Since the late 1990s, SRK Reno has been providing solid waste landfill design and permitting services for small municipalities in Nevada, Utah, and California. Services have included the design and permitting of landfill and transfer stations, preparing and implementing closure plans, the design and installation of landfill gas and groundwater monitoring systems, quarterly groundwater monitoring, and construction and reclamation and closure bond costs.
Recently, Mono County, California asked SRK to prepare a final closure and postclosure maintenance plan and detailed closure design for the Bridgeport Landfill. The landfill is situated in the eastern Sierra Mountains at an elevation of 6,500 feet, and more than half of its annual precipitation falls in the form of snow.
For final cover, California regulations require a minimum of 18 inches of compacted low-permeability soil with at least 6 inches of growth media or erosion cover on top, or an approved alternative. Lacking an economical source of fine-grained soil, Mono County directed SRK to evaluate alternatives for the final cover. Unsaturated zone modelling indicated that typical store-and-release soil covers would not work for the anticipated high precipitation at the landfill’s high elevation, and so SRK explored geocomposite and geomembrane cover alternatives. Compatibility testing with geosynthetic-clay liners and available site growth media indicated the natural salt contents in the native soil were too high and would adversely affect the liner’s bentonite component.
SRK and the County decided on a structured geomembrane product from Agru America called SuperGripNet®, which incorporates an integrated studded drainage medium on the top with a spiked lower surface to increase interface friction angles in contact with the subgrade, overlain by a heat-burnished geotextile to facilitate overliner drainage. Overliner collection drains (gravel and perforated pipe) were installed at specified intervals to facilitate drainage and stabilize the overlying 18-inch growth media layer. Overliner drains were routed away from the final closed landfill surface and into the site’s perimeter stormwater control system.
The alternative cover design provided the client with a creative solution for closing the landfill in snow country and a relatively cost-effective alternative, using available soil material.
