be backfilled and compacted in 250mm layers to a 4m depth.
It is tested for in situ density and air voids content as compaction proceeds.
As each metre thickness is laid, the surface is tested by 600mm diameter plate load testing.
Cragg explains that the results typically show 10mm settlement for a load of 100kN/m 2,15mm for 150kN/m 2and 25mm for a 300kN/m 2load.
In contrast, at the southern part of the site, the soil is required to function as a growing medium and has undergone de-compaction to encourage plant growth. Here, Scott Wilson has designed a bird scrape and hide, ponds, woodland and meadow.
Encia senior principal engineer John Coggins says: 'We wanted to create a soil that allows rainfall to infiltrate. It's the opposite of what we're doing in the areas earmarked for development where we want the surface to shed water.
One of the reasons that landscaping schemes like this have failed in the past is because they have used heavy plant such as dozers to lay the material so that the soil becomes compact, but we have used a backactor to keep the soil texture open and aerated.'
The main earthworks part of the scheme is due to finish this month.