Rock solid Landfill Liner Protection

Landfill disposal will become an increasingly premium disposal option, as regulations tighten to divert waste instead for recycling and energy generation. With limited space available, and the need to ensure absolute containment of toxic leachate, the use of highly durable dense asphaltic concrete as a lining systems looks set to spread.

For any keen gardeners out there, the process of building a garden pond is probably a familiar one. First dig yourself a nice big hole in the ground, line it with an impermeable sheet of material and fill it with water. Pretty soon you'll have your own little aquatic ecosystem of fish, amphibians, insects, invertebrates and a variety of plant life. What could possibly go wrong?

Well let's just hope you installed that liner properly for a start. Preferably with some nicely padded underlay. Forced by tonnes of water to take the shape of the underlying hole you dug, it is vulnerable to punctures from stones and other sharp objects in the ground. It needs to be strong, pliable and properly installed. But even then it has a finite lifespan, over the course of which it will slowly perish.

One day your pond will spring a leak. The water will begin to slowly drain into the ground beneath, leaving you to either keep it topped up with water, or to take on the not inconsiderable task of emptying the pond to replace the liner.

Now let's imagine for a moment that it was not a few thousand litres of innocuous, if somewhat smelly, water that filled the pond in your garden. Instead it is filled with a toxic cocktail of rotten organics, plastics, batteries, paint, tyres, domestic appliances and a million other household items - all of which have been leaching a vast array of chemicals into a poisonous soupy liquid for decades. You're probably wishing you'd spent a little longer choosing and installing a better liner, right?

It may be an unlikely scenario for your garden pond, but on a much, much bigger scale, it is a scenario familiar to landfill owners, operators, designers and builders.

Improving standards

In the developed world, gone are the days when a landfill was simply a hole in the ground. Concerns over groundwater contamination and greenhouse gases have led to tight regulation.

While there are numerous options for lining landfills, including clay and a range of polymer linings such as HDPA, at some sites factors such as the geological characteristics of the ground, or the hazardous nature of the materials being disposed, call for something a little more robust.

Two examples are the Châtillon Landfill, situated in a former gravel pit to the south of the city of Fribourg, Switzerland which accepts fly and bottom ash from waste incineration, and the 38 hectare Westmill site located in a working sand and gravel pit in Hertfordshire, UK. Faced with similar problems in terms of needing to protect the environment with a high quality liner, both sites turned to Swiss Dense Asphaltic Concrete (DAC) specialist, WALO.

Claimed to be completely impermeable and massively strong, DAC has much in common with the tarmac used to pave our roads, and has become a widely accepted solution to landfill engineering in Switzerland and Germany.

According to David Wilson, managing director of WALO UK, in essence a DAC landfill lining system consists of two layers - an asphaltic binder layer and the dense asphaltic concrete. The asphaltic binder is usually placed onto a permeable crushed rock sub grade and is used as a stabilising layer onto which the DAC is laid, usually to a thickness of around 80mm.

In addition to replacing the lining at the Châtillon Landfil the client was looking to increase the available void

"It isn't as impermeable as the DAC and that's really because the DAC is laid at about 180°C to 190°C and if there's any water present, either from precipitation or from the construction process itself, as soon as the dense asphaltic concrete came into contact with the water it could create steam that may create a bubble inside the DAC, which would be a potential weakness. Instead that steam is forced into the binder layer which is porous below the DAC," explains Wilson.

Drawing a comparison with road laying materials, Wilson explains that DAC uses fresh aggregates, sands and fillers, as does normal road asphalt. The difference is that for the sealing asphalt of a landfill liner, the company is looking for slightly different properties. For a road pavement a certain degree of stiffness is required, where as for a landfill liner a certain degree of flexibility is desirable to accommodate any settlement within the landfill cell.

"Most importantly, what we're looking for is impermeability so we tend to put more bitumen into our dense asphaltic concrete," adds Wilson. "It's laid in a very similar way to that of a road pavement. The difference is we have the capability of doing it on relatively steep slopes of up to 1:1.6. We have the capability of laying slopes of slightly in excess of 150 metres."

"We use very large winches," explains Wilson. "A paving machine will be in the region of eight to 10 tonnes in weight, with anything up to six tonnes of asphalt. We're talking about big pieces of equipment designed for paving on very steep slopes."

The big advantage of being able to line such steep sides is that it enables the void available for disposing of waste to be maximised.

But its ability to build tall and steep is not the only advantage WALO's DAC liners offer in terms of exploiting the void. According to Wilson, for a hazardous waste landfill site installing a clay liner, the Landfill Directive can call for anything up to five metres of material above a HDPE layer.

"The reality is that the impermeability of DAC means we can use a standard system, so instead of five metres plus the thickness of the liner system, we would be looking at a total thickness of 340mm. So there is a significant void increase," he adds.

When it comes to withstanding the harsh chemical conditions found within a landfill site, the company says that in terms of its resistance to chemical attack it is necessary to take bitumen's thermoplastic characteristics into consideration. The action of acids with a pH value of <4, or high volumes of solvents, have the potential to increase the temperature of the bitumen slightly, which could be linked to a slight softening of the bitumen's structure.

Biffa's Westmill site is located in a working sand and gravel pit and every year receives up to 350,000 tonnes of municipal waste

However, WALO also notes that the pH of most municipal solid waste landfill leachate is usually in the range of 4.8 to 5.2. It also says that experiments have shown that any softening of a DAC barrier through such an attack would only affect a few mm in the top surface.

Furthermore, according to Wilson while ordinarily there would be a 300mm layer of gravel type material on top of the DAC, to act as a leachate draining system, even if heavy construction equipment where to travel directly on the DAC layer, it is comfortably strong enough to survive unscathed. Something which is a risk for HDPE and other membrane linings.

When asked how DAC might stand up to one of the arch nemeses of landfill sites, fire, Wilson is confident: "The point to remember is that 93% of the DAC lining is made up of aggregates, anything up to 10 to 14mm all the way down to very very fine fillers. But it's graded in such a way that it's so dense that even without the bitumen it gives a permeability of about 10-8, which is pretty good because the Landfill Directive is looking for 10-9 with clay.

"By the time you put the bitumen in you're looking at around 10-13 or 10-14," he adds. "Even if there was a fire, there might be some softening of the bitumen, but it wouldn't come to any harm and experience has shown us that the top one or two mm may or may not get damaged, but the reality is it's 80mm thick. We've never ever in fifty odd years experienced any damage from that sort of incident to a DAC liner."

According to WALO, as with all barrier systems, the cost of lining a cell with DAC depends upon its overall size, base and slope areas, the length and angle of the slope, its overall height and the topography of the site.

Clearly such a robust lining system is not a 'cheap' option, but the company claims that taking everything into account, such as the increased void space created by using a much thinner lining system, the speed of laying, the steepness of sides and the alternative uses that a cell with a DAC lining system can be put to, the price is comparable to other lining products.

Conclusions

The developed world may be moving increasingly away from landfill as a favoured means of disposal, but it is far from leaving it behind altogether. The sites at which we continue to bury our waste are not found in our gardens, but incinerator ash and other toxic wastes have the potential to leach pollutants into the underlying groundwater.

As our understanding of the threats posed by landfills has grown, increasing efforts have been made to ensure that what goes into these holes in the ground stays there. With the volumes of waste being sent to landfill declining in most developed nations, and the cost increasing, in the future there will be less demand for landfill capacity. This will drive higher standards at the sites we do have.

While vastly different in scale, garden pond and landfill site liners need to serve the same purpose: containing liquids and protecting the ground. Lining landfills with highly resilient dense asphaltic concrete may not be a new idea, but as landfill disposal becomes a 'premium option', it may well be something that will become more common.