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Beginning in 1999, Envirocon performed seven consecutive RCRA pond closure projects on elemental phosphorus ponds at a former plant near Pocatello, Idaho. The plant was on the Fort Hall Shoshone-Bannock Reservation. Envirocon utilized crews consisting of over 90% tribal members through the Tribal Employment Rights Ordinance (TERO) who worked more than 400,000 manhours without a lost time accident closing 73 acres of process tailings ponds using an innovative in-situ closure method.
Initially, the work was performed while the facility was operating, bringing a heightened focus on following health and safety protocol due to the inherent risks in producing elemental phosphorus. The ponds held a soft, low strength sludge that contained elemental phosphorus and phosphene gas. The presence of these constituents required that a large portion of the initial fill work scope to be conducted in Level B personal protection (PPE) and flash protection suits.
In general, the work for all seven phases involved a two-phased process that required initial fill of the ponds followed in 1 to 2 years by the construction of a multi-layered RCRA compliant cap. The reactive nature of the sludges mandated that they remained covered continually. The objective of the initial fill process was to cover the sludges with sand and other aggregate materials while leaving a water blanket over them to prevent them from contacting ambient air. Once initial fill was completed, liquids were removed using pumps and wick drains. When subsidence decreased to the level allowed by the EPA approved closure plan, each pond could then receive a RCRA compliant cap consisting of multiple synthetic liners and other components.
Initial fill work began with placing a geotextile fabric over the pond sludge. Drainage collection pipes were installed during deployment of the woven-geotextile liner. The collection pipes at each pond were attached to a manifold with pumps and flow switches. This system facilitated the removal of liquids from each pond after placement of the initial fill. A covering of sand or graded rock was then placed over the geotextile fabric using a variety of conveyor systems. The initial sand fill was placed on six of the ponds using two, specially designed bridge conveyors. The bridge conveyors were designed and fabricated for this project. The bridge conveyors spanned 250 and 225 feet and, were fed with feed conveyors up to 120 feet in length. Envirocon was responsible for complete assembly, testing, operation, and maintenance of the prototype equipment. Hundreds of thousands of tons of sand were placed using the bridge conveyors.
Initial fill was placed on other ponds using truck and crane-mounted conveyor systems, due to the shapes of ponds which prevented use of the bridge conveyors. The truck and crane conveyors were used to place overlapping acres of sand until there was sufficient fill strength to access the pond with LGP equipment. Each pond was filled using this method, and was completed similar to the other six ponds once the LGP equipment could access the pond. Wick drains were installed after the initial sand fill was placed in each pond. Over 100,000 linear feet of wick drains were installed. The majority of wick drain installation was performed in Level B personal protection.
The final caps were 7-foot thick RCRA compliant caps installed over the initial fill. Each cover consisted of four separate geosynthetic layers and seven different geomorphic zones. The four geosynthetic layers were a geosynthetic clay liner, 60-mil HDPE liner, drainage net, and a non-woven geotextile. The final cover contained a seep collection system below the geosynthetic liner and a drainage collection system above the geosynthetic liner. Gas monitoring wells were installed around the perimeter of the pond and temperature monitoring wells were installed prior to constructing the final cover. Geosynthetic boots or seals were required around the temperature monitoring wells to insure final cover integrity. The covers were then seeded and surrounded by concrete lined "V" ditches constructed around the perimeter of the ponds.
One 11-acre pond was too large to allow preloading with the specified equipment without first dividing the pond in half. This was done by constructing a 790-foot slag dike using an 80-ton crane with a clam-shell and a long stick excavator with a 55-foot reach. Extreme care had to be taken during the installation of the dike in order to prevent the creation of mud waves or other conditions that could allow the sludge to contact ambient air and spontaneously ignite. Once the dike was constructed, Envirocon placed the preload fill using the bridge conveyor as with the other ponds.
Envirocon successfully completed seven phases of initial fill and final cap installation over seven consecutive construction seasons without an OSHA-recordable injury using 90% TERO labor. Once Envirocon demonstrated its ability to conduct the work safely, competitively, and in full compliance with the RCRA approved closure work plans, many of the later phases of work were awarded on a negotiated sole-source basis.
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