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.

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 Secudrän® XX
geonet 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.

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.

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.

If the cover layer for the double lined sealing system is a sand with
gravel (gravel portion <30 %), a Secutex® polypropylene (PP)
nonwoven geotextile with a minimum mass per unit area of 400 g/m², is
recommended for the protection layer. For coarser grained soils or
projects with challenging subgrade conditions, mechanically bonded PP
Secutex® protection nonwovens with a mass per unit area of up to 1,200
g/m² may be used.

Due to the aggressive nature of the chemicals above the two-layered
Carbofol® sealing system, mechanically bonded HDPE Depotex® protection
nonwovens are recommended.

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) or Depotex® (HDPE) protection
nonwovens to prevent mechanical damage during the installation and
subsequent operation in the facility. For extremely sensitive
environmental protection projects, high quality double lined
geosynthetic systems with Secudrän® XX geonets as a monitoring layer
from Naue Fasertechnik ensure economical, long term containment.