Inside the bizarre ‘twin’ tunnels dug by Victorian drills in North Wales


Centuries of slate mining have left extraordinary features in the Welsh landscape. Among the strangest of these are twin tunnels shaped like binoculars that were dug by some of the world’s first drills.

Their existence in the vast slate mines of Gwynedd is a story of Victorian ingenuity and, ultimately, heroic failure. Despite their promises, steam tunnel boring machines were unable to match the efficiency of poorly paid miners equipped only with poles, hammers and gunpowder.

In the mid-1860s the machines were tested in three mines. One was used to cut an opening at Cookes level in Maenofferen, Blaenau Ffestiniog, where clove imprints can still be seen on the slate floors.

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The most famous is at Abercwmeiddaw, near Corris in South Gwynedd. Here, strange twin tunnels reach about 30 meters into the rock. Cut in 1864, they look like a pair of giant binoculars.

A recent visitor was Nic Parry from Shrewsbury, who said the experience of exploring the “Twin Girls’ Tunnel” more than matched their billing. “It was one of those places where you get there and think wow!” she says. “It’s as if these patterned lines were drawn. It was really quite amazing!

“When you’re inside, you tend to wonder about their real history. Nature has started to reclaim them, which I think adds to the beauty of the place.

An earlier section of the tunnel can be seen in the distance

The first tunnel boring machine was designed by Scottish engineer George Hunter, whose father James had developed industrial stone planers at Arbroath. Their industrial cutters were widely adopted by the slate industry – even though James was to lose a leg in the process.

One was commissioned by the Braich Ddu quarry near Trawsfynydd in 1863. Boasting four discs, each 4 feet in diameter, it has been described as “the largest machine of its kind ever made”.

As the so-called “Hunter saw” was made at Arbroath, the only way to get it to the quarry was by sea, then by boat on the River Dwyryd, followed by a four-mile road trip, and finally by road. carriage along a two-mile-long narrow-gauge streetcar.

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To oversee its installation, George Hunter and his family left Arbroath for Maentwrog in the Ffestiniog Valley. Here he came to the advice of Sir William Fothergill Cooke, co-inventor of the electric telegraph who used his newfound fortune to buy a house at Aber Iâ, present-day Portmeirion, near Porthmadog.

As Sir William had invested heavily in Maenofferen’s career and in the career of Hafod Las, Betws y Coed, he wanted to see if new technology could increase their profits. And as it happened, George had just patented a prototype tunnel boring machine that, in testing at Arbroath, had cut a five-inch-deep ring in a stone wall in less than five minutes. Sir William quickly provided funding for its construction and shipping.

Sir William Fothergill Cooke, co-inventor of the electric telegraph, who partnered with George Hunter to design and patent the first tunneling machines for use in the Welsh slate industry
Sir William Fothergill Cooke, co-inventor of the electric telegraph, who partnered with George Hunter to design and patent the first tunneling machines for use in the Welsh slate industry

Essentially, the TBM was a rail-mounted turbine that spun rotating cutters. For the cutting edge, steel bolts were used with sharpened conical heads. The machine could be set for a cutting diameter between 5 feet 6 and 6 feet 6, or about two meters. The cutting head rotated slowly, at one to two revolutions per minute.

Once in place, it was locked in position. Typically, the machine would take about three to five hours to carve a 2-foot-deep groove in the rock. At this point the cutter head was removed from the newly cut groove and the whole machine was brought back to the tunnel entrance to allow the miners to pass.

They could then start hacking into the kernel at the head of the tunnel, a process that could take several hours. Only then could the machine be pushed back into the tunnel, put back in place and the next carrot cut.

Cooke & Hunter machines produced a smooth, lined finish
Cooke & Hunter machines produced a smooth, lined finish

The first tunnel dug in this way was probably at Maenofferen, where a single bore 30 feet long can be seen. But it was a laborious operation. The main weakness was the inability to extract the core without removing the machine. Over the next few years modified Cooke & Hunter drillers were patented, but the central problem remained. Sir William admitted it: in a patent, he described how the machine was “inactive for nine out of 12 hours”.

His radical solution was to dig multiple tunnels in parallel. After digging a bore, the machine would be moved to an adjacent rock face and a second cross bore. Once done, it was moved from a third adjacent rock wall before being put back on the first: the idea being that it could be constantly used.

For this, a cradle was needed to move the machine between each parallel tunnel. The process was tried with some stubbornness in Abercwmeiddaw, creating his telltale tunnel of binoculars. At Maenofferen, a strange “quadruple bore” tunnel was dug in 1868, forever known as “Cooke’s Level”.

Unlike the Cooke & Hunter stone cutting and dressing machines, which were widely adopted in the slate mines of North West Wales, the tunnel boring machines did not justify their investment. Finances were never Sir William’s forte, as JG Isherwood noted in a paper presented at the National Association of Mining History Organizations conference at Bangor University in 2014. He wrote: ” Cooke may have had good ideas, but may have lacked business acumen. .

“At the end of 1867-beginning of 1868, his enthusiasm led him to an investment in the Llanberis Slate Company Ltd which was to cost him dearly. The company was already somewhat dodgy and had spent much of the shareholders money for little profit developing the Gallt-y-Llan slate quarry near the end of Llyn Peris.

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Despite the excitement over the arrival of a drill at the quarry, by November 1868 the venture had failed and Sir William was severely exhausted. In a few years he would lose what little he had left.

In 1870 he founded another stonemasonry business in London, shortly after retiring from leading roles in the Maenofferen and Bettws firms. Despite raising £40,000 in funding, the new company would fold within four years. By 1879 he had died, leaving an estate worth just £17.

The start of the Abercwmeiddaw Twin Tunnel, showing how a second bore was started in an attempt to solve the problem of core removal
The start of the Abercwmeiddaw Twin Tunnel, showing how a second bore was started in an attempt to solve the problem of core removal

George came out of it a little better, but not without sorrow. Of her five children born in Maentwrog, three died, possibly from typhoid. Shortly after moving to Aberdyfi, as tenants of the Abercwmeiddau Quarry, a sixth child was born in 1870.

In a final effort to perfect his tunnel boring machine, he filed a patent in 1882 for a machine driven by compressed air. But the world had caught up with him and passed him by. In 1880, work began on the first attempt to dig a Channel Tunnel. This used a top rotary boring machine capable of cutting nearly half a mile per month.

Although both boreholes were 2,110 meters long, the project became plagued by fears of an underground invasion by the French. After several injunctions, it was finally abandoned in 1898.

Cooke & Hunter machines have been consigned to become footnotes in the history of tunneling. In his 2014 article, Mr. Isherwood, a leading authority on their development, wondered what might have been. “If tunnel boring machines and over/under cutters had been tried in any other material than Welsh slate, and in a locality more conspicuous than a remote corner of Wales, perhaps their names would have been associated with the first attempt at a Channel Tunnel, rather than the inventors Brunton, Beaumont and English?

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