Item A: Public Hearing onMetro Plan Amendment (Delta Sand and Gravel)

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Gravel Pit Makes Liabilities Into Assets "mud" to maintain trench wall stability. Soon after excavation, the trench is backfilled with a soil bentonite clay mix that creates a subsurface "wall" that is impermeable to groundwater migration. Throughout the past two years, the Golden, Colo., office of Envirocon has used slurry walls to construct three very different types of water storage facilities. These projects include the following: a slurry wall around three mined-out gravel pits that were being converted to water storage; a slurry wall that surrounded a cooling pond and formed the core of the pond's impoundment dam; and a slurry walVgeocomposite clay liner system that was placed on the front face of a dam as part of an engineered water containment system. The focus here is on constructing a slurry wall around three coalescing gravel pits to create one, large municipal water storage facility. Water storage project In February 2004, Envirocon, working in conjunction with Englewood, Colo.-based GEl Consultants, completed the Hazeltine Slurry Wall project in Henderson, Colo., for Denver Water, which provides municipal drinking water for the city of Denver and many surrounding communities. Page 2 of3 slurry trench. An excavator will then take this material and place it into the slurry trench. The slurry trench backfill must have a permeability of 1 by 10-7 cmls or less. An excavator \vith a long stick digs out the slurry trench. The white bags, laid out along the trace of the slurry "rail, are dry bentonite clay. Bentonite is added to the trench backfill to create a subsurface "wall" that is impermeable to ground-\"ater migration. The Hazeltine project is located adjacent to the Platte River a few miles north of Denver. The Hazeltine slurry wall surrounded three mined out gravel pits, was 14,480 linear feet long and ranged from 27 to as much as 85 feet in depth (528,274 square feet of slurry wall face). The purpose of the wall was to create a single water storage facility out of the three adjoining gravel pit lakes. During construction of the Hazeltine project, crews faced two challenges. The frrst issue was the extreme depth of the slurry wall- up to 85 feet. The fact that one of the pits had been excavated right up to the edge of the Platte River created the second challenge - the need to construct a 55-foot-wide slurry wall working platform. The working platform provides a flat working surface to excavate the trench and mix soil with bentonite clay prior to backfill. To build the slurry wall working platform at Hazeltine, Envirocon placed and compacted 286,000 yards of fill and then su~cessfully excavated the slurry wall through the platform fill material. Three Envirocon slurry wall excavation crews constructed the 14,480-foot-Iong slurry trench using Caterpillar 365, Komatsu 1100, and Komatsu 750 excavators. The larger excavators, equipped with lon~ stick booms, were used for the deeper portions of the trench. Multiple slurry wall crews allowed Envirocon to complete the Hazeltine project in five months. During construction of the cutoff wall, the water level within the gravel pits began to rise. To facilitate groundwater infiltration testing, Envirocon successfully pumped approximately 1 70 million gallons of water from the newly formed Hazeltine reservoir for discharge to the Platte River. From liabilities to assets Constructing slurry walls around former gravel pit lakes has provided much-needed municipal water file://X:\0290 Delta Sand & Gravel\04-0020 Pit Exoansion\Hvdrualic Barrler\Gr~v~l Plt M 11 /1 /?()()h