European Directives

Arguably, the most important European legislation relevant to landfill containment is embodied within the Council Directive of 17 December 1979 on the protection of groundwater against pollution caused by certain dangerous substances (80/68/EEC) (CEC 1979).

The Annex to this Directive contains two lists of generic as well as specific compounds or substances.

List I contains the most damaging substances and their direct discharge to groundwater, that is without percolation through an unsaturated zone, is prohibited. Indirect discharge of List I substances is permissible only where prior investigation shows the groundwater to be permanently unsuitable for other uses. Direct and indirect discharges of

List II substances may be authorised only after prior investigation and should be limited so as to avoid pollution of the groundwater by those substances (Article 3.2; CEC 1979). List II includes, amongst others, ammoniacal nitrogen (NH3-N), and it must be assumed that Article 3.2 would encompass landfill leachates, since this constituent is singularly the most long lived and potentially problematical gross component of landfill generated liquors.

The severity of the Directive is mitigated by two factors:

Groundwater (Article 1.2(a); CEC 1979) is understood to be water that is below the surface in a zone of saturation, implying that indirect discharge through an unsaturated zone, in which attenuating processes remove the
polluting potential, is acceptable;

An exemption is made (Article 2(b); CEC 1979) for discharges that contain substances in lists I and II in a quantity or concentration so small as to obviate any present or future danger of deterioration in the quality of the receiving
water.

Taken together, these Articles suggest that controlled release of contaminants is permissible, so long as it is by indirect discharge and that attenuation in the unsaturated zone protects the groundwater.

More recently, a proposal for a Council Directive on the landfill of waste (91/C 190/01), submitted to the Commission on 23 April 1991 tightened the provisions of the Groundwater Directive by laying down landfill containment criteria irrespective of site specific conditions and knowledge of attenuation mechanisms.

Annex I (Section of the draft Directive requires that, with the exception of landfills for inert wastes only, the following containment criteria must be met, either by the presence of in situ natural strata, or to the same level of hydraulic containment by engineering measures (synthetic liners or amended soils):

• minimum thickness of 3 metres;
• maximum hydraulic conductivity of 1 x 10^-9 m sec.

For engineered liners in clay the thickness is normally 1 metre, minimum at a maximum hydraulic conductivity of 1 x 10^-9 m sec.

See also http://www.geofabrics.com/docs/UK_Environment_Agency_regulation_15.pdf

Disclaimer: This post is provided for educational purposes only. Readers must not rely on this information for landfill design projects, or any use where reliance is placed on the accuracy of this information. This page may not be updated. ALWAYS refer to the up to regulations issued by the local regulator. Use of this site is prohibited for any use in connection with compliance with regulations.

At the Landfill CQA blog we regularly receive requests for information from people who are developing landfills in industrializing nations where they are still largely unregulated.

They want to know what the essentials are for responsible landfill design and operation, and they have little inclination to read the detailed contents of much which is published in the developed nations guidance as it often seems unachievable within what they can afford to finance.

Also, with so much uncontrolled tipping they will often see themselves as unable to attract much waste anyway, if they are too far ahead of their competitors and have to make more than a fairly nominal charge to pay for environmental measures.

The message of this posting is that it is not in the long-term interests of the industry in these nations for operators to confine themselves to the absolute bare minimum of environmental protection and monitoring, and apply that slavishly everywhere.

The message must get home that unless operators can demonstrate that they are prepared to go further than the bare minimum required to show that their landfill practices are probably OK, where this actually matters they will lose all public trust.

If they are not prepared to go on to give a thorough demonstration that their practices are environmentally sound, then they will lose public support as local awareness of environmental quality develops.

History will then be repeated. Just as happened in the developed world within the last 20 years, they will have lost the trust of the local residents. Once that occurs, it will be difficult for their politicians, in a few years time as their national economies develop, to avoid the very prescriptive and not necessarily always essential, requirements of detailed legislative rules, By rules I mean those such as are in the EU Landfill Directive.

By “industrializing nations” we mean growing economies and rising prosperity, and in these conditions, as wealth grows, so will the environmental awareness of the population. A wealthier public will, in time, come to require the strictest imaginable conditions on landfill operations regardless of the circumstances, if they see the land despoiled by tips.

Draconian measures applied late, once much environmental damage has been done in any locality by poor landfill practices, will not be in the best interests of environmental protection. Politicians and lawyers are not good at making flexible regulations on complex environmental matters, and “one size fits all” usually means much miss-spent money, and effort; which if applied elsewhere could be far more beneficial to the health and welfare of the population.

There are plenty reasons why there should be debate within the industrializing nations about whether it is a better technique one way or another to allow wastes to degrade in highly contained, highly controlled sites, or whether instead the products of degradation should be allowed to be diluted and dispersed into the environment.

There can be some interesting technical debate, once environmental scientists and waste experts apply their minds to his. However, there should not be an unnecessary degree of polarisation on these issues. Some sites will always require full containment, others will not, depending on many local geographic, geological, and climatic factors, to name just a few.

There is no perfect containment site – a containment barrier always leaks to a lesser or greater degree, and will break down in time, and in reality we don’t know with any accuracy how long that time is likely to be.

The objective should be simply to keep the products of wastes in the landfill until they have degraded to a harmless state.

Equally, there is no such thing as a perfect dilute and disperse site. Instead there is a range of possibilities varying from a reasonable degree of containment to a rather low degree of containment, and the best type of containment to use in any particular situation will always depend upon a number of factors. Perhaps, the most important factor is the vulnerability of the surrounding environment.

The consideration of containment in big landfill sites versus less engineered dilute and disperse sites is perhaps the best example of the need for flexibility.

The issues for operators and regulators are not whether it is technically better to contain or technically better to dilute and disperse.

The real point is that whichever method is to be used, it must be demonstrated to be an environmentally sound way of managing the wastes that the facility is designed to cope with.

That means spending money on surveys and monitoring before the landfill site is developed. Quite probably what should be done is considerably in excess of what is currently required for regulatory purposes in most industrializing nations.

It means being prepared to concede that if it is not possible to demonstrate fully that containment is not needed, then containment should be required (while conversely accepting that containment is often not necessary).

This may not be welcome news to operators, but unless they heed this warning, the writing is on the wall.

If waste management operators, anywhere, truly want to build lasting businesses from operating environmentally sound landfills. If they aim for landfills to continue to be used as the low cost option, and the main way of waste management in their countries, then those in charge of landfills had better make sure – starting right now – that they are demonstrating that they are doing it properly.

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.

The often proposed alternative landfill liner system to Low Permeability Clay Layer or HDPE Lining Membrane consists of replacing the default design of compacted clay liner (CCL) with a Geosynthetic Clay Liner (GCL).

There are currently two major types of commercially available GCLs. One type consists of bentonite encased between two geotextile fabrics and the second type consists of bentonite glued to a HDPE geomembrane.

The type of clay typically used in GCLs is sodium bentonite. Sodium bentonite is the name given to the highly plastic clay mineral montmorillonite, with sodium as the primary exchangeable cation.

Bentonites used to fabricate GCLs are processed in an unhydrated state such that they appear to have a granular consistency. However, upon hydration with water, the bentonite swells to form a continuous clay layer.

GCLs are shipped in rolls typically 3.7 to 5.3 meters wide and 25 to 60 meters long. They are installed by unrolling to form panels. Adjacent panels are overlapped, and for some products, powdered bentonite is placed between the panels at overlaps.

Large-scale laboratory testing has shown that, when installed in accordance with the manufacturer’s specifications, GCL overlaps are self-sealing and do not create a preferential pathway for liquid flow.

GCLs have been used in liner systems and cover systems for landfills, surface impoundments, and tank farms, as well as in other structures. When used in landfills, GCLs are often substituted for the compacted low-permeability soil component of a composite liner. The function of the GCL in the composite liner is identical to that of a compacted soil liner, which is to provide a low-permeability barrier to liquid flow through any defect in the overlying geomembrane.

The GCL material is manufactured under strict quality control (QC) guidelines. The QC requirements include conducting index and performance testing on both the supplied materials and finished product at specified frequencies. After the material is approved at the manufacturing plant, care is taken to keep the rolls dry, not stack them too high, and keep them from damage during handling.

Prior to acceptance in the field, information concerning the manufacturer’s name, product name, lot and roll number, and length, width, and weight must be submitted to the on-site CQA representative, who will verify all records.

To analyze the leakage through a composite liner utilizing a GCL instead of a CCL, D’Arcy’s equation is utilized based upon an assumed design hydraulic head over the liner.

The leakage through a membrane liner has been found to be closely correlated with the hole defects. In a recent paper a defect size per acre of 1 cm2 was assumed, however assessments of defects and their likely frequency and size vary widely.

The US Fabricated Geomembrane Institute (FGI) offers its popular short course, “Constructing with Fabricated Geomembranes“, on 23 October 2009 in Lakewood, Colorado at the Sheraton Denver West Hotel. This course will be presented by Timothy D. Stark, Stan Slifer, John Heap, Daren L. Laine, Bill Shehane, Stuart Lange, Andrew Mills, Gary Kolbasuk and other speakers. Course participants are eligible for 6 PDHs.

Those involved with the design, construction, operation and closure of potable water and irrigation ponds, floating covers, canals, landfills, waste water lagoons, secondary containment, golf course ponds, decorative applications, corrective action activities at closed sites, etc. are encouraged to attend this course. Participants will gain a broad knowledge of what is required to properly design, specify and construct with fabricated geomembranes and advantages of fabricated products over rolled geomembranes.

The fabricated geomembrane information will cover manufacturing, formulation, fabrication, shipping, installation, long-term performance, wedge welding, testing of field geomembrane seams, updated ASTM testing of geomembranes, and design and installation of various applications, such as floating covers, canals, decorative and irrigation ponds, and secondary containment.

AGENDA

The 23 October short course will unfold as follows:

7:30 – 8:00 am Registration / Continental Breakfast
8:00 – 8:20 am FGI Introduction, Activities, and Research
8:20 – 8:40 am Fabricated Geomembranes vs. “Rolled Goods”
8:40 – 9:30 am Manufacturing Fabricated Geomembranes
9:30 – 9:45 am Break
9:45 – 11:00 am Fabrication and Installation
11:00 – 11:30 am Leak Location with Fabricated Geomembranes
11:30 am – 12:30 pm Lunch on Your Own
12:30 – 1:30 pm Floating Covers and Potable Water
1:30 – 2:00 pm Canals, Decorative and Irrigation Ponds
2:00 – 2:30 pm Wastewater Ponds
2:30 – 3:00 pm Break
3:00 – 3:30 pm Secondary Containment
3:30 – 4:30 pm Case Histories
4:30 – 5:00 pm Summary and Questions
5:00 pm Geomembrane Welding Demonstration

REGISTRATION

The registration fee for industry professionals is $100 and the course is free for government employees and students. All receive one day of instruction, short course notes, refreshments and lunch. Industry professionals should register by 25 September 2009 to get the best rate. See the online event registration page to start the process: http://fgi.eventbrite.com/

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 need to manage leachate and landfill gas will continue until the wastes contained within a site no longer have the potential to cause problems in their specific location. In fact the pressure to maximize landfill gas and thus improve the amount energy obtained from waste is bound to continue to rise.
This fact is emphasised by the Waste Regulations and Environment Agency Permitting requirements, with landfill operators being legally obliged to introduce and maintain long-term aftercare regimes which, in the case of landfill gas control and leachate management, may have to continue for many decades.

It is not possible to define exactly the point at which wastes can no longer be considered to pose a potential environmental threat. The period of time necessary for a landfill to reach environmental stability is very much related to the nature of the wastes and the rate of the decomposition processes at work within the body of wastes.
In many ways, the development of modern landfilling techniques, and particularly the move towards containment landfills, can tend to slow down rather than speed up the rate of stabilization of the wastes.

It is a recognised fact that the rate of stabilization can be maximised by raising the moisture content of the landfill, but this is hard to do without allowing a zone of saturation to develop within the landfill. This may result in several metres of leachate being allowed to develop above the basal liner.

Such an approach has significant benefits since it is much more likely that stable, methanogenic conditions can be established at an early stage, recirculation of leachate is made easier, and the processes of leachate stabilization and landfill gas production can be better controlled.

However, this approach is in direct conflict with the engineer’s wish and overriding need to protect the liner system by minimizing leachate heads and preventing infiltration. This conflict, which is a real one and not just theoretical, has to be addressed by the landfill industry.

As the industry moves towards the concept of “Bio-reactor” landfills it is essential that the desire to control the complex processes at work within the landfill – particularly in respect of leachate management and gas enhancement – does not ultimately conflict with, and thereby prejudice, the need to maintain the integrity of the engineered structure. This highlights the need for the landfill scientist to work closely with the landfill engineer in order to achieve an acceptable degree of compatibility.

At the end of the day the principal aim should be the protection of the environment. There is no reason why, with careful design, this aim should not be achieved whilst at the same time optimising the benefit to be gained from collecting and harnessing a valuable resource in the form of landfill gas.

Leachate recirculation has tremendous benefits by reducing the strength of the leachate, especially with a very young leachate where the free-of-charge anaerobic digestion it receives in such a landfill as it percolates through the saturated layers is excellent pre-treatment.

Of course to achieve recirculation one actually has to have, if you like, a reservoir to pull on within the base of the landfill and therefore by definition one would have some standing leachate level there to pull on. The other point of course is another, in a sense, problem that is that of the hydraulics of heavily compacted waste at the bottom of a landfill, which is really quite impervious, and actually trying to get water to pass through such waste in a controlled manner is a tough one which neither the industry or its regulators have got to grips with yet.

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…

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.

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.