Environmental Engineering Reference
In-Depth Information
On the basis of the above observations it becomes fundamental to know the
service life of the geomembranes in order to optimize the landfill liner design. The
design life of the geomembrane is influenced primarily by the synergic effects of
chemical and physical stresses over an expected period of time [ROW 98]. Primary
considerations are the effect of temperature and tensile stresses over and above the
calculated design value (e.g. stresses induced by wrinkles).
On the basis of experimental data, field monitoring results and direct
observations from literature, Rowe [ROW 08] reached the conclusion that the
service life of a geomembrane is related to the type and amount of antioxidant used,
the presence of stress concentration (e.g. at wrinkles, due to indentation by stones,
etc.) and stress crack resistance. Based on existing data, the service life of a properly
formulated HDPE geomembrane is projected to be in the order of 150 years at
temperatures of around 20°C
(e.g. the primary liner of a municipal waste landfill)
and 350 years at a temperature of about 10°C (e.g. the secondary liners). These
service lives have been predicted assuming, in general, good working conditions in a
well-managed landfill and in particular: i) good design and construction practice; ii)
the specified minimum oxidative time and minimum stress crack resistance
indicated by Koerner
et al.
[KOE 93]; and iii) negligible tensile stress concentration
in the geomembrane.
Rowe [ROW 98] also noted that an increase in the temperature of the liner may
substantially reduce the service life of the geomembrane by up to 10 to 20 years.
Given the above indications, it is acceptable to design a landfill liner with a
certain confidence on the performance of geomembranes in the medium- and long-
term (i.e. 50 to 350 years). Moreover, this conclusion is strengthened by the fact that
in many cases after landfill closure (assuming that a low permeability capping
system has been used) and at the end of the leachate collection system service life,
the seepage velocity through the basal lining system is mainly governed by the
capping system, annual precipitation and climate conditions of the region (i.e.
hydrological balance of the landfill). The bottom barrier layers only play a role in
terms of diffusive transport. This means that the main function of the geomembrane,
i.e. the reduction of seepage velocity and therefore limitation of advective transport,
can be fully exploited during the active life of the landfill and active management of
the post-closure period (on average up to a maximum of 50 years). This time span is
fully compatible with the service life of a well designed and installed bottom barrier
geomembrane. After this period, the amount of leachate reaching the aquifer below a
well performing landfill is mainly governed factors other the basal lining system.
Using the long-term simplified model and previous equations, it is possible to
investigate the effect of the number of wrinkles (with holes) per unit area of the
geomembrane on the performance of composite liners including a CCL or GCL.
