The rate of flow (flux – Q) per unit area of liquid through a liner is a function of the hydraulic conductivity (k) and the hydraulic head (i), and can be approximated by applying the following equation, attributed to Darcy:

Q=ki (1)

In the case of vertical flow under gravity, the hydraulic head (i) is defined as the height of liquid standing over the liner divided by the thickness of the liner. If the liner is perfectly drained and no leachate accumulates above it, but the liner remains saturated, then the hydraulic gradient is 1.0 and the flux of liquid is independent of the thickness of the liner. However, if a layer of liquid is allowed to accumulate above the liner, the head (i) is greater than 1.0 and the thickness of the lining layer exerts an influence on the flux.

European Community requirements, and many other national standards, specify a maximum hydraulic conductivity of 1 x 10^-9 m s” and the Table below shows the maximum permissible leakages assuming either perfect drainage or a 1 metre head of leachate above the liner.

Synthetic and composite liners.

Synthetic lining membranes possess hydraulic conductivities of between 1 x 10^-15 and 1 x 10^-16 m per second, and small samples of the materials may be considered essentially impermeable. The materials are laid as strips from rolls, or as sheets, with joining being achieved by on-site seam welding.

Quality Assurance checking during the installation of membranes is a standard procedure. Nevertheless, some flaws may be anticipated and the American Society of Civil Engineers suggests that an achievable performance level for such liners is a seepage loss rate of 200 litres per hectare per day (Bonaparte and Goss 1991).

A leakage rate of 200 litre per hectare per day is equivalent to an annual flux of 7 mm, for natural clay or amended soil liners.

There is very little information available about the motivations and origins of the design principles behind the work of a landfill CQA engineer, so we decided to write this (based on a 1994 paper by Harris, Knox and Walker).

CQA principles must logically be applied to all landfills where the wastes accepted potentially pose a risk of water pollution, mainly those accepting household, commercial and industrial wastes. These wastes account for a relatively small proportion of total waste arisings (approximately 20% of the UK’s total of 516 Mt (3), the rest being demolition waste, mining and smelting wastes, fly ash from power stations, sewage sludge and agricultural wastes). Nonetheless, this fraction presents the most intractable difficulties of the total.

Whilst the nature of the wastes deposited in landfills may have evolved through man’s history, the operational methods until recently, remained largely unchanged and unsophisticated.

Over the last forty years or so, a much wider understanding of processes involved in waste stabilization has been developed. This has coincided with a worldwide increasing environmental awareness leading to demands for environmental improvements.

These demands are well founded, in fact although clean unpolluted UK water supplies are, as everywhere, vital for the survival of the population some experts have suggested that as much as one third of all UK groundwater supplies are now contaminated to some extent by pollutants. If the rate of damage to our water resources was sustained for another century the situation would have become extremely serious for public health, even without the further pressures on water supply anticipated from climate change.

Legislation in the UK has matched public demands for change, by the implementation of Directives issued by the Council of the European Communities. Most importantly in the 1980s, the introduction of the Groundwater Directive has caused an evolution in the standards of site preparation works and operational practice being demanded for all new landfill projects in order to prevent pollution of the water environment.

This evolution was accelerated by the inception of the National Rivers Authority in 1989 and the introduction of their Groundwater Protection Policy in December 1992.

The changed requirements have led largely to the discontinuation of the “attenuate and disperse” concept of landfill with the emphasis now on “engineered containment and operational safeguards”.

This is generally achieved by the installation of either an engineered clay liner or a composite liner, so called because it combines the use of natural materials (e.g. compacted clay) with polymeric membranes, otherwise known as flexible membrane liners (FMLs). Using these materials, emphasis is placed on preventing the release of leachate into the geologic environment.

In addition, there are concomitant operational requirements considered necessary to limit further the potential for leachate release from the site. There are two broad components.

The first is concerned with the limitation of leachate production.

This can be effected by infilling in a series of cells sized on the basis of water balance calculations to (in theory at least) avoid the generation of leachate during the operational phase by utilising the absorptive capacity of the waste.

Rainwater accumulating in other parts of the site can be kept separate and discharged in an uncontaminated condition.

Leachate production is further reduced by progressive capping and restoration of each cell as it is infilled to final levels and by ensuring that these restoration layers are laid to a high standard to prevent rainfall infiltration.

The second component is designed to ensure that any leachate produced can be removed easily from the site.

The composite lining system for engineered containment is protected by a blanket of free draining material incorporating a perforated drainage pipework system. Not only will this prevent mechanical damage to the liner, it will also facilitate the easy removal of leachate, limiting the potential for building up a head of leachate in contact with the liner.

The ability to remove leachate easily from the site must then be supported by a reliable system for its disposal.

This is usually the discharge to public sewer with varying degrees of pre-treatment though more rigorous on-site treatment with discharge to stream is increasingly being used as technical and management standards continue to improve.

The broad concepts behind this approach have been accepted and practised in the UK since the mid 1980s.

However, operational experience continued to highlight design and installation problems. Subsequently further design and landfill construction (base and capping) guidance and regulations were introduced through enactment of legislation on site licensing, and then permitting (under IPPC Regs – now known as Environmental Permits), driven by the Landfill Directive, and the amended Waste Directive.

The CQC and CQA engineering carried out as part of Landfill CQA is tasked with ensuring that the final link in the chain is achieved by verifying that the complaint design is fully implemented and achieved or exceeded during site construction.

This is a serious responsibility for the landfill CQC and CQA professionals when one considers the extremely high importance highlighted earlier to all future generations that we get this right, and the water environment does not become further damaged by pollution from landfills.

Landfill CQA Actions up to the Start of Construction

Once the landfill design engineer has completed the landfill design and the specification has been substantially completed, it is possible to write the landfill Construction Quality Assurance (CQA) Plan, which the construction contractor will then be required to follow, and which will once completed be reported upon to the environmental regulator.

The end goal is for the environmental regulator to agree that the landfill has been designed to the required high quality standard of construction, and grant the waste management licence, effectively allowing the site to open and start accepting waste materials.

Each CQA programme is specific to the site and the detailed design adopted. It must reflect the  unique requirements of the particular liner installation.

The monitoring and tests to be carried out during construction should be appropriate to the materials chosen, and be focussed on the essential requirements for ensuring compliance with the specification, primarily ensuring that re barrier is as low in permeability in use as intended when designed.

However, the importance of liner integrity (or liner failure) at each site will vary according to the results of the site Hydrogeological Risk Assessment (HRA). So, you should base the most detailed checking on the results of the HRA just as the liner design itself will have been chosen to comply with the degree of engineered containment required by the HRA.

So, now that we have explained how site-specific variations can be very important and may change the CQA plan a lot, we shall describe the requirements for a typical CQA programme.

A typical CQA programme is  likely to necessarily include the following stages:

• Activities Before Construction, including liaison with the design engineer, a constructability review, preparation of a geomembrane construction specification and a pre-construction meeting with the installation contractor
• Construction Period activities, including monitoring of geosynthetic materials, subgrade, sampling, testing and repairs
• Post-construction activities, including provision of detailed as-built drawings and CQA report.

The stages listed above, before construction are examined in greater detail in the sections which now follow:

Activities Before Construction

The early involvement of the consideration of CQA and availability of materials/constructability within the design/build process is invaluable in ensuring that the installation of the design can be carried out without unnecessary difficulty.

The designer must check that construction can be achieved without compromising design requirements. For example, a clay cap would not be buildable in a part of the world where suitable clay was not available, and there are surprisingly many areas where this is the case.

Construction must also be devised to a programme and working methods to include only those geosynthetic configurations which can be properly monitored within the CQA programme.

The main stages of such a CQA programme are typically:

Constructability Review

A review by the CQA engineer to verify that the design methods and construction techniques chosen can be properly constructed and adequately monitored.  This stage of the CQA process will typically pay special attention to critical aspects of the design where deficiencies are most likely and the liner (barrier) would be most vulnerable, e.g. liner penetrations, pumping sumps etc.

Geosynthetic CQA Plan

The the CQA engineer prepares this document. In it are set out in detail the tasks of the CQA programme and the essential records and other outputs to be generated by it.  Among its other uses, this document is typically used to demonstrate to the local environmental regulating authority the scope and level of CQA to be adopted in order to give them confidence that all necessary checks will have been undertaken.

Geosynthetic Construction Specification

A crucial element in the CQA programme which sets out requirements for both the materials and the workmanship involved in the liner construction. To provide the specification the design engineer will have carried out a detailed selection exercise during which he will have identified the most suitable material for the liner. Once identified these will be worked up in more detail as his detailed requirements for the chosen material in the specification.

Now that the material has been chosen and described, the minimum requirements for fabrication of the selected material into a liner, and the programme of checks must be stated by the engineer. The checks will be devised in a way that ensure that the minimum requirements will complied with.

The key to a really good design will be the extent of close co-operation between the design engineer and the CQA engineer. Both must work together to produce a practical, workable specification.

Pre-construction Meeting

The idea of this meeting is that it can be a valuable opportunity for the design engineer, CQA engineer and geosynthetic installation contractor to verify that all parties have the same understanding of the specification. If there are any misconceptions found between the members of the team it is important that these are all ironed out and resolved before the work begins and membrane materials arrive on site.

This is as much as we can provide in this article, however, all the aspects of landfill barrier Construction Quality Assurance discussed above must then be followed through at a level of detail equivalent to that seen already, and once the work is complete a compliance report is prepared by the Engineer, which is sent to the environmental regulator for them to grant the permit to work/open the landfill.