More than twenty years ago, the founders of the PVC Geomembrane Institute (PGI) and the succeeding entity Fabricated Geomembrane Institute (FGI) set out to evaluate the long-term durability of geomembranes in actual field use. They wanted to exhume numerous samples of exposed geomembranes for testing on a periodic basis without ruining the integrity of the installed primary liner system. The Minnesota Department of Natural Resources (MN DNR) became a partner, and the study was started in October 1995. The study selected a MN DNR mine settling basin in the town of Hibbing, Minn., for the long-term study.
The main objective of this ongoing project is to investigate the 30-year durability of PVC geomembranes and PVC chemical fusion and hot wedge-welded seams. At the beginning of the study, samples of three 30-mil (0.8-mm) geomembranes from three different manufacturers were placed along the bottom of a double-lined mine settling basin at the reclamation site. The weather extremes and the harsh winters with many freeze/thaw cycles provided an excellent venue to test the durability of these geomembranes. The settling basin contains mine drainage that has been neutralized with magnesium hydroxide from mine waste rock piles. Three strips of PVC geomembrane–originally 6-feet (1.8 m) wide and 50-feet (15.2 m) long—were installed in 1995. After exhumation and sampling events two, four, five, ten and twenty years after placement of the samples, the test strips are shorter but still buried in a 6- to 12-inch (15–30-cm) thick sand cover at the bottom of the basin. Each test strip contains a single seam that runs the entire length at the centerline of the test strip. Two of the strips have chemical seams, and the other has a 1.5-inch (3.8-cm) wide single wedge welded seam.
Geomembrane samples were previously collected and tested after two, four, five, ten and twenty years in the basin, and the materials still meet the original specifications. Following the retrieval of the 20-year samples, it was determined that there was sufficient space available in the basin to install additional materials. After discussions with the MN DNR, it was decided to install eight new materials with dimensions of 6-feet (1.8-m) wide and 25-feet (7.6-m) long. These samples also contained a seam at the centerline, but all seams were created using a thermal welder. Therefore, on Sept. 21, 2017, the FGI and the MN DNR installed eight additional geomembrane material samples on the bottom of the mine settling. The eight new geomembranes installed are:
- 36-mil reinforced polypropylene, provided by Cooley Group (CG)
- 30-mil single film laminated woven coated polyethylene, provided by Owens Corning Inc. (OC)
- 30-mil single scrim coated woven coated polyethylene, provided by Intertape Polymer Group (IPG)
- 40-mil unreinforced LLDPE, provided by Raven Industries (RI)
- 30-mil XTRM Ply ethylene interpolymer alloy (EIA) formulation geomembrane, provided by Engineered Polymer Technologies (EPT)
- 30-mil XR-5 ethylene interpolymer alloy (EIA) formulation geomembrane, provided by Seaman Corporation (SC)
- 30-mil PVC geomembrane, provided by Plastatech Inc. (PI)
- 30-mil smooth HDPE geomembrane, provided by GSE Environmental (GE)
The FGI will perform exhumation, sampling and testing of the newly installed materials after five, ten, fifteen, twenty and twenty-five years. In addition, large archive samples of each geomembrane were retained before installation for archival purposes. These archival samples were shipped to the University of Illinois at Urbana–Champaign for storage.