Woodsmith Mine Tunnel

York Potash Ltd^{[note 2]} put forward the idea of mining potash and polyhalite on the moors above Whitby in 2010. Originally, three options for moving the mineral out were considered; a pipeline, a tunnel or a railway. Whilst the intended end location is Wilton (for delivery to the Redcar Bulk Terminal [RBT]), the developers also considered the Ports of Hull and of Immingham to the south. However the geology south through the Yorkshire Wolds was prohibitive for tunnelling as the chalk bedrock carries aquifers that are important for the region and would also need extra access/ventilation shafts, create more overburden and be of a significant extra cost than a shorter tunnel to Teesside.^[5] A tunnel to Hull would stretch for 60 miles (97 km) and need an extra 62% of tunnelling effort, time and expense.^[6]

Transporting the product out via a railway line involved crossing the River Esk at Larpool Viaduct, and then along the Esk Valley Line via two reversals in Whitby to gain the route towards Middlesbrough.^[7]

A pipeline would have involved turning the potash and polyhalite into a slurry^[8] which would have required a more labour and technically enhanced process at both ends of the transport system.^[9] A pipeline would also be above ground and cause a lot of environmental damage.^[10]

Both the pipeline and rail options were dropped in favour of a tunnel with conveyors in it during the planning stages, and through various submissions, the accepted plan was approved in June 2015.^[11] The company, now Sirius Minerals, agreed to pay £130 million to the North York Moors National Park Authority to "protect and enhance the environment". This would be over the whole life of the project.^[12]

To avoid damaging aquifers across the route, the tunnel is designed to stay deep within the Redcar Mudstone Formation.^[13] The Redcar Mudstone Formation is less permeable to water and the route also avoids any former ironstone workings in the Redcar and Cleveland area.^[14]

The cost of the tunnel was estimated at £1.1 billion in November 2018,^[15] and when complete, the tunnel will be the longest that is wholly within the United Kingdom.^[16] The conveyor will be robust enough to transport between 10 and 20 million tonnes of polyhalite when the mine is in full production with the raw mineral being transported to Teesside for granulation and onward shipping.^[17][18][19]

The tunnel will run in a north westerly direction from the Woodsmith Mine site, passing under the River Esk, the Esk Valley railway line, the A169 and then up across the Esk Valley to its first ventilation/access Point at Lady Cross Plantation. It then crosses moorland and runs alongside the A171 road for some distance crossing under it just south of Lockwood Beck reservoir where the second access point is. It then turns in a more north/north westerly direction through Tocketts Wood and under first the A173 road and then the A174 until it arrives at the Materials Handling Facility (MHF) at Wilton.^[42] After granulation, the finished product will be taken on another conveyor (this time above ground) to the export site at Redcar Bulk Terminal.^[43]

The company claim that the entire route does not travel under any housing, but it does come close to some barns.^[44]

• Length - 23 miles (37 km)
• Overall diameter - 20 feet (6 m)
• Internal diameter - 16 feet (4.9 m)
• Minimum depth - 390 feet (120 m)
• Average depth - 820 feet (250 m)
• Maximum depth - 1,180 feet (360 m)
• Width of conveyor belt - 47 inches (1,200 mm)
• Speed of conveyor - 13 miles per hour (21 km/h)^{[note 4][46]}
• Gauge of maintenance railway - 2 feet 11.44 inches (900 mm)^[47]

The original plans detailed a tunnel with an internal diameter that was 14 feet (4.4 m) with a tunnel lining of 9.8 inches (250 mm). This was increased to a diameter of 16 feet (4.9 m) with a tunnel lining of 14 inches (350 mm). The overall diameter of the tunnel will be 20 feet (6 m).^[22] This also increased the cost of the whole project (including sinking the mineshafts) from £3.6 billion to £4.2 billion.^[48] The increase in diameter has also led to a slower progress rate than before; the TBMs tunnelling would be reduced from covering 66 feet (20 m) to 56 feet (17 m) per day.^[49] The average depth of the tunnel will be 820 feet (250 m) as it makes its way north eastwards towards Wilton.^{[citation needed]}

The 47-inch (1,200 mm) wide conveyor^[46] will transport the mineral at the rate of 13 miles per hour (21 km/h) with a changeover between conveyors at the Lockwood Access Site.^[44] The Mineral Transport System (MTS) will consist of the longest underground conveyor in the world.^[50] The tunnel will also include an access railway for maintenance crews, an 11 kilovolt power system for the conveyor, fibre optic cables, a pumping main and a 66 kilovolt feeder cable to supply the mine site from Wilton.^[45][51] This is needed as there are no National Grid power sources near to the mine site.^[52]

Planning documents assume the gauge of the railway to be 2 feet 11.44 inches (900 mm), which was later confirmed to be the accurate gauge, and the caverns where the crossovers are located will also be furnished with sidings.^[47][53] A train maintenance depot will be sited at the Wilton end of the tunnel.^[54] In November 2021, two of the maintenance trains crashed causing damage to the tunnel concrete segments.^[55]

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