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An electrical integrity survey was required as the final stage of CQA on the geomembrane/geonet/geomembrane lining system in the large basin shown in Figure 1 before the facility was filled with water. The survey was required almost to the top of the 2 m high concrete wall at the tops of the 1:1 slopes.

Figure 1 Double lined basin with 1:1 slopes and vertical wall.

In such lining systems the geonet leak detection layer is usually backfilled with water to produce the required conductive medium between any leak and wherever the current return electrode can be placed in the water – usually down a side slope riser pipe into the secondary sump. Clearly it would be impossible to backfill the complete leak detection layer in this facility.

To resolve this problem a new electrically conductive nonwoven PP geotextile had been installed between the primary geomembrane and the geonet. The geotextile was accessed between the two geomembranes at the outside of the peripheral wall. The liner was not to be battened to the wall until the basin was almost filled with water and all corner-bridging removed.

The electrode of a simple 48 VDC power supply was attached to the geotextile (Figure 2).

Figure 2 Wire from power supply attached to plate clamped to conductive geotextile on outside of peripheral wall.

This one attachment location allowed a wading survey to be successfully performed in a small amount of water at the low end of the basin (Figure 3) and a water lance survey on the remaining exposed liner.

Figure 3 Leak at sandbag in standing water at low end of basin.

There was ample current flow through the geotextile to enable very small holes ( > 1 mm) to be found by the water lance technique (Figure 4).

Figure 4 Small leak (arrowed) found with water lance.

The owner tried several times to generate small holes in wet and dry surfaces to foil the system, but without success. At one location his slight surface damage generated no signal until it was ensured that the damage actually penetrated the geomembrane, whereupon the signal was quite significant.

Seven leak indication signals were found, from a 1 mm pinhole to a 40 mm long grinding gouge penetration along the edge of an extrusion weld bead. All were repaired.

Sensor IEP installed their DDS system of underliner electrodes in this facility (a very critical high profile project) and also performed a spark test survey on the primary geomembrane, again only possible due to the incorporation of the conductive geotextile. The wading and water lance surveys were performed by I-CORP for the owner independently of Sensor IEP.

The liner has shown no leakage during operation.

The Sensom conductive geotextile, made in France, is available in weights of 240, 360, 500, and 800 g/m2. Clearly, it is a candidate for bonding to geonet to make a conductive geocomposite drainage layer, and it would significantly increase the electrical conductivity of a GCL in composite liners. It may also have a role in accelerating dewatering when used as the filter of a prefabricated vertical drain (PVD).

For further information contact Ian Peggs at I-CORP INTERNATIONAL.