To fulfil its needed function a gas evidence membrane must have both a reduced permeability to landfill gases and sufficient strength characteristics to prevent tearing or puncturing during laying and subsequent construction activities. Secondary criteria influencing the choice of membrane are cost and ease of handling.

Factors that determine the permeability and strength of the membrane are the thickness and sort of material employed for its manufacture.

All design ought to be carried out with due consultation with government environmental regulating officers. For that reason, the following might not be suitable for your use.

Perhaps the most commonly recommended membrane is a 2.0mm thick High Density Polyethylene (HDPE). However, a 1.5mm really Lower Density Polyethylene (VLDPE) membrane has comparable puncture and tear resistance and although its gas permeability is higher it’s still very small and consequently acceptable. The VLDPE membrane has the advantage of getting slightly cheaper and much more effortless to handle in confined spaces and to lay more than irregular shapes.

Either kind is preferable to the use of PVC which doesn’t possess the essential criteria and, despite getting inexpensive, will need to not be utilized as a gas proof membrane. Gas resistant aluminium membranes can be utilized in buildings above the floor slab, but will require a screed or protection boards laid over the top to ensure protection.

It’s futile to bear the additional costs of installing a gas proof geomembrane if this membrane is incomplete or punctured. For that reason, the membrane will need to be integral with the damp proof course which must also have low gas permeability and all providers should ideally enter the constructing above the membrane.

Each effort should be made to design and style the building so that toilets and kitchens etc. are located adjacent to outside walls to ensure drains can pass directly via the walls and any manholes, rodding eyes etc. is usually positioned outside the foundation slab area.

The drains themselves need to be gas tight and also the layout should make sure that any subsequent ground settlement doesn’t open any joints in the drain pipes or pipe connections. Where providers puncturing the membrane cannot be avoided then these will need to be suitably sealed or puddle flanged.

Provision ought to also be made for service trenches and service ducts to be vented or sealed prior to the services enter the developing. The annulus within a duct has to not act as a conduit which will otherwise permit gases to enter the building structure.

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.

Inferior geocomposite drainage layers threaten landfill slips

Whether or not due to recessionary pressures on profits for contractors, or inexperienced contractors bidding outside their normal expertise and winning landfill/geo-engineering work, environmental experts ABG are reporting that inappropriate separation layers are increasingly being offered in drainage layer geotextiles.

These inferior materials crush, or simply bend under the normal soil loading and the drainage path between the underside of landfill capping sub-soils, and the low permeability capping layer which these drainage geotextile composites are intended to provide becomes non-existent.

The very real concern is that if these defective materials are accepted for use in the works, slip failures on the restored landfill surfaces will be inevitable during wet weather conditions. Water will build up on the layer between the top of the capping layer and the sub-soil creating a slip plane, and eventual failure.

The remediation costs after such slips, and disruption to use of the land, caused are to be avoided at all cost. Contractors and Designers and Site Engineers accepting geotextile drainage materials which subsequently block when the drainage path void becomes flattened and filled with soil, could also quite possibly be sued for negligence after such slip failures.

And yet, use of such materials is easily avoided by carrying out a simple test which can be carried out in less than 60 seconds on a small sample of any drainage geotextile composite offered. It is done by squeezing in the hand a sample (geomembrane, protection layer and the drainage stone (equivalent) layer) of the material between two resilient rubber pads to imitate the soft pressure exerted by the soil.

Inspection of the extent to which compression of the separation layer can be seen to occur is a good indication of their capability. Low performance of geocomposite drainage layers is due to combinations of drainage core compression and textile intrusion into the drainage core. Some products on offer will compress visibly to the point that the drainage void space can be seen to have been greatly reduced, and some very inferior samples show almost complete loss of open drainage voids.

Other more rigorous tests should also be considered appropriate to the application of these materials, but by use of this simple action alone the worst performing products would be discounted.

Goran Erak, Business Development Director for ABG, Environmental Geosynthetics and producers of the original Pozidrain product is very concerned about the loss of reputation of drainage geo-composites posed to the landfill remediation and restoration industry by the use of inferior products. He gave my company a set of rubber pads to use when we are offered these materials, plus a sample of their Pozidrain product, which shows no such problems.

Goran was also keen to point out that reliance on the supplier’s data on plate compression testing could also bring problems unless the supplier/manufacturer’s test protocol was checked in detail. Test results offered by some suppliers had been found to show compliance for stiff steel plate tests, whereas soft pads would give an entirely different and more accurate reflection of soil conditions in-situ. It is the requirement that standard flow capacity test must be carried out with soft platens, so any use of hard platens is a non standard test.

The following applies to the typical RCRA Subtitle C Landfill Cap System

Landfill Capping is the most widespread type of remediation since it is in general less pricey than other technologies and actually manages the human being and environmental risks allied with a remediation site.

Landfill caps can be used to:

* Reduce exposure on the surface of the rubbish facility.
* Avert vertical infiltration of water into wastes that would create contaminated leachate.
* Contain waste while treatment is being applied.
* Manage gas emissions from underlying garbage.
* Generate a terrain surface that can maintain plants and/or be used for additional purposes.

The plan of landfill caps is location specific and depends resting on the proposed functions of the system. Landfill Caps can range from a one-layer system of vegetated soil to a multifaceted multi-stratum technique of soils and geosynthetics. In general, less complicated systems are necessary in arid climates and more intricate systems are essential in damp climates. The fabric used during the assembly of landfill caps involve low-permeability and high-permeability soils and low-permeability geosynthetic products. The low-permeability materials reroute water and preclude its path into the rubbish. The high permeability materials move water away that percolates into the cap. Further materials could be used to increase slope stability.

The most significant components of a landfill cap are the barrier layer and the drainage layer. The barrier layer can be low-permeability soil (clay) and/or geosynthetic clay liners (GCLs). A flexible geomembrane liner is placed on top of the barrier layer. Geomembranes are usually supplied in large rolls and are available in several thickness (20 to 140 mil), widths (15 to 100 ft), and lengths (180 to 840 ft). The candidate list of polymers commonly used is lengthy, which includes polyvinyl chloride (PVC), polyethylenes of various densities, reinforced chlorosulfonated polyethylene (CSPE-R), polypropylene, ethylene interpolymer alloy (EIA), and many newcomers. Soils used as barrier materials generally are clays that are compacted to a hydraulic conductivity no greater than 1 x 10-6 cm/sec. Compacted soil barriers are generally installed in 6-inch minimum lifts to achieve a thickness of 2 feet or more. A composite barrier uses both soil and a geomembrane, taking advantage of the properties of each. The geomembrane is fundamentally impermeable, but, if it develops a leak, the soil component prevents significant leakage into the underlying waste.

For facilities on top of putrescible wastes, the collection and control of methane and carbon dioxide, potent greenhouse gases, must be part of facility design and operation.

More…

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.

Dense Asphaltic Concrete can be an extremely versatile product, suitable for many types of applications.

The ancient civilizations of Egypt, Babylon and Assyria all used bituminous lining materials and mortars for waterproofing and building, some more than 5,000 years ago, and many examples of their work remains intact even today.

More recently with the advances in hydraulic technology, asphalt has been shown to be an effective material for sealing Dams, Reservoirs, Canals, Water Catchments, Sea Defences, Coastal Groins, River banks and importantly for us - Landfill Sites.

WALO is a main UK supplier.

Current landfill site practice in response to the requirements of the Waste Regulations (UK) is to minimise leachate generation. To achieve this the operator fills the site, in a series of phases or cells which are raised as rapidly as input rates will allow to the top of the landfill, within small constrained areas, to reduce rainfall ingress.

When the operational cell is complete to restoration levels it is likely to be capped to prevent further ingress of rain water. Even the oldest waste at the bottom is therefore likely to be quite young and very little of degradation will have taken place to most of the waste.

So, under these circumstances significant settlement can then be expected, no matter how well the waste is compacted by the action of the site “compactor vehicles” (wheeled or sometimes tracked front-end shovels – often called “buldozers” by the public).

Settlement occurs due to the following mechanisms:-

i.The load on waste in the lower levels imposed by waste above it, particularly for deep sites, will be several times greater than that imposed during the initial compaction process using mechanical compactors. This will result in continued, physical compression compaction throughout the waste. This mechanism for settlement is likely to be predominant during filling and immediately following capping.
ii.The degradation process breaking down waste into a denser material.
iii.Volume reduction due to volatilisation (carbon emission in landfill gas etc). The production of gas will mean in very approximate terms a net mass loss of possibly 18% in total waste mass assuming 150m3 of landfill gas is in time extracted from each tonne of waste at 1.15kg/m3.
iv. Removal of leachate from lower levels of waste may also cause further settlement which is probably due to the pore water pressures being reduced.

Settlement is therefore predictable and must be catered for in the design of the gas abstraction system. It is the job of the landfill gas Engineer to assess the site “condition” and determine the potential for further settlement so that he can be satisfied a suitable design is proposed.

Determination of settlement rates and possible leachate levels is particularly pertinent to new sites and are important design parameters for the landfill gas Engineer to determine before he starts his system design.

If you need your landfill modelled for settlement, and settlement prediction provided, this is a service we provide regularly through our associated consultants. Just Contact us by email at news [at] landfillcqa.co.uk (Please replace [at] with @ ).

The use of a Geosynthetic Clay Liner (GCL) as the low permeability layer in landfill lining systems and restoration caps can provide a cost effective and readily Construction Quality Controlled alternative to natural clay. In fact this material may be the only option in regions where the local geology is such that no clay of suitable quality is available.

A GCL uses Sodium Bentonite, a dehydrated clay that has long been recognised as an ideal impermeable barrier material, and which will expand once in the soil to six or more times its initial volume.
Claymat is an example of this type of product which comprises a sandwich of bentonite between two layers of geotextile. The result is a thin, flexible (it arrives in rolls, clean, easily transported and installed) lining system. Finesse Claymat is as manufactured by ABG of Meltham, and there are a number of other manufacturers.

Layers of sand or fine gravel (150 to 300 mm) are often placed on top and/or below the GCL as specified by the landfill capping system designer to protect it from damage during installation or thereafter.

The GCL membranes on the market have been tested to provide both an excellent self-healing capability, and chemical resistance, and are accepted by most environmental regulators. A paper available from Thomas Telford Journals shows that permeability can be compromised if suitable treatment is not applied at overlaps, so clearly the CQA Engineer will need to take care about ensuring good site procedures on this matter. (See “Forensic analysis of excessive leakage from lagoons lined with a composite GCL”, At Thomas Telford Journals.)

Assessments of the environmental protection afforded by a landfill liner require that all underlying soil and geosynthetics components are considered in landfill contaminant migration assessments. The results described in a 2004 paper provides published data for laboratory GCL diffusion and sorption coefficients, required to perform contaminant migration assessments for five VOC contaminants commonly found in municipal solid waste leachate. Assessment of diffusion coefficients and clay-leachate compatibility assessment is also deemed necessary to ensure acceptable long-term performance. In fact GCL membranes were shown to give permeabilities generally significantly lower than those reported in the literature for compacted clay liner materials.

Pozidrain ground water drainage membrane is also often used in conjunction with a Geosynthetic Clay Liner as when laid above the GCL it reduces the hydraulic head and stress on the geomembrane and it also provides additional physical protection against puncture.

A significant number of landfill sites have already utilised the benefits of Geosynthetic Clay Liners and many use Pozidrain and equivalent products within these systems.

SITA Cornwall  News (January 2008) – reports: Improvement in compliance… Landfill

SITA Cornwall’s two landfill sites play an important role in managing the county’s waste. A great deal of work was carried out in 2007 at the landfills and even more is planned for 2008.

It was a busy year in terms of engineering at United Mines Landfill in 2007, with the construction of a new landfill cell, the diversion of a sewer and surface water system and the capping of an area of 600m2. The leachate plants at both United Mines and Connon Bridge sites were refurbished which assisted the significant reduction of leachate levels.

Local residents and councillors have joined a community liaison group at United Mines to discuss issues around the site and preparations have begun for an open day in spring.

The aim to cap a large area of United Mines and Connon Bridge confirms that major landfill construction will continue in 2008.

2008 will also see the temporary closure of the Connon Bridge Landfill site. Work has already commenced to improve the visual impact, increase gas and odour control and further reduce leachate generation.

Efforts will also be concentrated on reducing the sites’ impacts both on the local and global environment, with the development of a strategy to reduce the sites’ water and energy use.

(Clearly there will be plenty of Landfill CQA work in the region this year – Ed.)