Stormwater June 2012 : Page 41

the membrane system,” he continues, “which was there for two purposes: to reduce water losses from the reservoir, and to avoid its contamination. There are three known sites of contaminat-ed groundwater across the street—a couple of gas stations and a printing operation—so Charlotte-Mecklenburg Utilities was interested in making a bar-rier between the drinking water sup-ply and the potentially contaminated groundwater. However, there isn’t any contaminated groundwater on the site; we monitor it.” The reservoir is lined with a 30-ml polymer membrane. “That’s not as thick as a municipal solid waste landﬁ ll membrane, but fairly thick,” says Eller. “There’s 2 feet of soil atop that, as a secondary liner, and also as a counterweight, to hold the membrane down.” Below the membrane, Thalle Con-struction Company Inc. of Hillsbor-ough, NC, installed the underdrain system of perforated HDPE corru-gated plastic pipe (CPP). “The pipes are dual-walled, 6 to 18 inches in di-ameter,” he says. “Piping was placed underneath the membrane, to prevent uplift from the groundwater. When the reservoir is full, the water pressure atop the membrane would keep it in place. However, when the reservoir is emptied, groundwater pressure under-neath, and none above, could cause uplift of the membrane, damaging it.” Has the reservoir been emptied due to the recurring droughts of the past few years? “No, but the utility some-times pulls the reservoir down, to do maintenance,” answers Eller. “There’s maybe a two-day water supply in the reservoir; the main reservoir is a lake several miles away. The site is tempo-rary storage; it also allows additional sedimentation, and allows the water to be oxygenated.” Why use corrugated pipe? “Corru-gation makes the pipe with less mate-rial, but more strength,” he says. “Only the outside is corrugated; the pipe’s interior wall is smooth. The piping sys-tem was laid in trenches, with geocom-posite drainage material under the membrane, which brings water to the pipes. The pipes then collect any leak-age through the liner. Where the un-derdrain system pipes come to a central point, there are two solid HDPE pipes. This area is ﬁ lled with cement and bentonite grout, which further stops seepage. “For underdrainage, plastic per-forated pipe is the only choice any more,” concludes Eller. “You’d once get perforated clay or PVC pipe, but it’s labor intensive to get those ma-terials perforated; it’s much easier to perforate HDPE. HDPE pipes are eco-nomical; they don’t corrode, and as long as they’re not in sunlight they can last indeﬁ nitely.” For related articles: www.stormh2o.com/drainage-systems Storage Made Simple. CMP STORMWATER MANAGEMENT SYSTEMS Cost Efﬁcient, Smart by Design With sizes up to 12-ft diameter and a full range of ﬁttings, Corrugated Metal Pipe (CMP) stormwater management systems are the most versatile, cost-effective and durable option in the market. Advantages Install faster, are stronger and more reliable than other systems The larger sizes available allow for less obtrusive footprints Completely integrated ﬂow control devices and other custom ﬁttings Perforated pipe offerings for inﬁltration, retention, or groundwater recharge applications Available in all the different CMP material options to meet speciﬁc service life requirements For more information, visit ©2012 Lane Enterprises, Inc. ,ANE%NTERPRISES&#0c;)NC&#0e;s lane-enterprises.com June 2012 www.stormh2o.com 41