Building over former mines: a new guide for the UK

Dave Parry, Coal Authority mining consultant and information manager. Image: Coal Authority

Our mining legacy requires consideration across large parts of the United Kingdom. In Great Britain alone, nearly 15% of land is deemed to be in a mining area. Many urban and industrial centres have been located close to heavily mined areas, with the ten largest urban areas each affected by mining to some degree. Each exploited mineral type brings with it different considerations for both the natural environment and any proposed construction in the area of the mining activity.

The manual replaces the previous version (SP32) that was published 35 years ago and covers the impacts of mining activity, planning legislation and regulation, appropriate site investigation and techniques of ground treatment and construction design. It represents a major contribution towards the knowledge and record of the mining legacy and provides guidance to those, together with their advisers, who seek to invest in the built environment in those mining areas.

The key issues associated with abandoned mine workings are extensive and, in many cases, very complex in their detail. However, they may be summarised as events, risks and hazards associated with: collapse of shallow mine workings and surface subsidence, subsidence of mine entries like shafts and adits, and mine gas emissions.

Also, mine water emissions caused by sudden outbursts and discharges to waterways can lead to groundwater contamination. Another issue is historical methods of abandonment such as serviceability, and currently occurring failures.

For land potentially affected by mining, the remediation requires an understanding of the location and depth of mining and a key part of the process is a search of available mining records. However, older mining, particularly non-coal mining, is often unrecorded where it pre-dates statutory requirements for mine plans. Even where older records do exist, they may be incomplete or contain inaccuracies associated with the surveying and draughting techniques of the time. The new manual has a section dedicated to various available information sources and containing guidance on the interpretation of mining records.

It was a collaborative research project managed by CIRIA (Construction Industry Research and Information Association). CIRIA appointed a main research contractor, who in turn enlisted the services of other companies and organisations in industry and academia that specialise in various areas. A Project Steering Group was established, made up of representatives from stakeholder communities and industry experts, and a day-long workshop event was held to assimilate the views and opinions of specialists in all aspects relating to the mining legacy.

For the later stages of the project, a focus group was established for key chapters of the manual, these being practice-related chapters that needed to draw on the experience and expertise of practitioners, many of whom were members of the steering group. In addition to comments on these and other chapters, members provided significant contributions and authorship for various parts of the manual. A draft version was available online from mid-2018, allowing industry-wide comment and feedback.

The new manual retains much of the information from the original guide, but with substantial updates in dealing with the effects of the mining legacy. It also provides additional guidance on the planning regime, environmental considerations, legislative framework, and health and safety issues.

The depth at which mine workings were deemed to not require consideration in the context of stability at the surface has traditionally been determined with the aid of ‘rules of thumb’, particularly relating to the cover ratio of unmined strata above a worked area. The definition of ‘shallow’ workings has often been regarded as within a depth of 30 metres, regardless of situational variations.

These arbitrary ‘rules’ served well over time and may still have value as an initial assessment, particularly in the case of stratified minerals such as coal about which they have developed. However, the new manual promotes detailed evaluation on a project-specific, risk-assessed basis, considering all known and potential variables. It also promotes a risk-based, non-prescriptive approach to the mitigation and remediation of mining legacy issues

SP32 was first published in 1984 and proved invaluable to the industry and was still widely used well in excess of 30 years after being commissioned. However, it was specifically aimed at engineers and geologists, and was heavily weighted towards coal mining and construction.

Since the publication of SP32, the numerous issues associated with abandoned mines have progressively become better understood, with significant advances in knowledge and expertise. The deep coal mining industry has been run down, and this has coincided with increased urbanisation and regeneration of land in areas of former mine workings.

Major changes in legislation and town and country planning have been implemented, reflecting the increased awareness of the effects that mining has on public health, safety, and the environment. New materials such as geosynthetics are now in widespread use and techniques of grouting and ground consolidation have been better developed. It was therefore recognised, by stakeholders and support and advisory organisations, that a considerable body of knowledge and expertise was not reflected in SP32.

Legislation governing mines and mine workings has grown over many years, often in response to a major change or event, and the manual includes relevant background to statutes, as well as other controls. Variations in legislation across the four countries in the UK are recognised, particularly with regard to the various planning regimes in operation.

In addition to discussing the effects of past mining and the management of land and construction in mined areas, the manual also provides an overview of historic and more recent mining throughout the UK in terms of its type and location, and the methods adopted.

Practice-related chapters are supported by case studies, short examples and practice notes and incorporate a considerable amount of hands-on experience in support of the more prescriptive elements of the guidance given. An important part of the manual is the setting of a standard for reporting and record-keeping. This is something which has often been neglected, even though the accurate and detailed recording of information is extremely important for the maintenance and management of new and existing assets or structures going forward.