TEXTILES.ORG
By Ian D. Peggs
Introduction
A geoelectric integrity survey was requested on a new landfill cell with the following lining system from the top down:
- 18 in. sand
- Geotextile/geonet/geotextile composite (geocomposite)
- Primary geomembrane
- Geosynthetic clay liner (GCL)
- Geomembrane (rub sheet)
- Geocomposite
- Secondary geomembrane
- Prepared subgrade
There was no sand above the primary geomembrane in half of the cell, which was on a slope. There was a berm along the low edge, along with a sand layer below the primary GCL. It was determined that a successful survey could not be performed on this lining system.
Following are the detailed procedures taken to confirm that an effective survey could not be performed. These details, in turn, identify some of the parameters that need to be considered, and actions that need to be implemented, during both the design and construction phases of a lining system to ensure that if a geoelectric survey is required, it can be satisfactorily performed.
For instance, while it makes technical sense to encapsulate a GCL, it may make it impossible to perform an electrical integrity survey because there must be sufficient moisture in the GCL and access to the GCL.
Basis of electrical surveys
The survey technique is based on the assumption that the geomembrane is an electrical insulator. Consequently, the boundary conditions for a successful survey are as follows:
- A conductive medium above the geomembrane
- A conductive medium through the holes being located
- A conductive medium directly underneath the geomembrane
- No electrical connection between the media above and below the geomembrane other than through the holes to be located
An electric potential is applied between a current injector electrode placed in the medium above the geomembrane and a current return (ground) electrode in the medium below the liner. Current flows only through holes in the liner. A dipole (two-electrode) probe is then used to measure the potential gradients on the surface of the overlying medium (sand, in this case) and identifies the steep characteristic gradients associated with a leak.
An analogy is measuring the surface elevation gradients on pond water with a whirlpool at a large leak. Away from the whirlpool (leak), the gradient is essentially zero with a little background “noise” from ripples on the surface. As the dipole probe enters the whirlpool, the gradient increases to a maximum when the leading electrode is directly above the leak (in the center of the whirlpool).
As the survey probe continues to move, the gradient becomes zero when the probe electrodes are equidistant astride the hole. The gradient reaches another maximum, but of the opposite sign to the previous one, when the trailing electrode is over the hole. As the probe climbs out of the whirlpool, the gradient returns to the zero background level. This characteristic up/down/up signal can occur only at a hole. The survey identifies such signals and locates the center of the leak midway between the two peak signals.
Survey procedure and observations
In a double lining system, the current return electrode is usually placed down the side slope riser pipe into the secondary sump where it activates the conductive medium under the primary geomembrane. Thus, this conductive medium must be continuous from the secondary sump to wherever in the primary liner there might be a hole.
Since there was no secondary sump in the new cell, we previously arranged for two plate electrodes to be placed in contact with the GCL during construction of the lining system connected to wires that would exit the primary geomembrane in the anchor trench. The electrodes were placed halfway down the longer west slope ~180 ft. from each end of the ~750 ft.-long cell, shown here in Figure 1.
To calibrate the equipment that would define the spacings of the orthogonal grid pattern used for surveying, a ~0.25-in.-diameter hole was made in the primary geomembrane at a location 250 ft. south—about halfway up the sand-covered area of the west slope, as shown in Figure 2. The origin of the coordinate grid was in the northeast (bottom right) corner of the cell. Damp sand was placed in the calibration hole, a little water was added, the geocomposite was replaced over the hole and wetted, and the sand cover was replaced to reproduce the condition of the original primary liner.
A potential of 250 VDC was applied between an electrode in the sand (see 4 in Figure 2) ~200 ft. to the south of the hole and the previously installed current return electrode to the south (1). A survey traverse was made for +/- 50 ft. in a north/south (left/right) direction directly over the calibration hole, but a characteristic leak signal was not obtained—the signal was constant. The applied potential was increased to 500 V, but still no leak signal was generated.
In both cases, the current registered on the power supply was a very low 1 mA (the lowest scale reading). Normally this would be in the region of 20 to 50 mA. For instance, in a survey on a 3-acre cell at another site during the previous two days the current was 35 mA at 450 VDC. A low current would typically indicate that there are no leaks in the liner, but it should still be possible to clearly “see” the calibration hole.
Discussions with site personnel revealed that the two current return plate electrodes had been installed with different types of cushion protection between the plate and the GCL. The southerly one was cushioned with geocomposite and the northerly plate was cushioned with GCL.
The former could insulate the GCL from the plate precluding the required good electrical contact. To determine whether this was a factor in not “seeing” the calibration hole, the south electrode was disconnected and the connection was made to the north electrode. However, the resulting calibration traverse was again unsuccessful in indicating the leak.
Factors that could lead to the lack of a signal at the calibration hole were considered:
- Poor connections between current return plate electrodes and GCL
- Insufficient conductivity within the GCL along the bentonite layer, which was too dry
- Insufficient conductivity across the geocomposite
The conductivity of the sand was not in question.
To address item 3, a water truck was used to soak the exposed geocomposite along the top edge of the sand so that the water would drain under the sand and wet the geocomposite above the hole. The leak still was not identified.
To address item 1, a long-strip electrode was clamped to the GCL through a hole made in the geomembrane about 50 ft. to the west of the calibration hole (Figure 3).
A strip of the GCL’s upper geotextile was cut and folded back to expose the bentonite powder over an area about 2 in. wide by 8 in. long. Strip electrodes were placed over the bentonite and under the GCL and clamped together at each end of the exposed strip. The assembly was wetted to assure good contact and good local conductivity. With an applied potential of 500 VDC, the current was still indicating 1 mA and the hole still was not seen when surveyed. This implied that the GCL was insufficiently conductive.
To further address this concern (item 2 above), a 500 VDC potential was applied between the strip electrode (5) and the north current return electrode (2) on the GCL, a distance of ~80 ft. Thus, current would flow only through the GCL. The power supply still showed a current of only 1 mA. To ensure that the ammeter on the power supply was functioning correctly, the current was measured with a multimeter on the microamp scale. As should have occurred, the current did increase with applied potential but reached only about 6 μA at 500 VDC. This very low current clearly confirmed that the GCL was insufficiently conductive.
Further discussions revealed that the primary geomembrane over the east berm was constructed with sand underneath the GCL. Therefore, the GCL may have extracted moisture from the sand to make it adequately conductive, to the extent that it may be possible to survey the complete berm geomembrane and the associated pipe penetration boots. This was attempted.
Another calibration hole was placed in the primary geomembrane about 75 ft. to the south of the previous calibration hole (3), and halfway up the west side of the berm, as shown in Figure 4 (6). The GCL below the hole was not wetted and the hole was filled with damp sand. The geocomposite was placed back over the hole but was not wetted. Sand was replaced over the geocomposite and compacted by foot. Thus, the lining system over the calibration hole was in the same condition as the rest of the primary liner over the rest of the berm.
The injector electrode (7 in Figure 4) was placed on the sand cover about 100 ft. to the south of the calibration hole. The current return electrode (8 in Figure 4) was placed in the sand under the geomembrane (through a hole in the geomembrane) adjacent to the injector electrode. At 500 VDC, the current flow remained at the 1 mA reading and the calibration hole again could not be seen. Therefore the GCL had not absorbed sufficient moisture from the underlying sand to make it conductive.
The berm calibration hole (6) was uncovered and water was poured through the hole to wet the GCL. The underside of the geocomposite was wetted, placed over the hole, and the top of the geocomposite was thoroughly wetted. The sand was replaced and foot-compacted. At an applied potential of 500 VDC, the current flow increased to 11 mA. A detailed survey traverse was made for ±10 ft. across the calibration hole.
As shown in Figure 5, the up/down/up characteristic leak signal was obtained, but it was not very large. The signal due to the hole became significant only about 2 ft from the hole. On the positive potential side of the hole, the measured peak voltage (48 mV) barely exceeds three times the background signal (~15 mV) as required by ASTM D7002, “Standard Practice for Electrical Methods for Locating Leaks in Geomembranes Covered with Water or Earth Materials.”
Discussion
The calibration surveys and test measurements showed that the GCL under the primary geomembrane did not have the required conductivity to complete an electrical circuit between a hole in the primary geomembrane and the current return electrode attached to the GCL some distance away from the hole. Even where the GCL was underlaid with sand on the berm, there was insufficient moisture in the bentonite of the GCL to make it conductive. Thus, it was not possible to perform an electrical integrity survey on the primary geomembrane.
As demonstrated, it would be necessary to wet and hydrate the GCL under the geomembrane and to wet the geocomposite above the geomembrane in order to perform an effective survey. The latter can be done by rain or by using a water truck, but the former cannot be done without removing and reinstalling the geomembrane.
While wetting the geocomposite and the liner from above will cause some water to penetrate any leaks, the area of wetted GCL will remain very small (only at the leaks). While this will provide a conductive path through the GCL to the sand, it may or may not be sufficient to provide an adequate signal during a subsequent survey—the current flow is a function of the cross-sectional area of the wetted path. Previous surveys with only a geocomposite between primary and secondary geomembranes have proven that one cannot rely on the leaking water to provide an adequately conductive pathway under the primary geomembrane.
In this case, it certainly will not be adequate in the areas where the liner is not underlain by sand. Therefore, for an effective survey the complete GCL should be uniformly wetted. This is impractical after construction. During construction, however, as has frequently been done, the GCL could be irrigated with about four passes of a water spray just before the geomembrane is placed. Similarly, a new geocomposite primary leachate collection layer can be wetted before a sand cover is placed over it. However, geocomposites are rarely a problem after they have been soaked for some time with leachate in service.
Alternative methods for leak detection
There are few other options for locating leaks in the primary geomembrane. Blowing smoke between the liners and observing where it rises out of the geomembrane has been attempted, but not very successfully—one can never be sure just how far the smoke penetrates under the geomembrane.
Pulling a vacuum on the leak-detection system and using a sensitive microphone above the liner to listen for the noise of air being drawn into a leak has also been done. However, since this cell is connected to adjacent lining systems, the volume to evacuate would be unmanageable.
When water is actively passing through a leak and draining through a mineral layer under the geomembrane, a measurable potential low can be generated at the entrance to the leak flow channel. But the topography of this cell would not allow it to be flooded to generate active leaks. This approach would only work where there is sand under the GCL.
Perhaps the method with the most potential for success in this case would have been to insert a lighter-than-air tracer gas along the secondary leachate collection pipe and to monitor its emission through any leaks in the liner using a sensitive gas analyzer. This is done very effectively on landfill caps with the readily available methane and carbon dioxide gases. It would probably be necessary to pass the tracer gas through a long hose previously inserted in the LDS pipe. The hose would be drawn along the pipe as the survey is performed above the liner and above the location of the end of the hose where the gas is emitted. The holes that are presently in the liner could be used for calibration purposes and to assess the feasibility of such an approach.
Summary
In preparing for a conventional geoelectric-applied potential liner integrity survey as the final stage of liner installation CQA, it was found (as the result of several different calibration attempts) that an effective survey could not be performed on the primary liner. The GCL under the geomembrane was not adequately conductive.
Calibration could be successfully achieved only where the GCL was underlain by sand, and then only by thoroughly wetting the GCL through the hole and by wetting the overlying geocomposite. It would not be practical to wet the complete GCL as would likely be required where it is underlain by sand, and as would be essential where it is placed on the 30-mil-thick “rub” sheet.
This survey demonstrates the need to consider the structural requirements for an effective geoelectric liner-integrity survey during the design and construction phases of the lining system. Plan ahead with the four boundary conditions in mind.