SRK Reno recently designed and permitted a flotation tailings disposal impoundment in Nevada. The conceptual engineering design envisioned traditional grading, embankment construction, and a single synthetic liner overlain by a drainage system with a gravity outfall passing through the synthetic liner into a lined collection pond. In addition, the conceptual design required incorporating a diversion channel for upstream stormwater to accommodate 1-in-100-year, 24-hour peak flows for the regulated minimum design criteria. Closure measures for this design incorporated a 3-foot-thick store-and-release soil cover and semi-passive management of long-term recharge and draindown effluent flows.
The estimated closure costs for the conceptual design were similar to the project’s capital costs, mainly due to the costs of constructing stabilisation earthworks, the semi-active management of post-closure draindown, and the anticipated requirement to pay the reclamation bond in cash. To reduce long-term closure liability and therefore upfront cash bonding requirements, the design was modified as follows:
• The single synthetic liner was replaced with a double-synthetic liner, sandwiching a leakage collection and recovery system (LCRS)
• The site grading design limits the rates of rise of the tailings surface to generally less than 10 feet per annum, and provides 3 feet of closure cover material for the tailings area
• Material from the grading operation was used to construct an upstream diversion berm, which includes all material for closure cover installation
• The drainage system was reconfigured to flow to an internal sump with the water either discharged back onto the tailings surface or recycled to the plant
• If operations suddenly cease, water balance scenarios all show a net negative water balance within the first one to two closure years, under average climatic conditions.
The capital design allows for constructing a designed penetration of the secondary liner to gravitate leakage into a double-lined evaporation pond that can easily manage the maximum permitted LCRS flow of 150 gallons per day.
With these modifications, capital costs increased by about 20 percent, but reclamation costs decreased by about 70 percent, resulting in an estimated 25 percent reduction in short-term capital outlay for construction and reclamation bond payment for the tailings impoundment.
