The California Gulch mine reclamation project in Leadville, CO, posed two basic problems. First, snowmelt and runoff were eroding the area; three channels, instead of the one original stream, were carving up the hillsides. Second, because the area had been named a Superfund site in the mid-1980s, contaminants from waste rock needed to be contained.Todd White of Resurrection Mining Company in Colorado, which owned the site, explains the historical problem: “This was a remediation project for a historic mining district. Mining started in Leadville in the 1870s, and the land was worked until the 1960s. In the beginning, the area was mined for gold; when that ran out, miners dug for silver, then zinc – they kept working the rock until everything of value was removed. Back in the early days, the tails, which is the ground-up rock left when you remove the mineral you’re after, were discharged into the river systems. That practice resulted in this area being declared a Superfund site. Resurrection Mining is involved in the cleanup because, some time in our history, we were involved in mining there.”Directing water flow into one channel, instead of three, and then armoring that one channel, was the initial task. “We wanted to improve water quality and reduce sedimentation of the Arkansas River, which begins here in Colorado,” says White. “Snowmelt would come down through here in several braided channels, picking up sediment and metals on its way. Making one flow path decreases erosion and makes it easier to direct the runoff to a water treatment plant.”MWH Engineers of Steamboat Springs, CO, designed the project, and the decision was made to line the channel with Tensar Earth Technologies’ Triton Marine Mattresses. “We used these mattresses because it was the best option for the job,” explains Design Engineer Mike Ross. “Riprap was not an option. It was just too huge – several feet in diameter. We considered concrete and concrete blocks, but the channel’s pH levels are pretty low for a good part of the year, and concrete just dissolved. We were also trying to meet EPA’s requirements, and Tensar’s mattresses filled the bill.”
The mattresses’ physical properties made them a logical choice. “Other materials available, such as gabion baskets made of galvanized wire, can corrode due to the water’s pH,” White explains. “This material was resistant to corrosion.” “Because they’re made of HDPE [high-density polyethylene], the mattresses wouldn’t break down in the water,” Ross adds. “They’re also 99% [ultraviolet] stabilized, so the environment won’t degrade them.”The task of securing 36,000 ft.2 of channel area was started in August 2002 and completed the next month, ahead of schedule. “The project went a lot faster than I anticipated,” White relates. “Filling gabion baskets, for example, is time-consuming, but here there was a frame made.” By putting the prefabricated mattresses inside the frame, he describes, the Nielsons Skanska Inc. crews (the project’s general contractor/installer) filled twice as many mattresses at one time as had been anticipated. “Crews held each mattress open within the frame like a large envelope, made a funnel at the top, poured in neutral rock with no acid-generation properties – general riprap – then laced the HDPE fabric together. The guys were just flying. The project was finished in almost half the time, which certainly saved us on labor costs.”Constructed with Tensar Uniaxial Geogrid (UXTriton200), each mattress measured 12 in. thick x 5 ft. wide x 20 ft. long. Transverse compartments were created by specifying internal baffles at 1.5-ft. intervals, and each compartment was filled with 4- to 6-in. stones. The channel was excavated, and then a crane was used to put the 300 flexible mattresses in place. “The first time they picked one up, I was surprised the mattress stayed together. I thought the material might break, but it didn’t,” White says. The mattresses were not seeded, nor was any growth material installed, but White states, “I anticipate sediments carried by runoff will consolidate in the rocks, and then vegetation may establish.” The mattresses also allowed the channel to be secured for nearly any water event. “We designed it for a 500-year flood, which should handle discharge greater than 350 cubic feet per second,” Ross explains.“We wanted to eliminate the wastewater from going over the rock, to control all the water,” White adds. “We don’t anticipate replacing these mattresses.”How are the mattresses holding up? “California Gulch doesn’t flow year-round, usually only in April and May,” says White. “We had normal snowfall this year and saw runoff go through it for the first time during mid-May. Things look good so far.”
