The former Glyncastle colliery had been mined since 1875. It was abandoned in 1965 and following a collapse in the adit in 1994, water levels within the mine rose, accumulating dissolved iron of up to 60 mg/L. Contaminated minewaters began to escape from a number of locations including a collapsed crown hole in Resolven, and at an airshaft in the Clydach Valley.

Atkins proposed that if the contaminated minewater could be captured it would be possible to treat before release into the River Neath. Longbore’s solution was to drill into the lowest point of the mine from the valley below, allowing the minewater to flow out under its own head of pressure.

Complicating matters, although the water levels had to be lowered to below the current discharge points, if they dropped too far and air was allowed into the mine shafts, oxidation of the dissolved iron would take place within the mine and ferrous deposits would begin to accrete in the discharge pipelines and valves. A system to control the minewater flow therefore had to be installed.

But first, two significant issues had to be overcome. The major problem was that drilling into a mine system with a water level 17 m above the drilling rig resulted in a head of pressure over 170 kPA. If this was not contained, the entire mine would discharge. Longbore’s solution was to cement a surface casing into the bedrock, onto which a series of gate valves and rubber snubbing units were bolted.

Guidance was also critical. Although the location of the adit had been confirmed the width was unknown, so the target point for the drill had to be as close to the vertical bore as possible. In addition, the bore had to punch into the adit floor at the correct azimuth and inclination to allow sufficient pipe to be pushed into the adit to clear anticipated sludge on the floor of the mine.

Longbore’s American Augers DD-6 rig and associated equipment were mobilised for the job and rigged up at the end of July 2005. Drilling commenced with a 170 mm steel Milled Tooth tri-cone drill bit run on a downhole drilling motor, with an adjustable bent housing set to achieve the required build rates. The Silesian Sandstones, locally known as ‘Ironstones’, drilled at an average Rate of Progression (ROP) of 13 m/h to the casing setting depth of 43 m. The pilot hole was then opened in two passes to 305 mm and 510 mm.

The 355 mm SDR-17 surface casing was electro-fused together, with a cement basket mounted on the leading joint, and run into the hole. A tremie pipe was then run into the annulus and the surface casing was cemented in place and allowed to cure, providing a pressure tight seal.

The pressure control device and diverter system was then nippled up to the wellhead and the 170 mm drilling assembly was used to drill out the cement shoe. Drilling progressed at an ROP of 6.4 m/h until the adit was entered at a drilled distance of 71.7 m, 0.1 m to the left of the target and 0.5 m long. The pilot hole assembly was removed from the bore under pressure, and the hole opening assembly was used to open the bore to 305 mm.

The 200 mm SDR-11 Butt Fusion Welded drainage duct was prefabricated in one length and Longbore utilised their patented Pipe Pushing Technology© to insert the duct into the bore. An Inflatable Packer was used to isolate the annulus between the drainage duct and the surface casing, and cement was pumped into the void to provide a physical seal.

On completion of the first bore, the pipeline was valved and gauged and placed in operation to drain the minewater while the rig was moved and a second back-up pipe was installed using the same methodology.

The completed scheme has successfully captured the contaminated minewaters from their diffuse discharge points, and has enabled their controlled flow to a sustainable treatment system that is reducing the iron content to less than 1 mg/L.