Portland Stone
An added dimension
On 2 November 2001 Dorset County Council resolved to grant planning permission for a new site for the extraction of Portland stone from a part of the Isle of Portland known as Stonehills.
This decision heralds what should be the beginning of a new era in the production of this nationally important resource by underground mining, with all the environmental advantages such an extraction technique brings with it.
Portland stone has been extracted for use in buildings since Roman times, but its popularity was greatly enhanced when it became a favoured building material of such eminent architects as Inigo Jones and Sir Christopher Wren. Fine examples of buildings utilizing Portland stone can be seen in most major cities and towns, and London boasts many of the best examples including Mansion House, the Cenotaph and St Pauls. The latter is perhaps the best known building constructed of Portland stone, with tens of thousands of tonnes being used by Sir Christopher Wren in its construction.
In 1952 the demand for Portland stone led to the granting of extensive planning permissions for stone quarries on Portland. Approximately 330ha were granted permission, subject to only two conditions.
During the decades following the 1952 decision many quarries continued to be worked and new areas were opened up. In addition to the impacts associated with the proliferation of quarries on the Island, the shallow dimension stone beds were worked out and quarrying of deeper aggregate beds followed. Extraction techniques changed from traditional 'plug and feather' methods to extraction based predominantly on the use of high and low explosives and highly mechanized overburden removal; the latter with associated environmental impacts.
In addition, poor-quality 'cherty' reserves beneath the dimension stone reserves, together with the stone waste, began to be exploited as an aggregate resource. Overall, the production of aggregate and dimension stone, combined with a lack of effective planning control, led to an unacceptable level of environmental impact from the Island's quarries.
Two parties effectively control the accessible reserves on the Island. The Crown Estate owns a relatively small part (approximately 20%) of the permission area and associated reserves, which is let to Albion Stone Quarries, while the balance of accessible reserves is owned and controlled by Hanson Bath & Portland Stone.
During the early-to-mid- 1990s it became clear that, due to a combination of land ownership and historical factors, an imbalance existed in the reserves controlled by the two main operating businesses. The Dorset County Council Mineral Local Plan estimated, perhaps optimistically, that the reserves remaining under the 1952 permission were some 1.5 million tonnes in 1990. However, Hanson controlled the vast majority of these estimated reserves.
It became clear, therefore, that if Albion Stone Quarries' business was to be sustained planning permission for new reserves would need to be sought. Albion Stone Quarries therefore entered into discussions with the Crown Estate regarding the possible exploitation of 25ha of 'greenfield' Crown Estate land at Stonehills.
The Mineral Local Plan was first drafted in the early 1990s and was therefore based upon reserve estimates for dimension stone that had foundations in the 1980s. Policies for dimension stone were thus prepared against a backdrop of estimates of substantial reserves, together with the increased level of environmental impact. Not surprisingly, therefore, the developing policies for dimension stone extraction sought to establish a presumption against new quarries unless significant environmental improvements could thereby be delivered. Other policies sought to encourage the industry to investigate new, less environmentally impactive techniques, including mining.
Under these circumstances it was evident that the planning background to an application for new reserves was not helpful.
At around the same time that Albion Stone Quarries began to consider their need for new reserves, the Crown Estate's mineral agents, Wardell Armstrong, began to raise the possibility of large-scale underground mining of dimension stone, particularly where reserves were deeper. There followed a pre-feasibility report on mining prepared by Wardell Armstrong on the joint instructions of the Crown Estate and Albion Stone Quarries. That report concluded that mining should be technically feasible, recommended mining trials, and gave impetus to further investigation. However, the investigation of mining techniques was at a preliminary stage and the need for new reserves was acute. As a result, Albion Stone Quarries began work on two initiatives:
- The development of a trial mine in which to develop extraction techniques.
- The development of a combined quarry and mining proposal for the Stonehills site.
In respect of the former initiative, this resulted in an application in October 1998 for the establishment of a trial mine under land that was already permitted for quarry extraction, but in close proximity to a residential area. The application was permitted for trial mining in December 2000 and development is scheduled to begin this summer utilizing stone-cutting technology, including diamond-wire saws and 'jet-belt' stone cutters, that has recently been extensively trialled in an open quarry environment.
In respect of the second initiative, as part of the consultation process required by the EIA regulations, quarry proposals were put to the local authority and local residents. As part of the consultation exercise, local residents and any other interested parties were encouraged to complete a questionnaire and comment on the proposals. For a population that is subjected to an intensive and extensive level of quarrying activity, the results were surprising, with a high level of support for the stone industry and the new quarry proposals.
However, notwithstanding this general support, responses clearly indicated mining as a preferred extraction technique.
This factor, together with the planning policy background and environmental issues, led Albion Stone Quarries to drop their Stonehills quarry proposal and, in August 1999, to instruct Wardell Armstrong to develop a scheme for the underground mining of reserves at the Stonehills site.
The Stonehills site comprises 25ha of land sandwiched between the residential areas of Weston and Southwell and abutting the western cliffs of the Island.
The existing use of the site is agricultural together with footpath-related amenity. The land is predominantly flat, sloping gently from north to south in common with the whole Island, with a shallow bowl to the east.
Geological investigations carried out in 1999 showed the site to be underlain by Portland dimension stone strata with a commercially exploitable thickness averaging approximately 6m.
Portland stone has inherent geological and geotechnical qualities that make it suitable for a room and pillar method of extraction. In addition, the Portland succession includes a bed of very competent 'cap' rock that will provide a suitable 'roof’ for the mine. Detailed analysis has indicated that a mine based on 5m rooms and pillars and giving a 75% extraction ratio could be worked in way that would not result in collapse, subsidence or other forms of surface instability.
But without doubt, the most significant advantage that underground mining delivers is the low level of environmental impact.
Access to the mine will be via a 100m surface roadway and an open, 120m, 1 in 10 decline, which, together with a small number of low-level ventilation shafts, will be the only surface development. In total, less than 0.5ha (2%) of the surface of the site will be disturbed.
Following the initial eight-week construction period, all extraction activity and associated facilities (workshops, welfare etc) will be confined within the mine. Noise and dust impacts associated with the extraction will therefore be removed. In addition, extraction access rood techniques will avoid blasting so vibration will not be not caused.
The 'open gap' qualities of the surface will be maintained and will not be significantly affected. The mitigation of surface disturbance means that the visual, landscape, ecological, archaeological and amenity-related impacts will be very low.
Albeit of low order, the most significant impact, as is commonly the case, will relate to traffic. However, traffic impact analysis has shown that the local highways will be able to accommodate the relatively low levels of vehicles generated. In terms of environmental impact following the initial construction phase, HGV vehicle movements will be approximately 10 a day (five inbound and five outbound) with factory offcuts being backloaded to the mine for back-stowing.
Notwithstanding the results of the public consultation exercise referred to earlier, the mining application raised a number of concerns among local residents. These concerns are being taken very seriously by Albion Stone Quarries, but the company firmly believes that those concerned will ultimately recognize the benefits that underground mining can deliver in terms of environmental improvements.
The new planning permission was approved subject to 40 conditions and a Section 106 agreement. The latter will deliver the surrender of permissions relating to 'cherty' extraction and some limited dimension stone reserves.
Stone has been hewn from Portland for more than 1,000 years. It is therefore appropriate that, at the beginning of a new millennium, the stone extraction industry has taken a major step towards reducing environmental impact by modernizing extraction techniques.
The author, John Pears, is a partner in Wardell Armstrong and the Deputy Crown Mineral Agent
