By Randona Conrad
In a collaborative effort with the United States EPA, Nuna Innovations Inc. installed concrete cloth geosynthetic cementitious composite mat to address an acid rock drainage condition at the Sheldon Mine site near Walker, Arizona.
This site is identified as an EPA Superfund site and is located on unincorporated private land within Prescott National Forest. It is close to the headwaters of Lynx Creek and the Lynx Lake reservoir.
When in operation from the 1860s to the 1950s, the mine produced copper, gold, silver, lead and zinc. Waste was left in two main areas: the Sheldon Mine tailings pile and the Sheldon Mine waste rock pile. Identified as a radioactive site by the U.S. EPA, it was suffering from an acid rock drainage problem that was occurring due to stormwater becoming contaminated with lead, arsenic and heavy metals. This not only posed a threat to nearby residents, but also to the Lynx Creek watershed.
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In 1975, the Prescott National Forest and the University of Arizona, School of Renewable Natural Resources completed a site recovery project. They regraded eroded banks, added limestone and topsoil and seeded native grasses. A ditch was created to capture the contaminated run-off water from four tailings piles at various points. Due to water and wind erosion over the years, the soil cap and drainage system was in disrepair and no longer isolated the contaminants. About 99% of the drainage ditch around the tailings piles was unlined. Shotcrete had been used at the top of the steep slope but was highly deteriorated.
The goal of this project was to line the entire ditch and prevent surface and stormwater from coming into contact with hazardous chemicals. Shotcrete was considered, but rejected due to cost and performance considerations such as cracking and undermining. An HDPE liner was also considered but required extensive excavation, which would increase trucking costs to remove excavated materials. Also, specialist labour required to install the HDPE was expensive. After careful consideration, the EPA specified concrete cloth for this project.
Concrete cloth is a flexible, cement impregnated fabric that hardens when hydrated to form a thin, durable, water- and fireproof concrete layer. This allows concrete construction without the need for plant or mixing equipment. The geosynthetic cementitious composite mat consists of a three-dimensional fibre matrix containing a specially formulated dry concrete mix. A PVC backing on one surface of the cloth ensures the material is completely waterproof. The material can be hydrated either by spraying or by being fully immersed in water. Once set, the fibres reinforce the concrete, preventing crack propagation and providing a safe plastic failure mode.
For this project, a simple profile was excavated on the upper portion of the drainage channel to accommodate clean drain rock surrounding an 18” perforated pipe for the French drain. A simple vee ditch was excavated in the lower section. Roots were removed within the excavated area and a 30 cm anchor trench was excavated on either side of the French drain trench and vee ditch.
Concrete cloth was laid in a transverse layout with 100 mm overlaps, along the entire length of the excavation. Overlapping concrete cloth is the simplest method of joining two layers together. This is appropriate for the majority of ditch lining, erosion control and ground surfacing applications. Overlapped joints are compressed along the entire length, while the material sets, to ensure there are no voids between layers. This can be done using screws, sandbags, water weights, loose fill, staples, etc. In this case the joints were secured with 3/4’ screws every 15 cm.