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Muck and grout

About 13,000 bores using 193,000m of drilling is needed to grout old mine workings on a Scottish site. Report and photographs by Adrian Greeman.

The north side of Scotland's Firth of Forth can be bitter even at this time of year as a cold east wind sweeps off the North Sea. So work stabilising the ground for a housing estate in Kirkcaldy has not been easy.

Drilling crews for the 10 rigs that began the job in November, and the half a dozen completing about 13,000 boreholes have had to wrap up well. There is even a system of canvas screens rigged up to protect the grout batching plant and pump station at the top end of the site's shallow valley, making it look like a First World War observation post.

The teams from ground engineering contractor Consolidate have until August to fill in old mine workings lying beneath an up to 200m wide site, which runs for 1km down the valley towards the main road and the coast beyond. New housing will eventually go in here, abutting existing houses nearby. Developer on the project is Gladedale Homes.

But as with all housing developments in the region, the planning authority required a proper records search and ground investigation first. Not unexpectedly, given the shape of the ground surface, there was an old coal mine beneath the site.

"That is no surprise," says Harry Hannel from the client's engineering consultant David Murray. "The whole Scottish central belt has been a mining area for hundreds of years, going back to the monasteries, and I would not like to count the number of old pits round here." Often old workings have no records at all or, if they do, they are inadequate.

Two different seams run under the site. The top one, the SwallowDrum, is a single band of coal about 1m thick. The lower one, 2m down, is a double seam with two bands of coal – the Lochgelly Splint and Parrot – each about 1m thick and separated by about 1m thickness of ground.

The state of the seams is complex, says Consolidate site manager Neil Hands. First they run at an incline across the site, starting at about 5m depth in the rock and dropping to 22m, beyond which there is no requirement to fill them. Some 6m of overburden sits on the rock.

Then there are five faults running across the site. This means that the seam levels change abruptly at various points, up or down, where a block of land may have dropped between two discontinuities. Sometimes chance would bring the upper seam level with the lower so working simply continued.

It is not known how much of the mine was actively operated; much of the top seam remains intact, but the lower ones were worked extensively.

"The double seam could have been mined separately or as one," says Hannel, although it is not clear from the rather inadequate records of what was left behind. On top of that, he says recorded "abandonment plans" cannot be trusted because 19th century mine owners were notorious for evading regulations.

"They had a licence to remove a certain quantity of coal, leaving 'stoops' for roof support. But on the way out, after submitting records, the more unscrupulous would strip everything. Few inspectors would want to go down to check."

It is therefore unknown whether seams had compacted, remain open or have been fully or partially backfilled with mine waste. It is up to the grouting crews to work out what is there. Information is gleaned from the initial site investigation, by the speed the drilling is going and the arisings coming to surface in the water flush – coal chips are reassuring since it indicates a solid seam. "It takes a bit of skill and the crews have to be vigilant," says Hands.

Drilling is being done on a 3.5m grid across the site. The crews have an assigned section and first drill widely spaced exploration holes before filling in the grid.

Hands has six crews working with a variety of rigs, two older ones built on a Hymac base, one Delta-based machine and three new Casagrande C6s, the latest of which has a full safety cage around the drill rods. He likes the Italian machines, which he says work at up to twice the speed of the older ones.

Most of the site drilling begins with a 102mm bit through soft glacial tills and clays that overlie the rockhead to a depth of about 5m. A sacrificial plastic lining of 75mm in diameter is inserted to support this initial bore.

"We then use a 63mm bit down through that into the rock, which is a cycle of sandstones and mudstones, on towards the seams. If the top seam is unworked, and quite a lot of it is still coal, we go on to the bottom one," says Hands.

If the top seam is empty the crew drills 1m into the seam base to ensure they are fully beyond it. Grouting is then carried out, which comes from one of two on-site batching plants."The grout we use is a normal cement mix but with a lot of PFA [pulverised fuel ash]," says Hands. A 1:12 ratio is used to save costs. The PFA comes from a power station further up the Forth at Kincardine.

The mix has final hardness of about 1N/mm2, enough to give ground support and more than many types of coal. In the wet state it is aimed to achieve a 300mm to 600mm flowability in a Colcrete flowmeter, with the ideal being 450mm.

For pumping at the edge of the site, a slightly stiffer mix may be used in a few holes; there is no point in pumping grout out further than is needed.

Just how much goes into any one hole is almost an unknown, says Hannel. The pumps deliver about 6m3/h and pumping could go on for half an hour or two weeks, he says. "You only know when it comes to the surface." However, average take at present, although it may change, is 4.2t per hole.

Fortunately most of the first-level holes on the record chart in the site office have so far been filled in black. That means they are solid coal and the drill can go straight in to the second level where most of the grouting has been needed. In some places where it was worked, the voids collapsed almost fully a long time ago and the settlement has already reached surface.

If grout takes at the first level the drillers wait for it to go off and then re-drill to reach the second seam, with more pumping following on.
There have been a few awkward spots on the site. In places the firmer till gives way to sand and gravel, which has needed temporary steel casings to get through the overburden. "And we had a high-voltage line across the site and a sewer. A bit of directional drilling was needed to hit the right spot by starting at an angle to avoid these," says Hands.

In wet and windy conditions, it has not always been easy to set up the drilling quickly and make headway, he says. But finally spring is arriving and the mud will dry up.

Despite the issues, Hands hopes the job will work out well. Its value will not be clear until the end since it has an upper and lower boundary between £1.65M and £2.2M, depending on re-measurement of grout take.

If the quantities should happen to be very low he will come out further ahead.

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