Approaches to inform redevelopment of brownfield sites: An example from the Leeds area of the West Yorkshire coalfield, UK

Highlights

• Geological information can better characterise the near-surface in coalfield areas.

• This information can be used to identify problematic ground conditions.

• Artificial ground associated with coal mining or landfill can be quantified.

• A holistic approach from investment through to ultimate redevelopment is recommended.

Abstract

Government-led regeneration schemes and policies encouraging the use of brownfield land present a challenge, particularly in coalfield areas. Coalfields have typically experienced multiple phases of development and can be susceptible to a suite of problematic ground conditions that may be rooted in the near-surface geology or result from anthropogenic activity. Such problems, related to the nature of void backfill, undermined and unstable ground and the presence of contaminated land in the near-surface, may deter investment in the very areas earmarked for redevelopment. An understanding of previous developments within coalfields is required to identify potential geological hazards, so that regeneration proposals include measures that address these issues. Public records of landfill and site investigations, and minerals exploration including opencast mine plans can reveal the distribution, thickness and high-level descriptions of fill materials, although the coverage of data typically precludes a comprehensive analysis of entire cities. The best way to show the spatial distribution of fill materials is currently as a two dimensional national/regional scale dataset. Depending on the distribution of data points, however, 3D modelling can be possible, which is much more detailed and accurate. Focusing on the heavily urbanised county of West Yorkshire in northern England, the assessment of opencast coal mining on the landscape and benefits of quantifying the impact are discussed. We demonstrate how certain types of publicly available data allow a greater understanding of the interaction between human activity and natural superficial and bedrock geology. If successful, this approach can help lessen the impact of delays and increased financial costs caused by unforeseen ground conditions.

Introduction

This study underlines the need for planners and developers to have access to accurate information describing the shallow subsurface, and investigates the use of publically available information to better characterise previously developed areas and mined ground. This applies to the UK as a whole, but is particularly relevant to brownfield sites in coalfield areas, where problems associated with fill materials can adversely affect developments. The sources of data discussed in this paper are intended to complement, rather than substitute site investigations, and highlight potential issues that can be investigated further with site investigation. Fig. 1(a) shows the extent urban development within the West Pennine coalfield, where the landscape has been extensively modified through coal mining. Fig. 1(b) shows the extent of former opencast coal workings in the Aire Valley area, south-east of Leeds.

Over the past few decades, UK planning policy has prioritised the redevelopment of brownfield areas in an effort to bring vacant land back into use, as well as encourage urban regeneration and preserve greenbelt land (Department for Communities and Local Government, 2012). In 1998, the UK government introduced a national target of 60% of new housing to be built on brownfield land (DCLG, 2011). In 1997, 53% of new residential properties were built on brownfield land in Leeds, rising to 97% in 2006 (Brannen, 2012). Since 2006 the number of new residences built on brownfield land in Leeds has steadily fallen in response to adverse economic conditions, totalling 86% in 2011 (Brannen, 2012). The National Planning Policy Framework (DCLG, 2012) introduced in England and Wales 2012 removed the national target and enabled local authorities to set their own. The National Planning Policy Framework encourages the development of brownfield sites in preference to greenbelt land, provided that the land in question is of little historical or environmental significance. In this context, brownfield land is regarded as any previously developed site (DCLG, 2012). This presents a challenge, particularly in coalfield areas, where many planning concerns are related to underlying geology and the exploitation of mineral resources. Coalfield areas include some of the most densely populated areas of the UK, including parts of Strathclyde, the West Midlands, and South Yorkshire, parts of which have experienced multiple and strategically uncoordinated episodes of redevelopment since the onset of a modern coal mining industry during the 1700s.

The legacy of coalfield developments can lead to unpredictable and deleterious ground conditions related to both well documented and unrecorded surface and shallow mine workings that can have an adverse impact on regeneration schemes. Such adverse effects include extensive deposits of variable composition and thickness, resulting from a variety of materials tipped onto the ground surface, such as colliery spoil, rubble, industrial and domestic refuse (Waters et al., 1996). The margins of open pits and subsurface excavations may become unstable where not properly engineered or restored, leading to ground movements initiated by deep and shallow coal mining (Bell and Donnelly, 2006, Price et al., 2011). Coalfields can also be prone to landslides, caused by the alternation between more permeable sandstones and less permeable mudstones, allied with the development of topography over-steepened during glaciations (Cooper and Gibson, 2003). Human activity associated with the extraction of minerals for fuel and construction have induced further geological hazards, such as the deposition of spoil or the removal of natural materials from steep slopes. Different styles of mining are characterised by contrasting types of subsidence affecting the land surface, allowing predictions to be made regarding subsidence if the mining technique is known (Bell and Genske, 2001). For example, lowering the water table by pumping water from underground mine workings can cause subsidence of relatively large areas (Bell and Donnelly, 2006).

Contaminants derived from fill deposited in coalfield settings can migrate from fill materials into the surrounding bedrock. Methane gas emanating from domestic landfill material used to back fill opencast workings at Loscoe in Derbyshire was found to be the cause of a gas explosion at a residential property located 70 m away, with ground heating observed 100 m away from the site (Williams and Aitkenhead, 1991). Contaminants derived from the bedrock itself can also lead to possible groundwater contamination. Pyrite-rich lignite, for example, can cause acidification of the water in flooded surface workings (Younger et al., 2002), which can migrate and corrode cement foundations that are not resistant to sulphate attack.

Another issue that can affect brownfield site development is the increasing demand for underground space, from shallow infrastructure, such as sewerage networks, to underground storage for materials, such as waste and gas (Evans et al., 2009). New technologies can enable previously sterilised areas to be re-used. For example, water held in flooded underground mine workings may be contaminated, but could be utilised in ground-source heating schemes (Evans et al., 2009). In the deeper subsurface, regulations designed for a particular activity may preclude the possibility of other operations occurring in the ground either below or above. The use of underground space requires careful planning to ensure that conflicts on underground space are kept to a minimum and any potentially useful resources are maximised. Areas that are suitable for coal mining are not generally considered suitable for carbon dioxide sequestration because of conflicts in land use related to potential interactions between the host for CO₂ and deeper coal-bearing strata. Areas where coal seams are classed as unminable (over 1200 m deep and over 500 m away from mine workings) are considered most appropriate for carbon dioxide sequestration (Jones et al., 2004) and can be identified using modern 3D modelling and GIS (Geographical Information Systems).

A study of 5000 industrial construction projects revealed that half were delayed by more than a month, and all developments on redevelopment land had encountered unforeseen ground conditions although, only 2–3% of the total cost of a typical construction project is spent on site investigation to assess the geology and soil conditions (www.bexley.gov.uk). With the aim of reducing construction costs by 33% and build times by 50%, the UK Government, in partnership with industry, introduced the Construction 2025 strategy in 2013 (Anon, 2013). Industry standards, such as BS5930 (Anon, 1999), set out a detailed code of practice for site investigation, beginning with a desk study, but this is not a statutory requirement.

Preliminary assessments of ground conditions should take place at the planning stage, in order to keep the resource effort to a minimum and to avoid the delays associated with unforeseen ground conditions (Marker, 1998). In Britain, the statutory obligation to record the location of mine workings was established in 1850, and the compulsory documenting of mine plans dates from 1782 (Bell and Genske, 2001). Many pre-1850 coal workings are unrecorded and can adversely affect properties built above them. In Glasgow, for example, some developments are located in areas of shallow mining and the collapse of these old workings and the settlement of fill materials in these areas have resulted in damage to buildings (Browne et al., 1986).

Parcels of undeveloped brownfield land may serve to deter regional redevelopment and investment. It is therefore beneficial to regions with the potential for redevelopment that any barriers to improvements are identified and mitigated. Additionally, some areas previously considered unsuitable for mineral extraction or development due to environmental hazards are now being included in development plans as new technologies and incentives allow. The Environment Agency (England) are investigating new ways of treating water from mine workings to prevent contaminants reaching natural water courses. One scheme is the five hectare area Lamesley Wetlands in County Durham, north east England, where biological processes used in the treatment of sewage improve the effectiveness of mine water treatment. Lamesley Wetlands serves as a nature reserve and benefits the local community, and is included in a local regeneration scheme (Johnston et al., 2007). The economy is now exploiting innovative ventures for industrial activities or infrastructure in the shallow subsurface. This increases the need to better understand human impacts on surface and subsurface space (Evans et al., 2009).

Developers of brownfield sites in coalfield areas need reassuring that the relevant local authority has adequately assessed any land instability issues arising from former coal mining activity. For example, the heterogeneous nature of opencast fill materials, such as variation in sorting and grain size, can lead to differential compaction (Younger et al., 2002), although where properly engineered, opencast fill can have excellent engineering properties (Waters et al., 1991). Differential settlement can also occur at the margins of backfilled opencast sites, due to the different load bearing strengths of in situ bedrock and fill material (Waters et al., 1992). Extraction needs to occur prior to development to avoid unnecessary sterilisation of resources, such as coal, sandstone, fireclay and brick clay. The Coal Authority is responsible for licensing and permitting coal extraction and public safety arising from the legacy of coal mining. Local authorities are responsible for site remediation prior to development through Part IIa of the Environmental Protection Act, 1990, which requires contaminated land is surveyed for, identified and compulsorily remediated, and that the local authority and environmental regulators maintain a register of sites. Additionally, there is a requirement through the National Planning Policy Framework (DCLG, 2012) that unstable ground is identified as part of the planning process. The developer is subsequently responsible for ensuring that the site is safe and suitable for the proposed use. Therefore, the responsibility for a brownfield site changes over time, with the local planning authority maintaining an input at key stages.