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Polluted & Marginal Land -98

Polluted & Marginal Land- 98 takes place at Brunel University, Uxbridge, from 7-9 July. Ground Engineering previews the event with a selection of abstracts from conference papers, and profiles companies participating in the technical exhibition.

Alt-Mat: Alternative materials in road construction

Dr Murray Reid, Ground Properties and Underground Structures, Transport Research Laboratory, Berkshire

Alt-Mat is a collaborative research project partly funded by the European Commission. It involves eight research organisations across Europe and is co-ordinated by the Transport Research Laboratory. The aim of the project is to help bridge the gap between laboratory tests and field performance, and hence to encourage the use of alternative materials in road construction. The project focuses on unbound granular materials.

The current situation with regard to the use of alternative materials in road construction in the UK is reviewed and the activities to be carried out in Alt-mat are described. Input from interested parties is invited.

Introduction

In recent years, there has been increased pressure for the use of alternative materials in all branches of construction. This has arisen partly from pressures to limit the consumption of primary aggregates, which are often situated in areas of science and environmental value, and partly to utilise the alternative materials more productively than simply sending them to landfill. Figures produced by the Department of the Environment (1994) show that 24% of primary aggregates were used in road construction, with a further 8% in the repair and maintenance of roads. Clearly, an increase in the proportion of alternative materials used in these applications would have considerable environmental benefit.

Alternative materials may be defined as waste material from mining and quarrying, industrial by-products, recycling of existing construction and by-products of waste disposal systems. Examples include colliery spoil and china clay waste in the first category, blastfurnace and steel slags and power station ashes in the second, demolition and construction rubble and asphalt planings in the third and ashes from municipal solid waste and sewage sludge incineration in the last.

The UK Government is committed to a policy of sustainable development, and as part of this targets were set for the use of secondary/recycled materials in Minerals Planning Guidance Note 6 (Department of the Environment, 1994). The targets are 40Mt/year by 2001 and 55Mt/year by 2006. Recent figures produced by the Quarry Products Association (Anon, 1998) show that the current use of alternative material is estimated to be 43Mt, equivalent to 16% of the aggregate market (primary aggregates plus recycled material). In 1989 it was estimated that only 10% of the aggregates used in construction came from secondary and recycled material (MPG6, 1994).

The figures indicate that the target for re-use of alternative material by 2001 has already been achieved. Further estimates indicate that about 70Mt of demolition waste are generated annually, of which up to 90% 'hard' materials which can be used in aggregate/fill applications. Currently about one third is used on construction sites, one third for engineering purposes on landfill sites and one third is landslide. The Quarry Products Association states that up to 17Mt of demolition waste in this latter category could be reused. In addition to the existing 43Mt, this would give a total of 60Mt of alternative materials being re-used per annum, in line with the Government's target for 2006.

Despite this increase in the use of alternative material, concerns have often been expressed about the mechanical properties of some of the materials and the possibility that they could release contaminants into controlled waters. These fears have resulted in alternative materials often being used only for low value applications such as hard standing or bulk fill. If full value is to be obtained from the use of alternative materials then their suitability for higher value applications must also be considered. These applications would include aggregates in structural concrete, in road pavements as cement and bitumen bound materials, and in unbound form as the sub-base and capping layers of road pavements.

The latter application is the main theme of this paper. Research is required to assess the behaviour of alternative materials in these higher value applications in order to determine whether the concerns which have been raised are justified.

The regulatory role in the technical appraisal of innovations in landfill design

AE Moore, The Environment Agency, North West Region, South Area, Cheshire

Introduction

UK landfill technology has developed so that sites are no longer selected according to proximity to waste sources and available void space but geological setting and engineering design are increasingly important to prevent potential discharges to the environment. Sites are often remote from developments and greater understanding of new products, such as geosynthetics and new ways of using existing materials have provided the designer with new tools to meet the environmental objectives of the Environment Agency.

Innovations can reduce the environmental impact of landfills, maximise available void space and make a site more competitive in the marketplace It is not practical to have a policy for each technology so the regulator encourages the concept of risk assessment for the formulation of site specific protection standards. A design making use of an innovative technology must demonstrate through a process of technical justification that it can achieve the required level of environmental protection.

Conclusions

It is accepted, given the rate of technological development, that all parties to a landfill design will need to take a proactive approach to the issue of accepting innovation into the main stream. The regulator promotes the use of risk assessment in landfill design. Consequently, the designer will maximise the economy of design by incorporating new innovations where available.

Each proposal should be sounded out with the regulator before presenting a technical justification. The technical justification is an interactive process between operator and regulator including expertise from consultants, academics and manufacturers. The regulator will have a questioning approach to innovative design and not all technical justifications will lead to acceptance. It is to the operator's advantage, therefore, to keep the regulator involved in design issues.

New money for research is available from environmental bodies, funded by Landfill Tax and operator donations and regulated by Entrust. This should encourage academic institutions to initiate research proposals to meet the needs of the waste management industry. Where research requires full scale site trials the regulator may have a major part to play.

Regulatory policy is based on protection of the environment and reduction of risk associated with landfills. Future legislation from the European Community may have an effect on waste management practices but the principle of containment engineering for leachate and gas producing sites remains the normal approach. Consequently, to limit the environmental impact of the construction of

landfills innovative design and advances in technology are to be encouraged.

Brownfield sites - the way forward

Phillip Crowcroft, Aspinwall & Company, Shropshire

Brownfield sites are providing an increasingly large proportion of potential development sites in the UK. Against a background of new legislation, the industry is coming to grips with the challenges of developing such sites, adopting the principles of risk assessment and management to minimise liability for developers, funders and future landowners.

This paper reviews the issues to be dealt with in developing brownfield sites and provides a series of case studies demonstrating how successful development can be achieved.

Few professionals involved in construction industry in the last few years can have failed to be aware of the increasing orientation towards developing brownfield sites. Pressures to re-use land with an industrial past have come from a variety of directions, and those creating some of the pressure are often outside of the normal group of construction professionals. This paper review the challenges facing the industry and the key issues which must be dealt with to achieve a successful redevelopment.

Such issues encompass the technical methods of identifying and controlling hazards on a site, as well as legal, marketing and public relations factors which can be the downfall of an apparently well-executed technical solution. A number of case studies are given which illustrate the point drawn out in the discussion, and demonstrate how industrial land can be successfully re-used in accordance with the best principles of sustainable development.

UK government policy on contaminated land follows the 'suitable for use' approach, which accepts that a different standard of remediation is appropriate for differing end uses. Furthermore, the determination of the extent of remediation must be based upon a balanced risk-based assessment methodology. Both qualitative and quantitative approaches to risk assessment are valid based upon the source-pathway-receptor (pollutant linkage) model.

Remediation can be designed to focus on any one of the of the three components in a pollutant linkage. Remedial action can be of a technical nature, for example the insitu treatment of a contaminant to render it harmless, or non-technical such as restricting contact between a contaminant source and a receptor by changing the way a site is used. It may also be valid to follow a programme of on-going monitoring and inspection as a means of further evaluation or risks, particularly if there is potential for natural processes (such as natural bacterial action on organics in the ground) to improve the extent or degree of marginal contamination.

The concept of reasonable cost has been introduced by Government in the draft Statutory Guidance to Section 57 of the Environment Act (1995). This means that where action is required by a Regulatory Authority to deal with an identified area of 'contaminated land' as defined by the Guidance, then such action should be appropriate and cost effective, ie the minimum action necessary to control the identified unacceptable risk. It is open to the landowner to take the action further to achieve a secondary objective perhaps associated with redevelopment of the site.

This issue of cost highlights the two different regimes under which land which may be contaminated (as distinct from the Section 57 defined contaminated land). After implementation of Section 57 Contaminated Land Regulations, expected in April 1999, there will be two areas of legislation which drive the process: Section 57 of the Environment Act (1995) and Town & Country Planning Act (1990).

The former will be policed by local authorities and the Environment Agency, and will only affect a relatively small number of sites which can be demonstrated by the authority to be land: 'In such condition, by reason of substances in, on or under the land, that a) significant harm is being caused or there is significant possibility of such harm being caused, or b) pollution of controlled water is being or is likely to be caused.'

The Town & Country Planning Act will govern most other remediation actions on land which may be contaminated by virtue of that land being subject to redevelopment and hence requiring planning permission. Controls on investigation and remediation are achieved by planning conditions or legal agreements under Section 106 of the Act.

From the private sector standpoint, 'reasonable cost' is fundamentally linked to the value of the site and its permitted form of redevelopment. If the cost of remediation exceeds the value of the site, then the developer will either need to seek gap funding, a change in use to create a more valuable end product or a different site. Thus, for most development projects, a remediation scheme must be selected which delivers the necessary reduction in risk associated with contamination, without excessive expenditure.

Electroremediation of contaminated soils

Mary Page and Christopher Page, department of civil engineering, Aston University, Birmingham

The electroremediation of soils involves the use of low intensity direct current to remove ionic contaminants, such as heavy metals, radionuclides and some anions, as well as certain compounds. Even as far back as the 1930s, there were some laboratory and field studies of decontamination of soils using direct currents but most of the work aimed at understanding the relevant processes and developing a commercially viable technology has been carried out in the last 10 years.

The approach is especially useful for decontamination of low permeability soils which are difficult to treat by other methods. The electric field imposed causes flow of dissolved chemical species, fluid and colloidal particles. An outline of the main processes involved in electroremediation and their theoretical treatment is presented.

A discussion is given of the many factors that affect the decontamination process, such as the nature of the contaminants, their chemical reactions and their concentration, the effect of changes in pH as a result of electrolysis of the soil pore water, the structure and chemistry of the soil itself and the processing conditions. Attempts which have been made to solve some of the problems encountered during electroremediation and to improve effectiveness of decontamination are also considered. Examples of field trials and commercial applications are described and the future of the technique discussed.

Whereas the actual technology of transporting contaminants through soils by means of electric fields is well developed, much work is still to be done in determining the optimum conditions for electro remediation. In particular, further study is needed on understanding the chemistry of real soils, as opposed to pure clay minerals, and their interaction with different contaminants and possible enhancement fluids. Choice of enhancement fluid and electrode material are important areas of research.

At present, much effort is being directed towards developing ways of combining electroremediation with other methods of decontamination, especially bioremediation, and some successful projects have already been completed.

In general, with the exception of those described in this article, commercial applications of electro remediation are only just beginning but the technique appears to have considerable potential for use in the decontamination of fine-grained soils contaminated by metals and/or organics.

The legacy of abandoned mining in the urban environment in the UK

LJ Donnelly, S Dumpleton, MG Culshaw, SL Shedlock (British Geological Survey, Keyworth, Nottingham) and DM McCann (Edinburgh University)

Britain had a long history of mineral exploitation, with coal mining by far the most extensive. Mining has always had to contend with underground water. It was the developing of the steam engine which enabled large volumes of water to be pumped from the mines. When a mine is abandoned, the workings will flood, unless pumping is con- tinued. Because coal workings are often complexly interlinked underground, often from one colliery to another, it is necessary to maintain a network of pumping stations to dewater abandoned mines, to prevent the flooding of currently operating collieries.

Mining permanently alters the state of the ground. In many coalfields several coal seams have been extracted in extensive, overlapping 'stacks' of workings. Subsidence fractures extend upwards, causing not only damage to property on the surface, but also creating pathways for minewater movement. Although disused mine shafts may be filled and sealed, underground workings, especially access roadways, may remain open long after abandonment, providing many miles of interlinked conduits through which large volumes of minewater may flow.

With the recent closure of so many coal mines, sometimes the abandonment of entire coalfields with the attendant cessation of pumping, rising minewater contaminated with dissolved pyrite oxid- ation products is now seen as a major threat to the environment. Uncon- trolled minewater discharges into surface water courses, frequently highly ochrous and acidic, are now well documented; more are expected following complete coalfeld closure. Additionally, water supply aquifers overlying abandoned coalfields may be at risk if pathways such as fractures, faults, shafts and boreholes allow the passage of minewater up into these higher beds.

Contaminated minewater emissions can have a direct impact on building foundation and water supplies. Furthermore, discharges of ocherous water will affect biotic life in streams and rivers, which may have a significant influence of the food chain.

Less well documented is the effect of rising groundwater or minewater levels on ground stability. Renewed mining subsidence may be initiated, which can occur several years after primary mining subsidence has been completed.

Rising minewater can induce the failure of support pillars and mine roadways, increase settlement in areas of collapse, affect the geotechnical properties of weak rock causing them to shrink or swell, close fractures and other rock mass discontinuities and gradually deteriorate remaining roof sup- ports. Similarly, the reactivation of geological faults is a process which commonly occurs during

mining.

This reactivation occurs in phases from months to years, separated by periods of relative stability. However, an increase in pore fluid pressures and the alteration of the geotechnical properties of the fault zone, caused by fluctuating groundwaters is sufficient to induce a further phase of reactivation.

The combined affect of subsidence and fault reactivation has financial and hazard implications. Urban regeneration is usually undertaken in areas where mining subsidence and fault reactivation is presumed to have been completed. However, site investigation should consider the possibility of abnormal ground deformation which may be induced by groundwater rebound.

Mine gases, especially methane and blackdamp, possibly carbon monoxide, hydrogen sulphide and radon may be expelled from shafts and boreholes ahead of rising minewater. The gases may constitute a health hazard (risk of explosion, suffocation, radiological hazard, etc). Mine gases may also be transmitted laterally via joints and fractures; points of emission at the surface may not necessarily occur directly from shafts or boreholes. Further investigation is required to determine whether the methane can be used as a resource.

Site investigation surveys, in particular geophysical techniques are useful for the delineation of near surface mine workings. Geophysical surveys should be designed, commissioned and carried out by suitably qualified persons. The role of geology and geophysics in site investigation is of paramount importance. This should include monitoring and prediction of likely rebound levels for minewater.

Failure to carry out thorough geological investigations at an early stage of urban regeneration can result in additional costs from 'unforeseen ground conditions'.

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