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.

In theory, there are 2 main kinds of modern landfills, the quarry-type below ground and the land raising-type. In the quarry or pit type the seeping groundwater and/or leachate must be piped to vertical shafts which are placed in the waste to keep the landfill dry.

Bauku telescopic well schematic

Circular pre-cast concrete wells can be specified which are raised with the waste with each “cell lift” but they are stiff and unable to withstand the inevitable movement of the waste as it settles around it.

So, what very often happens is that the shaft “shears” and the rings are pushed off-centre. As soon as this happens the necessary occasional man access need to maintain such wells has to cease for safety reasons, sooner or later the a pump becomes stuck in the well or it becomes silted, and maybe due to concerns about landfill gas intrusion from damaged joints during maintenance – it cannot be cleaned.

The sad fact follows that damaging effects on the shafts, usually due to the settlement in the waste commonly of up to almost 40 percent by depth, destroy almost every other type of shaft construction.

Therefore the German company Bauku developed the supposed telescopic shaft about twenty years back. Here the individual shaft elements are stacked above each other flexibly and can move with the waste as it settles.

Generally the shafts have a diameter of 2000 mm or more as maintenance must be carried out in the shafts. This is done by lowering breathing apparatus equipped specialist access contractor’s operatives down the shafts on wire ropes.

At the base of the shaft there are pumps for the transport of the seeping water as well as the entrances to the leachate pipes laid on the landfill liner, which need to be cleaned at regular intervals.

The Bauku telescopic shafts permit construction heights of almost one hundred m in the waste mass.

The dump site Merchernich at Cologne is a reference case to exemplify a very tough installation project.

These days, as Bauku state in their web site, HDPE “PROFILEEN telescopic shafts are found in all pit-type disposals of the Federal Republic Germany and with the adoption of the European (EU) standards and Construction Quality Assurance guidances more neighbouring nations are also thinking about this cutting edge product when planning their waste disposal (landfill) sites.

Telescopic shafts may also be integrated into the existing landfills later so that the standard of the many old rubbish heap sites can be improved significantly. Bauku also inform us at their web site, that such projects were carried out by us on a large scale in Britain in the last few years.

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.

Landfills nowadays each contain huge amounts of organic materials and hold a huge potential to pollute the local groundwater for generations in the containment systems upon which their design is based fail to function as intended.

The engineering of a landfill is no different to other engineered structures, in fact in many ways, especially due to its pollution potential it may be more important that it does not fail when compared to some other structures.

Landfill base liners are by nature buried once constructed and the opportunity to do repairs is extremely limited. Also, other structures may show visible signs of for example leakage, whereas a landfill may leak underground undetected for a long while until the damage is realised and by then there may be a substantial pollution plume already on its way underground to flow out into a river, or pollute a well or drinking water borehole.

The lining of a landfill is the foundation of a major civil engineering structure. If you think of a foundation of a tall building and how importantly engineers view the correct design of the piling for the foundations, you should then think of a landfill lining as equally if not more important.

Just as for the foundation of a multi-storey building great care is taken throughout the construction, the Engineer in charge of a landfill construction would be negligent if he did not require adequate checks to be made on all aspects throughout the design and installation of a landfill liner (or capping).

Carrying out all the necessary checking that the design is implemented and results in a properly built liner (or cap) in a methodical manner and without omissions and then to be able to show others subsequently that the quality of the materials used and the way they were placed will make a proper lining which is as the designer intended everywhere it is laid, is called Landfill Construction Quality Assurance (CQA).

CQA can only be applied once a competent design engineer has completed a design process which has resulted in a detailed specification for the materials to be used, and the thicknesses, depths and positions etc, of these materials when they are used.

This is what is called landfill geomembrane CQA, and it is normally carried out under the overall supervision of a client or purchaser’s professional representative (eg “Engineer”) who appoints an experienced CQA Engineer to carry out Construction Quality Control (CQC). The role of the CQC is the checker of the checker/tester which is usually the construction Contractor, assisted by an expert subcontracted testing laboratory.

The CQA Supervisor is best appointed to someone outside the construction Contractor’s organisation to ensure his/her independence.

Whilst geomembrane materials are relatively impermeable even when compared with low permeability clays, they will transmit a small amount of water even when perfectly installed.

The vapour transmission rates of the geomembrane materials used vary for different fluids, but for water they normally have a permeability in the region of 1×10^-15 m/sec. This sounds like a very low leakage rate, which of course it is, but for the large areas involved at most landfills the end result can be in the tens of cubic metres of leakage every day. This really does not matter in fact because during the design stage the lining designer will have ensured that this leakage will, by natural attenuation and dilution, cause minimal risk to the environment.

It is only if leakage rates increase substantially above this rate that problems will occur.

Unfortunately, if a landfill design is poorly carried out without a great deal of care being paid to construction quality (especially if only one thickness or one type of single barrier will be used), leakage can be hugely increased.

Just think how quickly a bath empties if you inadvertently knock the plug out while bathing!

In the realm of CQA, knocking the plug out without noticing when you did it would be called a lining defect.

It stands to reason therefore that leakage rates through a geomembrane are very significantly increased by the presence of even a few defects, and defects when present must be found and repaired before the job is finished.

In CQA plans in these defects are methodically identified and then as much as possible completely eliminated.

In CQA the defects that are usually identified and which the installer must prevent come from several sources, which typically take the form of:

• Defective geomembrane sheeting

• Defective seams resulting from inadequate seaming methods, or poorly trained installation staff

• Damage to the geomembrane during construction due to inappropriate subgrade materials or from construction plant or careless site personnel

• Damage to the geomembrane after burial due to inappropriate subgrade and/or cover materials, or due to excessive loading.

Does all his intensive CQA checking work?

The answer is yes, but it is never able to consistently always produce a perfect result – what human activity ever is?

It does appear to be worth doing, as US Studies (Giroud JP and Bonaparte R, Leakage through liners constructed with geomembranes, Geotextiles and Geomembranes. Vol 8, pp 27-67. 1989) have shown that the frequency of defects in geomembrane installations can be significantly reduced by the use of rigorous construction quality assurance.

However, what this also means is that unless the surrounding ground around a landfill is known to be a clay which is very impermeable and which itself will retain the leakage, or the surrounding geology comes somewhere close to this ideal, a composite liner (geomembrane (2mm HDPE say), plus a clay liner below it is necessary.

A combination of the best CQA practise and a composite liner will then be considered capable of achieving the intended and very essential protection of the locality from the pollution capability within any modern landfill.

( Article inspired by the paper by D Hall and P Marshall, Golder Associates in The Planning and Engineering of Landfills, Midland Geotechnical Society, 1991, UK)

The above is provided for educational and entertainment purposes only, and the reader must not rely on the content of this article to plan or design a landfill or the CQA measures applied.