Stoke Hills Stone mine at Limpley Stoke, just south of Bath is one of the the numerous sites where Bath Stone has been quarried underground. Previously known as Hayes Wood Quarry, the mine was reopened for stone production in 1983 by the Bath Stone Group and is now one the premier Bath Stone quarries. It commands a 95% market share of Bath Stone production, producing high quality block for architectural use known as Stoke Ground. Stone is extracted from two beds; the ‘Top bed’ which is around 1.6m thick and that which was that traditionally extracted at the site. The more durable ‘Base bed’ is thinner, at around 1m and separated from the top bed by a thin clay band. The author was able to attend a guided tour of the working mine in May 2010.
The workings at Stoke Hill are very extensive having been worked over hundreds of years and accessed from an adit below the BSG offices (NGR ST 776607) Another sloping entrance at NGR ST 778607 was created during the war when the site was requisitioned for ammunition storage. Substantial brick pillars around the central part of the mine date from this time. When the mine was reopened in 1983 it was this sloping adit that was first used. In the first instance stone was won by extracting the base bed stone from the floor of the existing network of passages. Production has now progressed to a new sector known as the 29’s where stone is extracted from both beds at the same time. This section of the mine is on the far side of the Midford fault that had originally deterred further underground expansion. The roadways leading to these new workings slope up through the fault zone to accommodate the throw of several meters on the fault. The fault itself can be clearly seen in the roof and walls as an open rift around 1m wide.
As with the old workings, the new workings are still on a pillar and stall plan but production is highly mechanised and the passageways much wider and higher than in the old section. Stone is removed on a face about 3.5m wide and 2.5 m high. Stone is removed to a depth of around 1m from the face at a time and the roof is then stabilised with rock bolts and mesh before moving froward to the next cut. To extract the stone cuts are first made using an electrically powered stone chain saw. This makes cuts slowly and remarkably quietly through the stone at floor level, at the parting between the beds (approx 1m above the floor) and at roof level where twin cuts are made (coinciding with the top of the top bed) so that a gap can be created above the stone to the depth of the block. Vertical cuts then separate the face into blocks of the desired size. After being marked to record location and orientation in the bed (essential information for the stonemason) blocks are split away from the back face by introducing hydraulic flat jacks into the saw cut.
Blocks can then be removed with a forklift. Blocks are graded and trimmed if necessary underground so that only squared blocks of saleable quality are taken to the surface. This is done by tractor and trailer along a haulroad that weaves its way though the old workings to the adit just below the BSG offices and stoneyard.
The old method of extraction involved cutting a vertical groove between each block and then introducing a vertical saw cut behind the block before splitting it away from the bed with wedges. The modern method is both less labour intensive and provides better quality block for the customers. The modern mine also has underground workshops to maintain the machinery and modern health and safety practices require there to be extensive monitoring of the roof, forced ventilation and refeuge chambers.
And in case this all sound like a lot of effort to go to, it is worth noting that the new build and conservation market are prepared to pay handsomely for this stone. A typical rough block measuring 1m x 2m x 2m and weighing 2 tonne will be worth over £2000.
- Plan of the modern workings south of the Midford Fault
- Plan of the mine showing new workings at the top, old reworked section (maintained for ventilation etc) in black and abandoned workings. The adit entrance is on the extreme right
- Main adit
- Sawing the face. Upper cuts are where stone is removed so that block can be removed. Lower cut marks boundary between Top and Base beds. Note the roof support system
- Splitting blocks with a flat jack. Note markings on blocks.
- Trimming blocks underground before transport to surface.
- Underground workshop