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Paving the way through mine stabilisation

BRITAINs largest-ever mine stabilisation job has generated 7.3 million of work for Midlands contractor P Forker.

The work at coal mines in Merthyr Tydfil which date back to the mid- 18th century is needed to stabilise the ground in advance of building work on the A470 trunk road improvement scheme for the Welsh Office.

A 4.5 km stretch of the new road crosses a series of old coal and ironstone workings, some of which are as deep as 40 m.

Consulting engineer Rendel Palmer and Tritton has used the results of site investigations carried out over the past 20 years to design the scheme.

The first investigations conducted in 1974 and 1976 were followed by a desk study by mining consultant Geoffrey Walton.

Norwest Holst carried out further investigations in 1990, which included 50 cored bore holes and 228 geophysical surveys.

The ground investigation and desk study has proved to be very accurate in depth prediction of the seams, said Byron Broadstock, resident engineer at Rendel Palmer and Tritton.

But despite the information available it was still difficult to predict the volume of voids that needed filling.

As a result, the original estimate of 220,000 tonnes of grout dropped to 150,000 tonnes.

The seams have been in a greater state of collapse than would have been expected, said Mr Broadstock.

The stabilisation area covers the footprint of the road, and has been divided into individually treated sections.

In the first stage of stabilisation, the contractor drills a double row of holes at 3 m centres around the perimeter of the section.

The holes are then grouted to effectively seal the area.

The contractor then drills and grouts holes on a 6 m primary grid.

Depending on how much grout these primary holes take, the resident engineer may then order a secondary grid at 3 m centres.

P Forker started on site last May and with its 18 drilling rigs and 55-strong labour-force will have drilled 10,000 holes by the end of the contract.

Drilling and grouting takes place on three areas at any one time to make sure that plant is not standing.

Before drilling begins, the contractor inserts a 90 mm diameter steel casing through the overburden to prevent the hole collapsing.

A 70 mm diameter hole is then drilled down into the seam.

Mains water, which is delivered to the whole length of the site through a pressurised pipe, is used to flush the cuttings from the holes.

P Forkers technical director Peter Watson said: We chose to flush with water rather than air because it gives better drilling characteristics and it is cleaner.

He also points out that water is a much quieter option, which is a major consideration on the Merthyr Tydfil site.

Noise is a very sensitive issue, with houses very close by and a contract requirement that noise levels are monitored every day and site hours are rigidly adhered to.

Because of the size of the operation, P Forker set up its own batching plant on site.

The main ingredient of grout is pulverised fuel ash (PFA), which is delivered to site from the nearby Aberthaw power station.

The computer-controlled batching plant pumps grout through a pressurised pipe to a reception agitator in the area to be grouted.

Four grout pumps then send the grout to the holes down a 50 mm diameter tube.

Grout for the perimeter consists of four parts PFA to two parts sand to one part cement.

The infill grout is weaker: 10 parts PFA to one part cement.

Because of the element of the unknown involved, P Forker monitors all nearby watercourses on a daily basis to make sure no grout escapes through undiscovered adits.

And some of the holes are dye tested before grouting.

The only big surprise to date has been the discovery of an old tunnel that originally contained a railway and watercourse leading to the old collieries.

The mine stabilisation contract was the first of three contracts to be awarded by the Welsh Office on the 40 million A470 trunk road improvement scheme.

The other two are for the road construction itself, with the first of these going to Christiani and Nielsen, which started on site last November.

The A470 was extended from Cardiff to just south of Merthyr Tydfil between 1969 and 1985.

This last section will connect with the existing A470 north of Cefn Coed.

Kristina Smith reports from South Wales on the UKs biggest mine stabilisation contract