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Quality control of landfills and backfills

Successful backfilling and landfill must satisfy regulatory authorities as well as owners and users, so independent quality management is needed. John F Thompson, principal adviser, geotechnics at the Applied Environmental Research Centre looks at the philosophy behind the regulations, and suggests principles for rigorous construction quality management.

Quality control of backfills and landfills is becoming increasingly rigorous as more stringent requirements are placed on operators to produce a completed landform and meet certain standards of environmental and engineering control. Some organisations that provide independent verification, and who probably feel that they can work to a prescriptive formula for supervision, testing and reporting requirements, often poorly understand these demands. The work is regarded as bread and butter stuff with little technical interest. However, the successful performance of this independent role depends on an understanding of what is to be achieved and its proper purpose.

The major objectives of quality management in this context are to reconcile potential conflicts of interest between site operators, regulatory authorities and end users of the restored land, and to ensure the long term requirements of performance and durability are satisfied.

Landfills

Department of the Environment Waste Management Paper 26B states 'the overall philosophy ... is that of a construction project, carried out over an extended period, to build a desired landform using waste materials and incorporating appropriate measures for environmental protection' (WMP26B, paragraph 1.13). Protection of the environment might include containment of biogas, leachate and waste to prevent nuisance to neighbours, prevention of pollution of ground and surface water, treatment or containment of pollutants in the ground, and measures to monitor the external environment. WMP26B says these environmental protection measures 'will inevitably change during the life of the site and the design should be flexible to allow for the implementation of enhanced environmental standards'. This paper should be studied with WMP26E (currently in draft) to understand the regulatory authorities' view of what they would like to be achieved by completion.

These principles are in direct conflict with the primary aim of the site operator, who wishes to maximise his income by the final placement of waste materials. Environmental protection and monitoring, restoration and aftercare are costs that are likely to be expended over a considerable period of time, and which can only be covered by income accrued and invested during the first few years. This is a risk to the operator because environmental standards might change with time or the measures put in place might deteriorate, either of which could create unanticipated costs. Both the design and subsequent control must ensure these risks are minimised.

Paragraph 4.16 of WMP26B makes it clear that independent construction quality assurance is a requirement of the regulatory authorities. The quality manager has a key role in ensuring durability and trying to prevent costly surprises in the future. That this role needs to be independent should go without saying, given the different intents of the site operator and the regulatory authorities.

As the cost of quality control usually falls on the site operator, the level of supervision needs to be reviewed regularly to ensure the quality requirements are being met without being unduly burdensome. There may well be room for innovation, even in what are usually regarded to be mundane tasks of observation, sampling and testing. Quality management is necessary to reassure both operators and regulators that the requirements of the environment are being met without excessive cost.

Backfills

Planning authorities have the power to require applicants to demonstrate that a new site will be restored satisfactorily upon completion. They are also entitled to expect a high standard of restoration, and can refuse permission if they are in any doubt in this regard. To propose an independent quality management system might demonstrate intent, and to carry it out should reassure the Mineral Planning Authority (MPA) that its required standards are being met. It should reduce risks associated with the backfilling operation by testing and documenting the works as they proceed, and certifying the end result. This might also enhance the value of the finished site.

Department of the Environment Mineral Planning Guidance Notes 3 and 6 both state that 'restoration and aftercare will be required to make mineral workings fit for beneficial after-use and environmentally acceptable'. MPG6 and MPG7 go on to say 'applications for extraction of minerals such as aggregates need to include information which demonstrates that the site will be restored satisfactorily', and that 'MPAs should consider whether to grant permission if there is serious doubt whether a new extraction proposal can meet this requirement'. MPG3 makes similar statements. Clearly, public policy is to require an acceptable restoration scheme to be implemented when the mineral reserves are exhausted, and there is the very strong suggestion that planning permission should be refused unless such a scheme is possible.

Annex A of MPG7 allows the MPA to put quite stringent conditions on the placement of the backfill and subsequent surface restoration. It can specify the location and form of storage mounds, the timing, methods of placement and the layer thickness of various materials, the placing of ditches and fencing, as well as maintenance proposals. Independent construction quality control would, in many cases, prove the most reliable way to demonstrate that the planning conditions are being met.

Backfilling of quarries and opencast mining operations with mineral waste is usually permissible. In MPG14 (quoting the Environment Act 1995) this is called 'mineral-working deposit', and is defined as 'any deposit of material remaining after minerals have been extracted from land or otherwise deriving from the carrying out of operations for the winning and working of minerals in, on or under the land'. The properties of the mineral waste, whether it will be required to support structures, the effect of inundation by groundwater, the specification of the compaction process, and whether it is inert, are essential to the effectiveness of the restoration scheme. Different layers within the ground are likely to have different properties, and such variability poses its own risks. Flexibility must be a part of the restoration procedures, and independent verification must be a continuous process if these risks are to be minimised and confidence given to the regulatory authorities and end users.

Once the proposed scheme has been agreed and passed by the MPA, it is necessary to ensure the work is carried out to the required standard. This might lead to a conflict of interest between the excavation contractor, who wishes to extract the useful materials as economically as possible, and the end user and MPA, who are more interested in the restored site. Independence is again the key requirement for quality management, supervision and testing of the operation to minimise conflict and give confidence in the end product.

Quality management

If the risks associated with future performance and changes in environmental requirements are to be minimised, it is essential that a rigorous quality plan identifying the objectives of the work is prepared. It needs to outline the purpose of the works, the methods to be employed to achieve them, and the certification required on completion. Certification requirements need to be closely defined. For example, are we just assuring construction to design and specification, or are we also assuring durability or minimum load bearing capacity? The certification requirements will govern the work needed to provide the specified assurance. It is vital that a plan is prepared in advance of the work if the outcome is to be satisfactory.

In all cases it is necessary to provide a degree of quality management that will reassure all interested parties. The absolute minimum is at least one suitably qualified person resident on site with a more experienced chartered engineer readily available, and a fully equipped laboratory capable of the necessary geotechnical testing.

The duties of the site engineer will be to observe the work, to ensure that material is placed in layers at the specified thickness and compacted in accordance with the design, licence and planning requirements, and to liaise with the client's and contractor's site personnel. The site engineer must be able to discuss any relevant issues with the chartered engineer, will be required to carry out or supervise insitu density and other relevant site tests of com- pacted material, and to identify representative samples for testing in the laboratory.

The chartered engineer should be experienced in the supervision of earthworks and understand the properties of earthwork materials. This senior person will need to visit the site regularly to review progress of the work with the site engineer and ensure that the specification is being complied with. Liaison with the client, and possibly the regulatory authorities, will be necessary to make them aware of any issues that arise.

The laboratory should be equipped to carry out all the relevant soils tests, including moisture content, Atterberg limits, particle size, compaction, permeability and if needed triaxial shear strength and consolidation. These tests would be carried out on samples provided by the site engineer.

The results of the quality management should be reported at regular intervals or at the end of the stage of the work. The report should highlight the results of the tests and site observations, and conclude on their significance in the context of the project. The end result, on completion of the work, should be required certification to give confidence in the future performance of the environmental protection measures and final restoration.

Conclusion

Clearly there are occasions when the independent verification of the installation of landfills and backfills is necessary to satisfy the regulatory authorities and end user of a site. This is not a secondary exercise, and requires a thorough understanding of its purpose. An independent engineer or organisation with the required experience of working with the ground should carry out this work. This engineer should prepare a quality plan before any work is carried out, and should be willing to certify that the work has been carried out satisfactorily on the basis of the proposed end use and necessary environmental controls.

Although QA/QC testing imposes an expense on the site operator, if carried out properly it should be seen as an opportunity to provide assurance of the future performance of the works. This can be used to satisfy the regulatory authorities, and might enhance the value of a site that is used for redevelopment.

References

Waste Management Paper 26B (WMP26B), Landfill design, construction and operational practice, Department of the Environment, 1995.

Waste Management Paper 26E (WMP26E), Landfill restoration and post closure management consultation draft, Department of the Environment, August 1996.

Mineral Planning Guidance Note 3 (MPG3), Coal mining and colliery spoil disposal, Department of the Environment, 1994.

Mineral Planning Guidance Note 6 (MPG6), Guidelines for aggregate provision in England, Department of the Environment, 1994.

Mineral Planning Guidance Note 7 (MPG7), The reclamation of mineral workings, Department of the Environment, 1996.

Mineral Planning Guidance Note 14 (MPG14), Environment Act 1995: review of minerals planning permissions, Department of the Environment, 1996.

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