Portland Stone, a type of limestone that has been quarried in the UK since Roman times, and industrially over the past 400 years, has proved popular as a cladding for both traditional and contemporary buildings. It has been used on prestigious buildings such as the Tower of London and Buckingham Palace.
Formed by sedimentation during the Jurassic era, the rock often features inclusions of marine organisms and other fossils that can be attractive. The caverns created by mining the stone – mainly on the Isle of Portland in Dorset – (hence the name) could now become the site of an exciting new visitor attraction which exploits what would otherwise be exhausted mine workings.
TAKING THE PUBLIC BELOW GROUND
Eden Portland (the company behind the initiative) envisages using the caverns to create an underground attraction, a ‘cathedral’ telling tales of biodiversity. The company which runs the Eden Project, Cornwall is planning a ‘theatre of science’ that explores ‘the importance of biodiversity, the threat of extinction and what it means to be human’. It would be part museum, part immersive spectacle and part educational facility and no doubt, huge fun. It would be a mini underground city telling the story of life on Earth. Extinct species, fossil displays and other attractions experienced in cavernous, prehistoric-style spaces under dramatic lighting could form a memorable experience for visitors.
“We will tell the biggest story of all: the evolving story of life,” explains Sebastian Brooke, director of Eden Portland. “It’s a story four billion years in the making and, as far as anyone knows, is unique in the universe. According to the world’s biologists, a big chapter is unfolding right now with biodiversity loss worldwide; with its mines, its cliffs and its mysteries, Portland is an amazingly vivid setting for the telling of it.”
The potential of such an immersive attraction would be huge and could attract visitors nationally and internationally. In addition to regenerating the local economy, it would be a fine example of sustainable development and a showcase of how underground space can be put to creative use.
At present, Eden Portland is trying to secure £30m (around US$41m) funding for the project and its success in achieving this will determine whether the project ever gets under ground. A previous proposal which had planned a dinosaur museum and mass extinction monitoring observatory (MEMO) failed for lack of funding. Let us hope Eden Portland fares better.
THE MINING STORY
What makes such an inspirational development possible is the extraction of the Portland stone and the caverns that result. That in itself is a story that many tunnellers will find of interest.
The proposed attraction would sit between two working mines – Bowers and Jordan – and it would also extend beneath a cemetery and a cricket pitch. The mines are leased by the Crown Estate to Albion Stone, the company which, for the past 30 years, has quarried from the local Basebed, Whitbed and Roach seams in which the mines are located.
Mining takes place within a 9m-thick layer of Portland Stone. Above this is an eight-metre-thick capstone layer of Purbeck Limestone which has never been exploited – and for good reason.
Managing director Mike Poultney, whose grandfather took control of the business in 1952, tells T&TI: “Geologically, the capping is very interesting but aesthetically is highly unattractive. It looks like petrified polystyrene.” So not exactly what you would use for prestigious buildings. Topping it all is a very porous layer of rubble. But water ingress into the mine is not a problem, says Poultney. “The mine has large joints so water ingress is not an issue, it drains like a sieve.”
EXTRACTING THE STONE
Mining the stone is achieved using the ‘room and pillar’ method – ‘rooms’ of stone are extracted and they leave in place huge, untouched pillars supporting the roof. Typically, extraction aims for a maximum rate of 75% – this was determined by Albion Stone’s mining consultant Wardell Armstrong, although, says Poultney: “In reality, we achieve nearer to 65%.”
Cubes with sides of around 1.5m are cut using Fantini GU50 tunnel chain-saw rigs which can make accurate vertical and horizontal cuts with their 2m-long, 40mm-thick blades studded with synthetic diamonds. The cuts subdivide the face into blocks. Shearing the partially-cut cubes away from the inaccessible rear vertical face – the final attachment to the bedrock – is achieved using Hydrobags. These thin steel envelopes are slipped into the cuts and inflated under twenty-bar water pressure to shear the rock. Once a clean break is made, a fork-lift removes the block to be taken to Albion Stone’s nearby facilities for further cutting. It is a carefully controlled operation from start to finish, all undertaken to minimise damage to the stone.
Supporting the cavern roof is achieved by 2.4m-long roof bolts, mesh and straps, installed as mining progresses. Roof bolts are also used to secure the pillars. Poultney says of the process: “A £500,000 Epiroc rig drills the hole, injects the resin, then inserts the bolt mixing the resin; then after the quick-setting resin has set, the rig tightens the bolt. All this is done in a total time of three minutes – and with minimal input from the operator in the cab.”
Sections mined are constantly monitored for movement, with the greatest so far being around 5mm. A reading of 10mm would trigger an action point which, Poultney explains, would probably mean installing more roofbolts.
Areas that have been exhausted are backfilled as mining progresses. Albion offered this measure, described by Poultney as ‘belt and braces’, for the planning application, at a time when they had no experience about likely movement in the mine. The inclusion aimed to placate local authority concerns about the possible creation of sinkholes, despite three different engineering consultancies having reported that their likelihood was in fact extremely unlikely.
Only 25% of the stone mined makes it to become large, finished stone blocks for facing masonry applications; the remainder – the mining offcuts – are used to backfill. “Although backfilling is operationally cheaper,” admits Poultney, “it does seem to be a shame to backfill those magnificent voids.”
Clearly, converting those “magnificent voids” to create Eden Portland will require more engineering and architectural input in terms of structural stability, access, fire, escape and ventilation. But it is not such a huge challenge for the underground construction sector.
The mine has a working life of around 50 years. When that time comes around, Eden Portland will hopefully have become an established visitor attraction.
