The most critical components of a landfill cap are the barrier layer and the drainage layer. The barrier (sealing) layer can be low-permeability soil (clay) and/or made from geosynthetic clay liners (GCLs).

A flexible geomembrane liner may also be required and if so, is placed on top of the barrier layer. In addition for stoney sub-soil materials a protection geotextile “blanket” may be needed, over and/or below the flexible geomembrane liner.

The soils used as barrier materials are usually clays that are compacted to a hydraulic conductivity no greater than 1 x 10-9 m/sec for UK landfills generally, but less stringent permeabilities may be justifiable and may be used where acceptable to the Environmental Regulator (UK – EA).

Compacted sealing layers are generally installed in 200mm minimum lifts to achieve a thickness of 600 mm or more.

Many people talk of using a composite capping system. A composite cap/barrier uses both soil (clay usually) and a geomembrane, and making best of the advantages of the properties of each. A geomembrane when installed is essentially if intact so impenetrable by water to be thought of as impermeable, but if it geomembrane barrier develops a leak, the soil component provides a second line of defence and prevents significant leakage into the underlying waste.

The purpose of landfill capping is to shield humans and the environment from the harmful effects of the landfill contents and limit the migration of the contents by reducing inflow of water from the surface, and greatly reducing gas escape. A cap will always restrict surface water infiltration into the contaminated landfill contents to reduce the possibility of contaminants leaching from the site after landfill closure.

A Landfill capping Landfill Capping is the most common form of remediation because it is generally less expensive than other technologies and effectively manages the human and environmental risks connected with a remediation place. That being said, the process of capping is still very expensive

Hydrogeological studies must be carried out to guarantee the drainage of any water that, by running between the membrane and soil mass, can reduce to zero the soil/membranes’ coefficient of friction. If such a situation was present the soil mass overlying the impermeable membrane would become unstable at many landfils subject to slippage in a veneer fashion.

The Geosynthetic material known as “Pozidrain” may be able to provide these functions with higher performance and lower cost than conventional crushed stone filters. Also, Pozidrain is also used by landfill operators who want to gain the revenue from every last once of waste into their landfill before landfill closure. The thickness of this material makes this possible as it is much thinner than a stone layer. This allows more waste to be put in the landfill before the planning consented top of site levels are attained.

Pozidrain has been specially designed to be compatible with both HDPE and clay liners and to give the optimum performance over the whole life of the landfill closure capping. Pozidrain will enhance the performance of the GCL or HDPE liners by providing an additional barrier that prevents the majority of the water or gas reaching the liner.

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…

One of the most common locations for landfills are worked out quarries and quarries suitable for landfill are an increasingly valuable resource for this reason.

In a growing number of cases suitable sites include rare geological exposures of mineral bearing rock, or strata of regional importance which need to be kept exposed after landfilling for educational and also often for historical reasons.  In the United Kingdom these features are identified at planning permission stage and usually allocated Special Scientific Interest (SSI) status.

These SSI’s can result in conflict between conservation and waste disposal interests. The geological feature is usually below the intended restoration level and results in a low point being left in the restoration profile where the SSI is present.

Where quarries used for waste disposal contain Sites of Special Scientific Interest, it is necessary to maintain safe long term access to the geological exposure. The landfill operators will wish to minimise sterilisation of void space for the waste. These objectives can be met by the construction of a structure which limits land take and which maintains a safe barrier to the waste material.

However, it is possible to minimise the conflict and to provide for these geological SSI’s without undue difficulty, as we will describe.

The following list of considerations is broadly based on research described funded by the Nature Conservancy Council in the early 1990s, and has led to the identification of engineering measures designed to optimise landfill void in quarries whilst protecting, in the long term, geological Sites of Special Scientific Interest.

1.    To provide long term, safe, unhindered access to the geological exposure with minimal sterilisation of landfill void space for waste, it is necessary to provide an engineered structure which limits land-take and which maintains a safe and  secure perimeter barrier to the waste material. Long term slope stability must be checked by geotechnical analysis, but it is not the sole design consideration since the access to the exposure must remain drained, be free of leachate and free of significant concentrations of landfill gas.

2.   The presence of a geological exposure in a quarry used as a landfill may have a significant effect on the design and operation of the landfill particularly with respect to leachate management. In some cases in nations where leachate levels are not controlled by landfill site licenses it will be necessary to maintain leachate in the landfill at a much lower level than if a geological Site of Special Scientific Interest was not present.

3.   Natural drainage should be provided where possible to prevent the accumulation of surface water adjacent to the geological exposure. Where this is not possible or the base of the geological exposure is below the water table, pumping may be necessary to facilitate access to the exposure. In such cases it will be necessary to maintain the level of accumulated water below that at which it  will flow into the landfill to prevent the generation of unacceptable volumes of leachate. In addition, it will be necessary to minimise the volume of water which may become contaminated by leachate so rendering it unsuitable for discharge to the surface water system.

4.    It may be necessary to take measures to prevent the movement of leachate from the landfill site through or beneath the waste retaining structure towards the Site of Special Scientific Interest where it  may contaminate accumulating surface and groundwater. The measures may include excavation of the base of the landfill site to a lower level and maintaining leachate below the level of the  base of the geological exposure, reducing the leachate level by pumping and the construction of a low permeability leachate retaining structure keyed into the low permeability materials forming the base of the site.

5.     Landfill gas is flammable, is explosive if ignited in an enclosed space, and can also create an asphyxiating atmosphere. In Europe gas hazard sites (such as landfills) are controlled by the ATEX Directive and national regulations, such as the UK’s Dangerous Substances and Explosive Atmospheres Regulations. Landfill sites in most nations now have gas control systems, and with adequate control it is considered unlikely that landfill gas will accumulate in significant concentrations adjacent to a Site of Special Scientific Interest. However, this may not always be the case and especially if no landfill gas extraction is provided on the site, the area should be monitored for the presence of methane and carbon dioxide prior to access, and ATEX Rules applied as appropriate.

6.   Where the landfill perimeter slopes adjacent to the geological exposure are engineered and graded to a profile of less than 1:3 access by visitors on foot across mown ground should present no significant problems if all visitors wear suitable footwear. Where steeper landfill perimeter slopes are designed an engineered access route may be necessary in the form of a graded path across the landfill or the geological exposure or a purpose made staircase from original ground level to the floor of the exposure.

However, if the above criteria are met, there is no reason why a geological SSI and a landfill cannot co-exist without a significant conflict of interest.

Although the following topic is not directly a landfill CQA issue we thought that it would be of real interest to many of our readers.

Compulsory site waste management plans (SWMPs) for construction projects over £300,000 in value, have been a legal requirement in England since April 2008, and are therefore needed for landfill works contracts. The intention is that the plans will forecast all waste produced on site and how much will be recovered or disposed of.

The SWMP should help businesses manage construction waste as part of a project rather than an afterthought. By planning for construction waste much earlier than has been the practice it should be possible to do much better in managing construction waste. Early experience that has been reported has suggested that in building projects it has been possible to achieve substantial savings which exceed the costs of producing and updating the site waste management plan.

A landfill site lining development or surface restoration project is undeniably a construction project, and almost all will exceed the threshold cost. Therefore, each project will need a site waste management plan, which starts with the designer and becomes the responsibility of the principal contractors on commencement of the work on site.

The regulations set out a range of offences relating to the failure to produce or implement a plan, punishable by a fine of up to £50,000 on summary conviction, or an unlimited fine on conviction on indictment.

On these landfill projects there will be little if any construction waste produced at all, apart from possibly some spoilt or unsuitable material, and the usual site facilities wastes. As ever, these otherwise laudable regulations make little sense in some applications, and in landfill in our view, we have such an example.

So how does a landfill contractor set about writing his site waste management plan after being awarded the contract and receiving the client’s design stage plan, for a landfill project?

What should the SWMP contain?

According to NetRegs, the level of detail that your SWMP should contain depends on the estimated build cost, excluding VAT. Their summary suggests the following:-

For projects estimated at between £300,000 and £500,000 (excluding VAT) the SWMP should contain details of the:

• types of waste removed from the site
• identity of the person who removed the waste
• site that the waste is taken to.

For projects estimated at over £500,000 (excluding VAT) the SWMP should contain details of the:

• types of waste removed from the site
• identity of the person who removed the waste and their waste carrier registration number
• a description of the waste
• site that the waste was taken to
• environmental permit or exemption held by the site where the material is taken.

At the end of the project, you must review the plan and record the reasons for any differences between the plan and what actually happened.

The contractor, or anyone else exporting waste from the site must still comply with the duty of care for waste.  However, because it will now be necessary to record all waste movements in one document, having a SWMP will help the site contractor’s management to ensure they comply with the duty of care.

For help in preparing your SWMP see the video on the Site Waste Management Plan hub page, or  Landfill Site Waste Management Plan page.

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.)

This summer was the second summer season, and the 4th stage of restoration capping at this Wexford County Council landfill during which Rowe Environmental provided capping construction quality assurance services.

The start of the works in June 2007 coincided with the uniqely heavy rainfall exprienced throughout the region that month. Although clay capping proceeded in between rain storms, problems arose with clay compaction due to the very waterlogged conditions on the landfill.

Civil Engineering Contractor Roadbridge used every opportunity to work whenever the site dried out in between rain showers, and the project was remarkable for not being not seriously delayed. This was despite the need for additional harrowing and recompaction of the clay over much of the site, in order to reduce the water content to acceptable levels, and the un-programmed addition of extra soils reinforcement to steep slopes within the restoration area.

The final stage of restoration capping at Killurin is programmed for next year when the recycling facilities and replacement residual waste landfill at the Council’s new Holmestown Landfill will be operational, and the Killurin Landfill closed.

The restoration capping works we completed this year
were similar to the Stage 3 of restoration which took place
through the summer of 2006.
Click on the image below to view
the Landfill Restoration slideshow for 2006:

 

The operator of a modern landfill site in responding to the requirements for minimising leachate generation is likely to fill the site rapidly 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. This means that even the oldest waste at the bottom is therefore likely to be in only the early stages of degradation.

Significant settlement can then be expected 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, graded compaction through the waste. This mechanism for settlement is likely to be predominant during filling and immediately following capping.

ii. The degradation process will break down waste into a denser material.

iii. The production of gas will mean a net mass loss of possibly 18% assuming 150m3/hr of landfill gas at 1.15kg/m3 is abstracted from each tonne of waste.

iv. Removal of leachate from lower levels of waste can also cause further settlement as pore pressures are reduced.

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

The CQA Engineer will verify the degree of settlement on-site through the contractor’s survey etc and ensure that the landfill design Engineer’s requirements are implemented during construction of the restoration capping.

We will occasionally post capping design information on this blog. We hope that you will find these posts interesting.

To guarantee a landfill cap which will drain adequately and for effective gas collection, the landfill cap should incorporate a drainage layer above and preferably also, a gas collection layer below the cap’s low permeability lining system. 

The geosyntheic “Pozidrain” may be able to provide these functions with higher performance and lower cost than conventional crushed stone filters. The Pozidrain is also favoured by operators who want to squeeze the last once of waste into their landfill as the thickness of this material is much less than a stone layer, allowing more waste to be put in the landfill before the planning consented top of site levels are reached.

ABG Geosynthetics tell us that:

Pozidrain has been specially designed to be compatible with both HDPE and clay liners and to give the optimum performance over the whole life of the landfill capping. Pozidrain will enhance the performance of the GCL or HDPE liners by providing an additional barrier that prevents the majority of the water or gas reaching the liner. Pozidrain geocomposite drainage layer has a proven record in landfill capping.

If you are still wondering what Pozidrain material is, think of a very big area of a membrane material comprising many “egg boxes” joined together. Across the face remote from the low permeability landfill seal is “tack” welded a geotextile filter material. The egg box profile creates a continuous void which allows water penetrating the filter to drain away. A very simple but useful idea.

The Landfill CQA technician/supervisor must at all times ensure that the Pozidrain supplied meets the specified requirements, and is laid in accordance with the specification. This is likely to involve the checking, and acceptance of the contractor-provided supplier CQA records, plus some additional independent testing for verification of factory test data.