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Great County Adit
between Redruth and Bissoe, Cornwall, UK
associated engineer
John Williams
date  1748 - 1872
era  Georgian  |  category  Mining/Quarrying  |  reference  SW762418
ICE reference number  HEW 1086
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), where he was then the major shareholder — or ‘adventurer’ as mine shareholders were known at the time. He had financial interests in many Cornish mines and was known as ‘The Great Mr Lemon’, giving his name to both Lemon Street and Lemon Quay in Truro.
The tin and copper mine at Poldice had been in operation since at least 1512. By the 1730s its two water-powered engines, and the large water wheel at Killicor Mine (Wheal Henry) to the east, were unable to pump the deeper levels dry.
In 1742, Lemon bought five Newcomen pumping engines for his various mines but his main plan for drainage was to drive a 4km long, near-horizontal tunnel from a portal at Wheal Nangiles south of Twelveheads to the western edge of the Poldice sett (mineral extraction boundary area). Water drained out of the adit by gravity and flowed through the portal into the Carnon River, which connects with Restronguet Creek and thence to the sea via the Carrick Roads.
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) north east of Todpool. The St Day branch — known locally as the Consols Adit — was then extended west past West Wheal Virgin to Carharrack Mine (SW738421) south of Crofthandy by about 1770.
As the mines in the area grew larger and deeper, the volume of water flowing into Poldice — the deepest of the mines in the area at some 183m — increased. By the mid 1770s, the pumping engines could not cope and the network of adits was enlarged further, earning the collective name, Great County Adit.
The Creegbrawse branch was continued north west past Wheal Busy (SW738445) north west of Chacewater, where it turned west to East Wheal Chance and then north to Halenbeagle Mine, and finally west to Wheal Hawke (SW714447) north of Scorrier. There were offshoots to other mines between Halenbeagle and Wheal Hawke, all completed by 1778.
Another branch was started at West Wheal Virgin and driven south into the United Mines — an amalgamated group of mines formed in 1782 by Sir William Lemon (1748-1824), grandson of Great Mr Lemon — in 1784. It reached Ting Tang Mine (SW730410) near Carharrack in the west and East Ale & Cakes Mine (SW748416) in the east by 1791.
Up to December 1793 the adits were extended even further with more branches to local mines. From Wheal Hawke the tunnels went west and north as far as North Briggan Mine, west to Redruth at Wheal Maria and south and west to Cardrew Mine. By the end of the 18th century the north branch reached Wheal Barberry, and a branch was built south from Wheal Hawke to Treskerby Mine (SW715435).
Another branch of the adit, from Wheal Peevor (SW708442) at Radnor south towards Trefula Mine (SW714422), was begun at the start of the 19th century. The network of adits west, south and north of St Day was also augmented between 1790 and 1820.
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.
Two exploratory tunnels were driven into Stencoose & Mawla United Mine (SW713460) near Scorrier in 1835, leading to the adit being extended from Wheal Barberry to Stencoose & Mawla by 1860. In 1845, the adit was driven west from Ting Tang Mine to Pennance Consols (SW713406), where it was 137m below the surface. This enabled the mine owners to exploit rich new lodes of ore at Ting Tang, and by 1865 the adit had almost reached Wheal Beauchamp (SW707406). Another branch was extended east from Wheal Busy through Wheal Daniell (SW747449) and on to East Wheal Busy north east of Chacewater by 1858, by which time Killifreth Mine (SW734442) was also connected to the system. Work continued sporadically until 1872, connecting outlying mines to the network’s many branches.
On 6th May 1843, the Mining Journal described the adit as “the most extensive, valuable, and systematic undertaking of the kind in Cornwall — perhaps in England, and we believe, but few in the world exceed it in importance”.
The County Adit Management Committee administered the adit until 1886 — it comprised the mine captains concerned and the mineral lords whose setts it crossed. Miners maintained the adit during the summer, when flows were lower. Surface drains were cut during the winter to divert runoff.
The adit’s cross-section varied over the years, though it tended to be slightly wider at the top than at the bottom. There were vertical shafts between adit and surface at intervals. The largest drives were the twin parallel tunnels of the main Poldice Deep Adit, which were 2.13m high and 610mm wide with arched roofs. Other 18th century tunnels were rectangular or coffin-shaped, 1.53-1.68m high and 510-610mm wide. The 19th century tunnels were arched or coffin-shaped, of similar height but a little broader at 610-760mm wide.
When completed the Great Count Adit drained around 100 mines in five parishes covering some 3,100ha. The total length exceeded 61km, with the longest branch being 8.8km to Cardrew Mine. Apart from at Pennance Consols, where it was deeper, the adit drained the land to a depth of 73-110m. In 1817-19 the volume of water flowing from the adit was 51 million litres a day, increasing to 66 million litres a day at its peak in 1839, and reducing to 59 million litres a day in 1869-71.
The total cost of the scheme has been estimated at around £250,000. The savings on pumping costs, since water was pumped to adit level rather than ground level, has been estimated at up to £20,000 per year. In the 1860s there were more than 30 steam engines pumping water into the adit.
However, the Cornish mining industry began to decline in the 1850s and by 1875 many of the mines along the adit had ceased production. The harsh winter of 1876 signalled the demise of the Great Count Adit. The weather led to flooding in the adit, releasing torrents of water laden with mining debris and silt, which was deposited downstream at Devoran to the south east.
The Gwennap mines faced too much competition from cheaper imports of copper from South America, and did not have the financial resources to prospect for deeper tin lodes, as had many of the Camborne mines. Only 10 of the mines along the Great Count Adit worked during the last three decades of the 19th century and by 1900 just three — Wheal Peevor, Killifreth Mine and Wheal Busy — were still in operation.
Limited periods of mining activity continued through the 20th century at a few of the mines but none of them are now working. There were also some works relating to copper precipitation and iron ochre collection at Bissoe, near the exit portal. The adit remains untended and by 1980 its outflow had dropped to 2.3 million litres a day.
Research: ECPK
"The Great County Adit” by J.A. Buckley
Penhellick Publications, Camborne, 2000
“A History of Copper Mining in Cornwall and Devon” by D.B. Barton
D. Bradford Barton Ltd, Truro, 2nd edition 1968
“Cornish Mining: Notes from the Account Book of Richard Trevithick, Senior”
by Arthur Titley, in Transactions of the Newcomen Society, London, 17th December 1930
reference sources   CEH South

Great County Adit