Portland is going underground - stone extraction in sensitive areas

Materials World magazine
,
24 Sep 2013

Mark Godden, Quarry and Mine Manager at Albion Stone, describes the challenges of mining in an environmentally sensitive and highly populated area, and techniques to ensure a future for dimension stone extraction on the Isle of Portland, Dorset.

A freestone is defined as a naturally occurring stone that can be cut in any direction without splitting along bedding planes or other lines of weakness. Freestones were much prized by early masons, who used hand tools to cut the stone blocks used to construct and embellish many of the UK’s best-loved historic buildings. The best freestone is Portland stone, which outcrops at a number of locations in southern England, but is most heavily quarried on the Isle of Portland near Weymouth in Dorset. Quarries have been operating in the area since at least Roman times and land-slipped outcrops adjacent to deep-water moorings allowed the relatively easy transport of stone blocks to distant markets, the most important of which was, and still is, London.

Portland stone is an oolitic limestone composed of minute spheres of calcium carbonate precipitated in a warm, shallow sea around 140 million years ago, A freestone is defined as a naturally occurring stone that can be cut in any direction without splitting along bedding planes or other lines of weakness. Freestones were much prized by early masons, who used hand tools to cut the stone blocks used to construct and embellish many of the UK’s best-loved historic buildings. The best freestone is Portland stone, which outcrops at a number of locations in southern England, but is most heavily quarried on the Isle of Portland near Weymouth in Dorset. Quarries have been operating in the area since at least Roman times and land-slipped outcrops adjacent to deep-water moorings allowed the relatively easy transport of stone blocks to distant markets, the most important of which was, and still is, London.

Portland stone is an oolitic limestone composed of minute spheres of calcium carbonate precipitated in a warm, shallow sea around 140 million years ago, at the very end of the Jurassic period. A very modern-day analogue for the type of environment where Portland stone formed would be the coastal margins of the Persian Gulf or the Bahama Banks. Portland’s structural geology was determined around 35 million years ago when Africa tectonically collided with Europe, forming mountain chains from the Pyrenees to the Caucasus. The outer tectonic ripples arising from this event produced gentle folding that can be seen today in the Weymouth Anticline, whose southern limb (where Portland is sited) dips at about 1.5° to the south-east. These earth movements also produced a series of well dilated master joints (or gullies, as they are known locally) through the Portland stone formation. The joints form major structural features that strike north-northeast, with an aperture of anything up to one metre and an approximate spacing of 20m. The stratigraphy of the Isle of Portland is quite straightforward and comprises a broadly shallowing upwards or progradational sequence of rocks, where the depositional environment changed through time from moderately deep marine to terrestrial. The freestone member forms the upper portion of the Portland stone formation with a thickness of around 10m, while the lower 20m of the formation is composed of numerous thin interspersed beds of limestone and chert, which contains no building stone but can be crushed and used as aggregate.

The Portland stone formation’s freestone member contains three economically important beds of oolitic limestone that exhibit increasing fossil shell content, from the relatively shell-free basebed at the bottom of the member to the very shelly roach at the top. Immediately above the freestone member lies the Purbeck group’s basal dirt bed and the hard algal limestone cap beds.

Portland stone has been quarried on an industrial scale since the late 17th Century when the demand for non-combustable building materials increased as a consequence of the Great Fire of London. It is estimated that some 36% of Portland’s surface has been quarried to date, with an estimated yield of at least 12 million tonnes. However, with an urban population of some 12,500 and an extensive built environment, the need to extract stone in a more acceptable manner has become paramount. Such environmental pressures led directly to the recent development of techniques for the underground mining of Portland stone. Modern open quarries have large geographical and environmental footprints, and the chance of obtaining new Planning Permissions for future quarrying activities on Portland are considered to be remote.

Albion Stone began planning for underground mining in the late 1990s. It was also at this time that the technology for extracting dimension stone underground became available with the development of mobile stone cutting machines in Italy. These machines allow the accurate in situ cutting of the freestone beds underground, thus eliminating any need for the costly and environmentally undesirable removal of up to 20m of overburden, which would be necessary as a precursor to quarrying.

Albion first began underground mining as an experimental operation in 2002 at its Bowers Quarry, where viable mining techniques were developed. A greater opportunity for mining arose in the seven acre Jordans Site, which Albion leases from the Crown Estate. The Jordans Site had a 50-year-old quarrying permission but is surrounded by residential, historical and industrial buildings that make quarrying the site virtually impossible.

Fortunately, old quarry faces resulting from adjacent quarrying operations during the 1950s provided an ideal location to construct two portals that would provide access to the beds of stone that Albion requires for future production. Preparatory work, included lowering the existing quarry floor by some 2.5m so that it coincided with the base of the Whitbed, allowed access to the prized stone. In 2005, the company purchased a Fantini GU50 cutting machine that is fitted with a chainsaw-type blade. The two-metre-long, 40mm-wide blade is studded with individual synthetic polycrystalline diamonds that can be replaced as needed and can cut into rock with high accuracy, allowing stone removal and facilitating underground development. The devised mining method involves making a number of vertical and horizon cuts to a working face, sub-dividing it into a number of cantilevered blocks that are attached to the bedrock only on their rear vertical faces.

Cutting a face takes approximately 12 hours using a cutting chain with a fairly low peripheral speed of typically one metre per second. Hydraulic pressure is used to loosen blocks within cut working faces, shearing them on their rear faces and setting them free. This is achieved using steel hydro-bags, costing £20 each. Hydro-bags (right) are slipped into the saw cuts and expanded under 20Bar water pressure, allowing a large forklift to remove the block for further processing at Albion’s nearby factory. Compared with older methods of dimension stone extraction that involved explosives, drilling and splitting using wedges, the modern methods used by Albion Stone result in blocks that are more dimensionally accurate with less wastage.

The three main dimension stone beds have different visual and weathering characteristics that are related to their petrology and fossil shell content. This variation gives clients some choice when specifying stone for new-build projects or when trying to match stone to existing buildings. The uppermost bed of Portland stone is the roach (origin possibly from the old French roke or roche, a boulder or cliff). Roach is found in a metre-thick bed and is full of casts and moulds of gastropods and bivalves, giving rise to a very attractive stone, particularly when cut parallel to its bedding. Below the roach is the whitbed (origin white bed), a layer up to 2.5 metres thick of fine-grained oolite with broken shell fragments 5–50mm in diameter. Whitbed can yield excellent freestone for external use, especially when a good weather resistance is needed. The lowest bed is the basebed, a micritic/oolitic homogeneous limestone with negligible shell content, usually regarded as the finest stone for detailed carving.

Mining at Jordans began in 2008 from two portals (see image, above) cut into an old quarry face. The mining method is room and pillar, based on a six-metre grid. The mine’s roof is a persistent bedding plane, associated with a paleosol called the basal dirt bed. The conjugate joint systems found within the Portland stone determines the orientation of the mine’s roadways and supporting pillars. The initial mining horizon is in the whitbed and roach, but recently work has started to bench mine basebed as well. Using the Fantini machines it is possible maximise yield from the mine by leaving the more jointed stone within the pillars and targeting the better (less jointed) stone in the production rooms. Roof bolting and, in areas close to dilated joints, mesh and steel strapping, is used to support the roof in underground roadways. It is fortunate that most of the joints encountered within the freestone beds do not propagate into the overlying lower Purbeck roof measures. The modern stone extraction methods used by Albion Stone do not involve any blasting, which preserves the integrity of the recovered stone. The mine is naturally dry and the waste stone is temporarily stored in a nearby quarry before being replaced in the underground workings on completion of mining.

Underground mining has enabled Albion to add significant reserves to its operations at Portland. It has allowed production in a sensitive location that would have been very difficult to quarry, and would no doubt have resulted in local objections to the operation. Furthermore, it has allowed high quality stone to reach a market for which there is no substitute. Most of the stone from Portland is destined for London, just as it was in the past. The long list of famous buildings constructed from Portland stone includes the Palace of Westminster (1347), Tower of London (1349) and St Paul’s Cathedral, where Sir Christopher Wren used nearly one million cubic feet in the rebuilding.

Stone extracted today is used in approximately equal measure for both new-build projects and for the renovation and upkeep of historic buildings. More recent new-build projects using stone supplied by Albion include the New London Stock Exchange in Paternoster Square, the Armed Forces Memorial in Staffordshire and the RAF Bomber Command memorial in London’s Green Park. Last year, the Olympic Rings were produced from mined whitbed for Weymouth Railway Station.

Although Portland’s resources are not endless, the new methods developed by Albion Stone will significantly prolong the life of Portland’s dimension stone industry with a minimal impact to those who live and work on the island.

Mark Godden is a Chartered Engineer and Fellow of IOM3. This article is based on a talk he gave to the MinSouth local society. To find out more about events and activities in your area, log on to www.iom3.org/local-societies