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.