What to look for (and to look out for!) when designing lining systems for electrical integrity/leak-location surveys.
By Ian D. Peggs
As the need to perform geoelectrical liner integrity (for CQA) and leak-location (problem resolution) surveys becomes more of a regulated requirement, it becomes more necessary to design lining systems so that single and primary liners of double lining systems can be easily surveyed. This requires advanced planning to ensure that effective, adequately sensitive surveys can be done at the time of construction and subsequently during the service period.
There are four major boundary conditions that are required for the performance of effective surveys:
- a conductive medium above the geomembrane
- a conductive medium through the holes in the geomembrane
- a conductive medium immediately under the geomembrane
- no connectivity between conductive media above and under the liner other than through the holes being sought
The last is frequently precluded in single liners by having cover soil in contact with subgrade soil around the periphery of the liner. When applying the required electrical potential across the liner it is necessary to have a complete electrical circuit between the injector electrode placed in the medium above the liner and any hole in the geomembrane, then through the hole itself, and then from the hole through the medium under the liner to wherever the current return electrode is placed. The current return electrode will be placed in the subgrade soil at the periphery of a single liner or in the secondary leachate (LDS) sump of a double lining system.
During a recent Liner Integrity Survey course at TRI Environmental Inc. the participants, after easily finding the test hole, requested that I place another random hole in the liner for them to find. I obliged, while they were at lunch, then they tried to find it. They couldn’t. Even when I guided them toward it they couldn’t. Then I tried and again could not find a signal at the hole!
We excavated the soil and found the hole where it was supposed to be. So why no signal? I poked an irrigation flagstaff through the hole and found a cavity in the subgrade about 25 mm deep. Thus, the hole was suspended above a depression in the subgrade; and there was no conductive medium through the hole in contact with the subgrade. A valuable lesson inadvertently given and learned.
Related features to keep in mind for an effective survey include:
- Survey before placing cover soil over edge of liner (Photo 1).
- Cover all concrete with liner where possible (Photo 2).
- Avoid batten strip fastenings; use cast-in embedment strips instead (Photos 3 & 6).
- Avoid metal pipe penetrations (Photo 4).
- If pipe penetrations are unavoidable, do not put them all together —separate them (Photo 5).
- Plan to cut a channel to expose a single liner at the top of haul roads entering the cell, otherwise the road will appear as a very large leak, consuming the current that should flow through the real leaks.
- Consider installing an isolating flap in cover soil between the crest of slope and anchor trench.
- Plan for wetting the cover soil during the survey.
- Leak flow-rate monitoring wells should be plastic so leaking water is not in electrical contact with surrounding soil.
- Stay away from wells and associated liner penetrations and monitor/pump LDS through side slope riser pipes.
- Install a stainless steel LDS pump retrieval cable so this can also be used to activate the subgrade conductive medium. This will avoid having to push an electrode on a wire down the riser pipe into water in the LDS sump.
- If wells are concrete, the plastic influent pipe should enter some way up from the floor.
- On double-lining systems, cover soil and effluent/influent pipes may be in contact with subgrade soil provided the LDS is totally isolated from cover soil/subgrade.
- Weld primary and secondary geomembranes together in the anchor trench.
- A geocomposite in the LDS between two geomembranes is not conductive. To avoid the need to back fill the LDS with water, use a GCL under the primary geomembrane, use a conductive geotextile on top of the geonet, or use a conductive primary geomembrane ensuring that fusion welding breaks through the conductive layer so there will be no false positives at the edge of the flap, and that there are conductive strips across seams on the underside.
- Backfilling the LDS with water takes time, can be costly, can damage the liner (if lifted), might require additional ballast on the liner, and still will not allow slopes to be surveyed. Conductive sheet and geotextile allow a full survey to be done immediately at any time.
- In large facilities, install current return electrodes under the liner as it is built, perhaps every 500 ft. Expose wires at the anchor trench.
- Remove all sandbags and other impediments to a survey probe being dragged across a pond liner.
- Use nonconductive tether lines for aerators or keep the wires out of the water.
- When using a water lance or the water puddle technique on exposed liner, the water must be retained on the top of the liner. On single liners, the water must not drain off the edge.
- Concrete support columns in reservoirs should be booted all the way up to above water level.
- Avoid underwater metal structures such as gratings.
- If a cushion/protection layer is required, such as under drainage stone or in sumps, use a geotextile rather than a geomembrane. The geotextile will still allow a leak to be closely located, but a geomembrane may provide a signal only along its edge, not above the leak.
There are many other details that could come to mind but the key is to remember the first four basic boundary conditions.
It is also extremely important to recognize that conventional design philosophy is that all single liners leak, so a drip or two coming out of the LDS is not a problem. A maximum allowable or Action Leakage Rate (ALR) should be specified with all liner designs and the LDS designed to handle that leak flow rate without further damage to the lining system.
For landfill bottom liners with a maximum 12 im. (300 mmI leachate head, the ALR is typically 20 gallons per acre per day (gpad) or 200 liters per hectare per day (lphd).
For wastewater treatment plant liners with 6 ft. (2m) of hydraulic head, it is typically 500 gpad (5000 lphd).
In summary, design for the probability of a geoelectric integrity/leak survey. The minimal effort required to do so will pay off handsomely when a survey is necessary, such as when the state requires you to reduce your leak flow rate from 700 lphd (70 gpad) to less than 200 lphd (20 gpad).