Use of pulverized-fuel ash in land restoration for the Millennium Coastal Park

The creation of the Millennium Coastal Park, a 22 km stretch of coastline at Llanelli in South Wales, has led to the transformation of the land blighted by industries of the 19^th and 20^th Centuries, including the old Carmarthen Bay power station site. The ethos when constructing the Millennium Coastal Park was to minimise the amount of material brought onto the site. In total 800,000 tonnes of pulverized-fuel ash from the old power station lagoons have been utilised for a number of applications including:

• Ground remediation - capping of contaminated industrial sites in the Burry Port area;

• Lightweight engineering fill - for backfill to land bridges and promenades;

• Lime / cement stabilised sub-base – for temporary roads and cycle ways;

• Landscaping - for creation of earth sculpture and a community woodland.

Ground remediation

Pulverized-fuel ash is a low permeability material, which can be used to cap sites where industrial contaminants must be contained. Typically, compacted pulverized-fuel ash has a permeability of 10^-7 m/s. When pulverized-fuel ash is mixed with lime or Portland cement a pozzolanic reaction, which further reduces the permeability and increases strength of the capping material, takes place over time. Over 400,000 tonnes of pulverized-fuel ash was re-used in ground remediation on the Millennium Coastal Park site.

Lightweight engineering fill

Approximately 300,000 tonnes of pulverized-fuel ash was used as a lightweight engineering fill for the two bridges on site, which span the railway. They provide access to seaward land on the site. The backfill to these bridges was pulverized-fuel ash placed and compacted to form a lightweight fill. In addition, the bridges are segmental precast units that contain pulverized-fuel ash, as a cementitious addition, to give added strength and durability to the structures. They are the largest of their type built with the UK.

Lime/cement stabilisation to form sub-base

For temporary haul roads, pulverized-fuel ash was stabilised with lime to form the sub-base. These have proven to be durable during the contract period. Trial areas have been produced of blends of pulverized-fuel ash, lime and Portland cement. This stabilisation process minimises the need to import primary aggregate sub-base materials to the site.

Landscaping

A series of pools, woodland areas, footpaths and cycle ways have all been formed along a four mile stretch of the coastline using pulverized-fuel ash blended with fertilisers for ground cover. Both sewage sludge and silt extracted from Burry Port harbour have been used to provide a source of nutrients for such landscaping work. Approximately 100,000 tonnes of pulverized-fuel ash was used including using compacted pulverized-fuel ash for an earth sculpture.

The direct savings in using pulverized-fuel ash compared to a comparable primary product are shown in the table below.

The £1.25/tonne cost for the pulverized-fuel ash includes transporting the material around the site and handling costs.

Reusing the pulverized-fuel ash on site provided potential savings of £1,280,000 in Landfill Tax costs at £2/tonne for Carmarthenshire County Council.

Indirect benefits

The reuse of the pulverized-fuel ash on site provided a number of additional environmental benefits:

• The avoidance of 800,000 tonnes of material going to landfill;

• By using 800,000 tonnes of pulverized-fuel ash on site, an estimated 40,000 lorry journeys and the associated environmental impacts have been avoided.

The pulverized-fuel ash was a very stable material and suited its applications well. Procedures are necessary to ensure that the material is damped down frequently to minimise dust that may be a nuisance to local residents. The pulverized-fuel ash is suitable topsoil media when blended with a source of fertiliser, with rapid growth of vegetation.

A wide range of other materials have been successfully recycled on this project, including silts from the harbour, sewage sludge, demolition waste from on site, and glass reinforced pipes recycled as drainage pipe.

Various tests were carried out on the pulverized-fuel ash to determine the moisture content and strength of the material. The material used complies with the Specification for Highway Works.

The Class 6R Selected granular fill was produced in accordance with the Specification for Highway Works