The barriers to underground mining of aggregates – An overview

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Executive Summary

Aggregates in Aggregates in the form of sand and gravel and crushed hard rock are essential to much present-day construction and can be expected to remain so for the foreseeable future. Since aggregates are heavy, high volume but relatively low-value minerals, it is desirable for economic and environmental reasons to maintain UK-based sources of supply as far as possible.

Supply is at present largely from surface quarrying, a pattern established in the early 20th century and greatly expanded since then. However, production from some significant sources is likely to be reduced in the period up to the 2040s as a number of high-output quarries are worked out and cannot be extended, or where pressure from other forms of development has on balance to take priority over possible new sites. In considering how aggregates supply could be maintained from domestic resources, one option is whether some could be obtained through underground mining. This could potentially access material not available through quarrying, either for environmental reasons or because of potential conflicts with existing or alternative surface development.

The mining option was briefly considered in the 1970s by the official Verney Committee, appointed by the government to examine the future supply of aggregates in Britain, as a long term possibility in support of its main conclusions. Verney saw considerable potential for mining in reducing the visual impacts of surface quarrying and the processing of aggregates, in cutting emissions of dust and noise and, if mines could be established in the aggregates-deficit area of South East England, by greatly reducing the need for long-distance haulage of supplementary supplies from more distant regions. However, Verney also identified potential problems, such as the need to leave material to support underground voids, the risks of subsidence and the need for pumping water from workings below the water table. The higher operating costs of mining could also be problematic, but Verney noted the possibility of offsetting revenue from the after-uses of mine voids. Overall, though Verney felt mining might only be worthwhile infrequently, it recommended that there should be a further, implicitly more detailed, examination of the geological prospects, and a joint study with the industry of the feasibility of mining at a specific site.

There was no official action in response to Verney's mining recommendations for several decades. Central and local government and the aggregates industry focussed instead on implementing the Committee's mainstream proposals for consolidating the emerging system of managed aggregates supply in England and Wales, in order to make the planning system work more effectively to facilitate the surface quarrying of aggregates. (There was limited pursuit of Verney's other long-term possibility of large coastal super-quarries, though with mixed success). In the late 1980s and early 1990s there was some interest in using the existing infrastructure of the closed east Kent coal mines as a platform for extracting the underlying carboniferous limestone in that area, but that came to nothing and the opportunity was lost when the mines were dismantled.

In the late 2000s the decision of the Aggregates Sustainability Research Programme (ASRP) funded by the Aggregates Levy to examine a range of questions regarding long-term future aggregates supply at last provided an opportunity to look in detail at the prospects for the option of underground mining – which had lain on the table since Verney reported in 1975. Two projects were commissioned:

• in 2009-2010, initial research investigated the economic feasibility of mining, based on detailed examination of specific geological prospects, and the costs of capital development and mine operation. It confirmed that there are potential resources of aggregates at depths commencing in the range 350-650 metres below the South East of England. This area, broadly east of a line from the Solent to the Wash, has long been unable to supply all its needs for aggregates. The study concluded that while in the long-term high-capacity deep aggregates mines in the South East would not be competitive in delivering aggregate to markets in that super-region relative to supply from a more distant reference quarry, short-term mining linked to optimistic assumptions about after use (retail on the surface and industrial underground) mining could compete. It recommended further work on a range of key issues;
• in 2010-2011 further research was commissioned to examine the barriers that would need to be removed if the mining option were to be pursued. This work was not completed, but its initial findings have nevertheless provided a valuable reference point for this overview.

Despite the availability of potential aggregate resources at depth in the South East, and the potential elsewhere for lateral shallow mine extensions to existing surface quarries in areas of exposed hard rock, mining expressly for the purpose of winning aggregates has not been undertaken in the UK. In addition to the issues of capital and operating costs, there are a number of constraints that have so far made the mining option commercially unattractive:

• the inevitable geological uncertainties about the extent, quality and "workability" of the target resource in any underground mining project, which only construction of a highly expensive shaft or decline (adit) could fully resolve, at the risk of the mine not delivering its expectations;
• management of the potential for surface instability and subsidence, in terms of compatibility with present and possible future surface development, dealing with concerns from surface landowners and reassuring the wider public, including the need for insurance-backed or trustee-funded financial guarantees;
• the expanding range of statutory environmental assessments since the 1970s, and the difficulty of providing through them acceptable public assurance on matters such as ground stability, vibration, hydrogeology, pollution control and habitat impacts; such assurance in advance of any mining necessarily having to be based on modelling alone;
• a range of planning issues reflecting the present-day understanding that, contrary to Verney's 1975 expectations, mining will not significantly reduce surface environmental impacts; a high-capacity mine would impose a severe additional load on local and possibly regional transport networks, and in practice most of the primary processing plant and all "added-value" manufacturing of road asphalt and ready-mix concrete would have to be located at the surface. While visual screening should be possible in the case of lateral mine extensions where there is an existing quarry void, that would not be available for a deep mine at a greenfield location. Public concerns that mining would open up the possibility of permanent after-use of the mine voids, in contrast to the reversion to previous agricultural or forestry use or passive conservation or recreational activities common in surface quarrying, would also need to be addressed. This poses a particular problem where the commercial viability of a mine proposal depends on a concurrent use of the land above the mine and a revenue from a specific after-use of the voids to be created, particularly in greenfield locations;
• the increasing difficulty in securing public and local political support which confronts any major development in the South East. The initial 2009-2010 study confirms this is the primary target area for high-capacity aggregate mines, in view of the strength of demand in the region, the resulting shorter haulage distances and the absence of surface exposures of rock of suitable quality;
• the cumulative deterrent effect on UK managements and overseas ownership of major aggregates operators caused by the high potential development costs and the costs, time and reputational risks in promoting an aggregates mine through an inevitable contested planning process, reflecting past experience and examples in comparable cases involving (amongst others) proposed new deep coal mines, the projected Isle of Harris coastal super-quarry, the continuing current controversy over major transport proposals in the South East, and the recent successful opposition to the proposed major new upper Thames reservoir in south Oxfordshire.
• the continuing ability of the Managed Aggregates Supply System to secure sufficient supplies of aggregate from a range of non-mined sources; surface quarrying, marine-dredged sand and gravel and recycled and secondary aggregate materials.

There appears to be three preconditions if the present barriers to aggregates mining are to be overcome:

• the costs of non-mined sources would need to significantly increase or their availability become significantly more constrained to encourage the commercial sector to consider more favourably the increased uncertainties, higher operating costs and capital investment associated with mining;
• better information is needed about the impacts of mining, to make it easier to satisfy essential regulatory tests (e.g. in being able to provide an acceptable level of information and assurance in Environmental Statements on matters such as subsidence, vibration, habitats and impacts on groundwater, hydrogeology);
• a supportive policy framework, both nationally and locally.

Short of major changes in the UK's economic and political climate, and if other sources of aggregates continue to be available, the barriers to mining therefore seem to be as firm, if not firmer, than at any time since Verney speculated about it in the early 1970s, certainly as regards deep mines to access concealed resources at greenfield locations. If market conditions became more favourable for the mining option, both the recent research and current planning and political realities suggest the best initial prospects would be through lateral underground extensions of existing hard rock quarries outside the South East.