Case Study 03

Using PFA/Fly ash for grouting.

Pulverised fuel ash (PFA), known as fly ash in many countries, has been used for a number of years within the West Midlands area for filling old mine workings. Back in the early 1980’s it became apparent that a major problem existed across the region from abandoned limestone mines. As a result a comprehensive government backed study was initiated to identify the full scale of the problem and consider a way forward. In view of the large volumes involved, the use of either natural virgin aggregates or traditional concrete mixes would have been inordinately expensive. Although PFA/cement grouts have been used extensively across the region for the treatment of abandoned coal mines and shafts they had not been used for large open void mine projects.

Over the following years, cost effective mine infilling solutions were developed, including the use of PFA / cement paste based on a 50:1 mix. This PFA ‘paste’ has a relatively low water / solids ratio and satisfactory strength. Consequently, since the late 1980’s continuing to the present day, a number of large scale infill contracts have been undertaken throughout the Metropolitan Boroughs of Dudley, Sandwell, Walsall and Wolverhampton in the West Midlands (UK) together with Shropshire County Council. Initially the Department of Environment (DoE) funded these contracts, and more recently English Partnerships, under the Derelict and Natural Land Stabilisation programme, to remove the blight of these limestone caverns. Mons Hill mine, in Dudley, is one of a long line of projects completed in the region over the last 17 years.

The Mons Hill site is a nature reserve and an area of special geological interest. The fossils found in the limestone are considered some of the finest in the world, especially the ammonites. In addition, housing surrounds the nature reserve, so the highest standard of protection to the local environment was required. However, the mine under the site was in danger of collapse and infilling commenced in June 2001.

Forkers Ltd is one of the main contractors who specialise in large grouting operations of this type. At Mons Hill a two storey container wall surrounds the production plant, see figure 1, and material storage area. This provides good environmental protection, reducing dust and noise, and gives good site security. PFA is supplied in a moist condition and up to 1,800 tonnes may be stockpiled. In order to prevent dust a series of fine mist spraying nozzles are fixed above the stockpiles and production area, maintaining the PFA in it’s moistened state, see figure 2.

The PFA is taken from the stockpile and placed in a hopper where, using a continuous weigh belt system, it is conveyed to a Belmix counter rotating paddle mixer. The Portland cement is laid onto the PFA on the conveyor belt using another weigh belt system. A computer automatically controls these belt weighers and the addition of water achieving the required mix and flow properties. Records of production and monitoring are made to ensure a well mixed, within specification product. The grout is regularly tested for flow and strength; however, the strength of the grout is typically only 0.7MPa. From the mixer the grout is fed to three Schwing concrete pumps, see figure 3. These pump the grout using a 125mm diameter steel pipeline to the injection points, see figures 5 and 6, which may be up to 1.5km from the production facility. This system can produce up to 3.5 tonnes per minute, or 1,900 tonnes per day, of grout.

Forkers Ltd have designed their ‘Groutmaster’ plant, where the grouting materials are never exposed to the air, ensuring dust free operation, see figure 4. The PFA is supplied dry in sealed tankers, similar to cement tankers. This is specifically designed for environmentally sensitive operations and can produce up to 300 tonnes of grout per day.

Because of the location of the site, its use and the requirements of the Environment Agency, Dudley Metropolitan Borough Council (DMBC) has carried out an Environmental Impact Assessment and comprehensive monitoring program. This involves the analysis of groundwater from numerous sampling points. Samples were taken prior to grouting and will continue for a long period of time to ensure no contamination of groundwater.

Because of the extensive precautions taken by the contractor, dust and water contamination from surface operations are minimal. In addition, the nature reserve wardens ensure the grouting operation does not impact on the wildlife and that nothing occurs that could put the reserve at risk.