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For no reason at all - The Great County Adit
It was begun begun in 1748 finally had about 40 miles of tunnels, draining over 60 mines. All the mines that used it are closed but it still drains the workings; according to Wikipedia that was over half a million gallons in 1980.
From "Engineering Timelines"
Great County Adit is an extensive system of underground near-horizontal tunnels built to drain tin and copper mines between Redruth and Bissoe in west Cornwall. Though the complex is not now maintained and the mines it connects to are no longer operational, it continues to drain groundwater in the area.
The adit complex was begun at the behest of William Lemon (1696–1760) in order to drain Poldice Mine (SW741429), ... {Lat Long for Google maps = 50.2430558607° -5.1691337963°}
/snip
The Poldice mine captain, or manager, John Williams (born 1714) supervised the work. The adit was hewn with hand tools and gunpowder blasting — rock drilling machinery wouldn't be invented until the mid 19th century. The scheme became known as Poldice Deep Adit, and by 1756 it was 2.4km long and reached Killicor, at the eastern edge of Poldice’s sett boundary near Hale Mills. It then ran north westerly along the Poldice Valley. Sometime before 1767 this tunnel was continued north west and then westwards to Wheal Quick at St Day. By around 1765 the passageways had been excavated as far as Creegbrawse Mine (SW745435) {50.2485610784° -5.1635368129°}
/snip
... it turned west to East Wheal Chance and then north to Halenbeagle Mine, and finally west to Wheal Hawke (SW714447) {50.2579423479° -5.2085003243°} north of Scorrier. There were offshoots to other mines between Halenbeagle and Wheal Hawke, all completed by 1778.
/snip
The increasing volumes of water draining into the adit network as more mines were connected to it, soon indicated that a second main tunnel was needed. Sometime between 1778 and 1803, a parallel adit was constructed between the portal and Groses Shaft at Poldice Mine to the north of the existing tunnel.
Great County Adit is an extensive system of underground near-horizontal tunnels built to drain tin and copper mines between Redruth and Bissoe in west Cornwall. Though the complex is not now maintained and the mines it connects to are no longer operational, it continues to drain groundwater in the area.
The adit complex was begun at the behest of William Lemon (1696–1760) in order to drain Poldice Mine (SW741429), ... {Lat Long for Google maps = 50.2430558607° -5.1691337963°}
/snip
The Poldice mine captain, or manager, John Williams (born 1714) supervised the work. The adit was hewn with hand tools and gunpowder blasting — rock drilling machinery wouldn't be invented until the mid 19th century. The scheme became known as Poldice Deep Adit, and by 1756 it was 2.4km long and reached Killicor, at the eastern edge of Poldice’s sett boundary near Hale Mills. It then ran north westerly along the Poldice Valley. Sometime before 1767 this tunnel was continued north west and then westwards to Wheal Quick at St Day. By around 1765 the passageways had been excavated as far as Creegbrawse Mine (SW745435) {50.2485610784° -5.1635368129°}
/snip
... it turned west to East Wheal Chance and then north to Halenbeagle Mine, and finally west to Wheal Hawke (SW714447) {50.2579423479° -5.2085003243°} north of Scorrier. There were offshoots to other mines between Halenbeagle and Wheal Hawke, all completed by 1778.
/snip
The increasing volumes of water draining into the adit network as more mines were connected to it, soon indicated that a second main tunnel was needed. Sometime between 1778 and 1803, a parallel adit was constructed between the portal and Groses Shaft at Poldice Mine to the north of the existing tunnel.