By Ian Peggs
Landfill caps prevent incident precipitation from becoming leachate that has to be treated, contain landfill gas that can be used to generate electricity, and prevent odors that annoy neighbors. Typically, geomembrane caps are covered with soil and vegetation for appearance and for the construction of parkland and recreational facilities.
However, exposed geomembrane caps (EGC) are increasingly being used as interim and even permanent covers for their cost savings and to eliminate potential problems with sliding cover soil layers.
Eight EGCs have been constructed on small and large landfill cells in the United States. Perhaps the most well-recognized EGC is the 17-ha (42-acre) cap at the South Landfill of the Delaware Solid Waste Authority (DSWA) — Photo 1.
This 0.91mm nominally green, polyester, scrim-reinforced, polypropylene geomembrane was installed in 1997-98. It has performed well for more than 10 years, surviving 145km/hr (90mi/hr) winds. Unfortunately, it has just started to degrade in the same way that other PP liners and floating covers have degraded after 8 to 10 years of service.
The other EGCs are made with HDPE geomembranes, one with LLDPE. Three in Florida have survived hurricanes.
At a new low-level radioactive waste landfill recently permitted in the U.S., the EGC is a polyester scrim-reinforced ethylene interpolymer alloy (EIA–PVC alloyed with ketone ethylene ester). This 15-acre EIA EGC has worked well since 1998, such that another cell was covered with exposed EIA in the fall of 2008. These are interim caps, but “interim” may be 30 years!
A new candidate that is receiving attention in many applications, particularly in cold regions, is prefabricated bituminous geomembrane.
Although new for caps in North America, BGMs have been used for many years to waterproof the upstream faces of dams high in the Alps and Andes, to line and cap municipal and low-level radioactive waste landfills, as base course barriers between major highways and sensitive aquifers, as separators under railroad ballast, to line mine process solution spillover lagoons in Canada (Breul et al., 2006), and more recently mine tailings ponds in Finland (A. Palolahti, private communication, 2007).