Bare Liner Integrity Surveys
Bare Liner Integrity Surveys are widely recognized as critically important factors in overall construction quality assurance (CQA) management for geosynthetic lined containment facilities. Construction Quality Assurance is a planned system of activities that provides stakeholders reassurance that structures were installed correctly and within the regulatory guidelines of local and regional governments. In the case of geosynthetic lined containment systems, regulatory bodies require adherence to specific leak rates for continued operations of these types of systems.
Bare Liner Integrity Surveys have become an important process in the CQA of geomembrane lined systems…
While careful quality control during construction of geosynthetic lined systems can greatly reduce leaks it will, unfortunately, not eliminate them. As a result, Bare Liner Integrity Surveys have become an important process in the CQA of geomembrane lined systems. Also known as Electric Leak Location Surveys & Electric Leak Detection Surveys, they are routinely performed on geosynthetic lined containments structures following construction and during usage. These resistivity surveys are performed on bare, soil covered, and water covered liners. Currently, electrical leak location survey methods are one of the only reliable ways to detect and find breaches in geomembrane liners during construction, and when the facility becomes operational.
The principle of electric leak location is straightforward: an induced electric current is generated above the liner and grounded to the subgrade, because the geomembrane acts as an isolated resistive sheet, current will flow more readily in the presence of a breach through the geomembrane. Thus, when a localized current high is located, it is typically associated with increased current density traveling through a hole in the geomembrane. While the principal for all electric leak location is the same, how the methodology is applied changes based on whether the geomembrane is exposed, covered, and how the system is constructed.
Bare Liner Survey Methods
Liner Integrity Surveys are routinely performed on bare geomembranes as part of the CQA process. Bare geomembrane survey methods can be extremely sensitive, reliable, and can easily pinpoint leaks. There are currently four different, but related survey methodologies, for exposed geomembranes: Water Puddle, Water Lance, Spark Testing, and Arc Testing. Determining which method, or methods, should be used is highly important as each method has specific site and labor requirements, varying sensitivities, survey time requirements, and limitations.
Water Puddle Method
The ‘Water Puddle Method’ is often the technique of choice for most new liner installations. This method has an average survey speed of 1200 m2 per hour per operator and can be effective in the majority of liner integrity surveys situations. Method limitations include problems with steep side slopes and poor liner substrate contact conditions.
The basic principal of this method uses an induced electric current generated above the liner and grounded to the subgrade. A mobile detection device, using an electrical current measurement system, is then deployed across the geomembrane to detect changes in current associated with leaks in the liner. The geomembrane acts as an isolated resistive sheet and current will only flow readily in the presence of a breach through the geomembrane. Measurements are made using an electronic assembly that produces an audio tone whose frequency is proportional to the flow of current at a leak site. The device (pictured above) consists of a horizontal water spray manifold with nozzles that mist water onto the liner. Puddles form from the sprayed mist and a squeegee device pushes the puddles of water across the liner surface. When water on the liner comes into contact with a hole through the liner the system detects a change in current and the operator is alerted. When a leak is found it is marked for repair.
Water Lance Method
The ‘Water Lance Method’ is typically used on uneven surfaces, extreme slide slopes, vertical walls, and can tolerate slightly wet and dirty sites. This method has an average survey speed of 900m2 per hour per operator and can be effective in the majority of liner integrity surveys situations. Method limitations include problems with poor liner substrate contact conditions, speed of survey, and high water volume requirements.
This method uses the same principle as previously; an induced electric current generated above the liner and grounded to the subgrade. In this case the mobile detection device consists of a lance or wand (tube), specialized ohm meter, pressure nozzle, and hose. A stream of water is charged and pumped out to the surface of the geomembrane. Measurements are made using a mobile electronic assembly attached to the lance which produces an audio tone whose frequency is proportional to the flow of current at a leak site. When the pressurized water comes into contact with a hole through the liner the system detects a change in current and the operator is alerted. When a leak is found it is marked for repair.
Spark Testing Method
The ‘Spark Testing Method’ can only be used on new construction bare geomembranes, requires a conductive-backed geomembrane, and the liner surface must be clean and dry. Its main advantage is that it is highly accurate in detecting ‘Pin-hole’ size leaks. This method has an average survey speed of 500m2 per hour per operator. Method limitations include speed of survey, and will only work on conductive-backed geomembrane.
The Spark Testing Method for electrical leak location uses a high voltage pulsed power supply to charge a capacitor formed by the underlying conductive layer, the non-conductive layer of the geomembrane, and a coupling pad. The area is then swept with a test wand to locate points where the capacitor discharges through a leak. Once the system senses the discharge current, it is converted into an audible alarm. The nonconductive (insulating layer(s)) of the geomembrane act as a dielectric in a capacitor which stores electrical potential across the geomembrane.
Arc Testing Method
The ‘Arc Testing Method’ can only be used on new construction bare geomembranes, requires a semi-conductive layer below geomembrane, and liner surface must be clean and dry. Its main advantage is that it is highly accurate in detecting ‘Pin-hole’ size leaks. This method has an average survey speed of 900m2 per hour per operator.
The Arc Testing Method for electrical leak location is to introduce a high voltage above the geomembrane, which is grounded to the conductive medium underneath the geomembrane. The area is then swept with a test wand to locate points where the current completes the circuit through a leak. A visible electrical arc is formed when the current completes the circuit and the current flow is also converted into an audible alarm.