Groundwater protection with double liner sealing systems
Double liner sealing systems are installed in industrial facilities where liquids are “stored, filled and transferred” or “produced, treated and used” and where aggressive hazardous liquids could lead to a groundwater contamination if released into the environment. With this system, a control system is installed between two Carbofol® sealing membranes.
If the requirements of the local water authority are more demanding with respect to the installation quality, monitoring capabilities and overall performance of the single lined system, a double lined sealing system may be used to reduce the risks associated with the containment of large amounts of potentially harmful liquids. In double lined sealing system, a monitoring or control layer is installed between two Carbofol® geomembranes.
Sealing
Double lined seals are selected where there are the highest safety requirements. A typical double lined sealing system consists of two – 2 mm thick HDPE Carbofol® geomembranes and an HDPE drainage Secudrain® lying between the two lining components, to act as the monitoring zone. UV-resistant Carbofol® HDPE geomembranes are an excellent choice to fulfill these stringent requirements and offer the added benefits of being resistant to rodents and roots.
Protection
To ensure the long-term quality of the double lined sealing system, mechanically bonded protection nonwovens are recommended above the upper Carbofol® geomembrane. With less than ideal subsoil conditions, it may also be necessary to install a mechanically bonded protection nonwoven beneath the lower Carbofol® geomembrane.
Performance Monitoring
The two-layered sealing system offers the flexibility to be monitored either continuously or periodically.
To enable the location of damage or faults within the liner system, the monitoring zone is typically divided into individual control fields. Each control field has two monitoring modes (one for vacuum checks and a wire for electrical monitoring). The division of the monitoring zones into individual control fields can also help limit the spread of contaminants in the entire monitoring zone by allowing quick and precise damage location.
After completion of the project, the entire monitoring zone is put under a negative pressure of 0.3 to 0.6 bar. The vacuum pump is located in a measuring cabinet that also contains the leakage indicator. A sudden drop in the negative pressure indicates a faulty control field in the system. Repair measures can then be initiated selectively and quickly.
Another monitoring tool is based on electro-resistivity measuring principles, where a unique conductive geotextile is installed in the monitoring zone between two geomembranes. The geotextile, along with a measuring and evaluating matrix-like structure are connected with the local data unit, which collects the resistance distribution within the special nonwoven geotextile.
If the conductive geotextile becomes hydrated due to damage in the overlying geomembrane, the electrical resistance of the geotextile in this area changes. This change in resistance releases an alarm, so that corrective actions can be performed in the specific area of the damage.
Conclusion
In industrial sites, where higher safety and monitoring standards are common, double lined Carbofol® HDPE geomembranes are typically required. The long-term performance and protection of the sealing system is enhanced by using Secutex® (PP) protection nonwovens to prevent mechanical damage during the installation and subsequent operation in the facility.
