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Fixing a hole

Detecting and remediating defects in landfill site geomembranes can help avoid larger problems later on. The only difficulty is finding them. Here are a few ideas on how to go about it

Finding a defect as small as a pen nib in an 80,000m2 landfill geomembrane cell is like looking for a needle in a haystack, and relies on some carefully executed testing. But a hole this size could lead to serious contamination of groundwater by leachate.

European environmental legislation is imposing ever tougher compliance standards and operational constraints on UK landfill sites. The EC Landfill Directive 1999/31, effective in the UK since June 2002, aims to increase waste recycling and recovery, and reduce potentially polluting emissions from landfill.
The Groundwater Directive 2006/118/EC has also introduced measures to prevent or limit inputs of pollutants into groundwater. While waste management changes are resulting in less waste being disposed of, at least without some form of pre-treatment, the need for landfill continues.

Thousands of tonnes of controlled waste from UK households, commerce and industry still need to be disposed of in this way. Some waste from sewage sludge is also placed in these sites, along with waste from mining and quarrying.

Landfill waste undergoes a process of biodegradation and stabilisation. Infiltration of rainfall, together with biochemical and physical breakdown, produces a liquid leachate that contains soluble components of waste. This includes undesirable dissolved organic chemicals, ammonia and metals. Leachate is collected in drains at the bottom of the landfill and treated.

The process also generates landfill gas, which can be collected via a pumped venting system to provide a source of energy.

Subject to strict planning and environmental regulations, landfills are commonly located on former quarries or excavations in superficial deposits.

Cell construction in the UK generally involves compacting ground and laying on it bentonite-enhanced sand, which swells to form a protective barrier around any leachate escape.

Site workers then install a geomembrane as the primary leachate barrier, with a 10mm thick woven carpet or geotextile on top to protect it and a 300mm to 500mm thick layer of stone on top. This gives further protection and acts as a drainage layer from which leachate levels may be managed via slotted plastic pipes. The cell slopes towards one end (to a sump) to allow leachate to be extracted.

The geomembrane and geotextile continue up the batters and are pinned into an anchor trench encircling the edge of the cell.

The liner system of a landfill cell is key to managing leachate and also helps to impede landfill gas flow. Escaped leachate into the ground is often difficult and expensive to remediate and may have long-lasting effects.

In the UK, containment of leachate is usually through installation of a low permeability geosynthetic membrane or geomembrane, typically made of high density polyethylene (HDPE).

HDPE geomembranes laid and welded in strips form a continuous barrier. This is done according to Construction Quality Assurance (CQA) procedures to ensure that the incidence of penetrating holes and other defects is kept to an acceptable level.

Conventional testing of welds and seams, such as spark testing, do not locate partially penetrating defects that may develop into leaks. Nor can these be performed after covering the geomembrane with the uppermost soil or drainage layer. Crucially, the physical impact of placing these materials can lead to the most significant damage.

Damage to the geomembrane can be caused by workers walking on the liner, tiny pinholes from staples securing the geotextile to its cardboard inner roll, cigarette burns or poor workmanship. Defects may also result from welding, knives, buckets, boulders from upper parts of the landfill excavation, scavenging animals and vandalism.

They may also be caused by physical impact to the upper stone/drainage layer during traversing by site vehicles, dropping of site equipment, wooden survey or electrode stakes or stone punctures.

Since the early 1990s, electrical leak location (ELL) surveys have been increasingly used in the UK to test geomembranes. Originating in the US, the technique relates to the direct-current electrical resistivity method and uses insulating properties of modern geomembranes to locate holes.

This is carried out either just after construction, when the cover soil or drainage layer has been laid, through one-off mobile testing or by permanent testing with embedded sensors.

The surveys are carried out over grids with uniform sample spacing and detect the presence and location of holes, as well as confirming such holes have been adequately repaired (along with local defects they may have been concealing).

ELL surveys rely on the geomembrane material – usually HDPE or polyvinyl chloride – having a high resistance per unit area compared with that of a leak path.

The resistive geomembrane lies between relatively conductive areas – underlying earth and the internal drainage/soil layers. While exhibiting very high resistivity, the geomembrane must allow a small degree of current flow to support the test rationale. But it will always
be minimal compared with the current flow through a hole.

In a mobile ELL survey, an electrical current – typically about 600V direct current – is applied to the ground between two electrodes, one buried within the material inside the landfill cell and the other buried in the earth outside it. A second set of non-polarising measurement electrodes is used to measure the electrical potential at closely spaced points, usually a distance of 0.5m to 1m.

As long as material within the landfill is electrically isolated from the earth outside it, negligible current should flow between the electrodes unless there is a hole in the geomembrane. In contrast, electrical current flow will increase through any point of leakage, indicating the presence of a hole.Accurate mapping of any changes in electrical potential enables lining splits, welding defects, as well the location of holes as small as 1mm.

ELL also tests liner under load, uncovering imperfections that may otherwise manifest themselves only when subject to service stress.

Historical contamination issues in the UK have seen enforcement of more stringent requirements for managing landfill leachate as well as guidelines on landfill siting, design and construction with geomembranes. But there is no specific legislation to regulate geomembrane testing.

For example, it is mandatory in CQA procedures for borehole construction around a landfill site to keep a check on the leachate ingress. But there is no provision to enforce appropriate standards of geomembrane testing to prevent this happening in the first place.

More guidance is also needed on preparing landfill cells for mobile ELL surveys. This would ideally include effective electrical isolation of the cell and construction of test holes as standard requirements.

For now, the landfill owner/developer must rely on an independent third party to carry out testing and assessment of the geomembrane.
Best practice includes rigorous site assessment to ensure the site is suitable for an ELL survey and that all parameters are in place for successful testing.

A blind test is usually carried out to assess site conditions and the likely success of the liner test. This usually takes the form of a test hole – whose position is not revealed to the site team.

On finding a defect, an assessment determines the response. A satisfactory one would indicate a sufficiently large anomaly was caused by the test hole. Should the response indicate a problem with site conditions, for example, cell isolation or cell moisture content, then such factors would need to be addressed prior to the start of the survey.

Once set-up is correct, a grid of electrical measurements is taken based on sample points at 0.5m to 1m spacings.

When negative and positive readings are found in close proximity – known as a dipole anomaly – testing is halted and the location is dug up to allow confirmation of a defect. The hole is recorded and repaired, which improves cell isolation in looking for further defects. The repaired area is then re-tested to make sure the problem has been successfully remediated.

Fugro Aperio's ELL techniques can confirm defects to within 0.5m, helping to locate pin-sized holes in cell liners, which could exceed 80,000m2. Of the 30 or more sites investigated by the company, defects have been found in virtually all cases. These have ranged from small tears and welding defects to large scale holes.

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