Due to rising water within mine workings a discharge has occurred within a block of coal associated the now abandoned Lancashire Coalfield, discoloured mine water was observed to arise from an old mine adit and discharging into the nearby watercourse of a local brook .
Drilcorp Ltd was commissioned by Coal Authority to drill and install a large diameter borehole within which to install a submersible pump and to utilise this borehole as an abstraction point to lower the water table in its surroundings and to reduce or stop pollution from the above mine.
The borehole is situated in open fields which will eventually be utilised for constructing a mine water treatment scheme in conjunction with the above abstraction borehole.
The work involved forming a 450m temporary access across an open field and a working area (on an incline) with aluminium track way so as to minimise damage to the grassland.
The site was set up including welfare facilities within a securely fenced working area to prevent access by the public from a nearby footpath.
The works involved drilling down to 18.4m depth at 675mm finished diameter with control measures to contain any arisings from the drilling operation to prevent pollution. Measures to contain what was thought to be a small artesian water outflow were also taken into account (particular consideration being given to the nearby water course)
The borehole was formed using a Hitachi 3600 fitted with a hydraulic driven telescopic auger unit. The first 9.80m of the borehole was formed using an 875mm diameter auger (through superficial soils to 1.1m and through coal measures including thick beds of extremely strong sandstone to 9.80m) which allowed the installation of permanent 762mm OD casing to 9.80m. The casing was formed and welded in 2 x 5m lengths and grouted externally from the base of the hole. The grout was allowed to set for a period of 38 hrs and the seal was then successfully pressure tested.
The hole was then continued through extremely strong sandstone into the mine workings using a 675mm diameter auger to a depth of 18.4m BGL. On completion, the borehole was reversed flushed until the returning water flowed clear. Drilcorp also undertook a CCTV survey which successfully clearly identified both the roof and base levels of the mine workings.
The borehole was lined with plain 609.6mm OD steel lining with a stainless steel screen section beneath (inside the workings). A top blanking plate complete with relief gate valve and pressure gauge was fitted to the outer and inner casing flanges as one sealed unit.
Two 2.10m diameter by 1.0m depth concrete manhole rings (set 400mm into the ground) were placed on top of the installation (with a limestone 10-20mm grade gravel placed internally as a base working surface inside the manhole rings) and a concrete biscuit with heavy duty lockable steel cover placed on top for protection from vandalism.
On completion of the borehole and installation, all plant was removed from site and the site fencing dismantled. The surface surrounding the borehole was then re-instated by Drilcorp Ltd. The aluminium track way was then removed and within two weeks, the grass had recovered and there was little evidence (apart from the manhole construction) of the works.
Spoil going into skip
Stainless Steel Well Screen
Finished Installation
