Gilt Edge Mine Closure

Problem

The Gilt Edge mine has been in operation since 1876 as a site for gold ore. During this time, a byproduct of the mining operations, sulfidic waste rock and spent heap leach ore, was accumulating in the surrounding areas. About 20 million cubic yards of waste material began to contribute to acid rock drainage (ARD), eventually leading to surface and groundwater contamination that was first detected in 1993.

Heavy annual rains pouring through the waste pile would bring contaminated ARD water into the local water supply, which cost millions per year to treat. To reduce the cost of water infiltration, a cap system was installed over the waste rock repository.

Solution

The State of South Dakota in conjunction with the U.S. Environmental Protection Agency worked together to design the initial project parameters. The working area, Ruby Waste Rock Repository, was designed to be a 65-acre cover over the spent ore repository. R. K. Frobel & Associates Consulting Engineers (Evergreen, Colorado) was brought in to provide technical assistance in designing the geosynthetic cap and providing quality control.

To provide adequate protection against water infiltration, the cap system utilized a structured geomembrane with an integral drain layer. This barrier would be installed over the 65 acres of waste rock. One challenge with this large area was controlling the stability of the cap system along with the flow of water along the slope, which extended over 550 meters. A series of nine 8 meter sideslope benches were created. These drainage ditches would be capable of controlling the flow of water for even 100-year storm events.

Originally, the project specifications—e.g., site conditions, slopes, longevity requirements, and acidic nature of the surface—made 60-mil high density polyethylene (HDPE) geomembrane a natural choice. However, the heavily textured surface of the rolled crushed rock posed significant friction. The additional toughness and resistance to stress requirements led to 80-mil linear low-density polyethylene (LLDPE).

The final cover system comprised six layers. At the top, was a 75-m thick processed topsoil seeded with native grasses. Beneath that, a 900-mm layer of rock and soil. A 450-mm layer of crushed 25-mm rocks separated the soil from a 10 oz./yd.2 grade of nonwoven geotextile. The last two layers comprised an 80 mil LLPD structured geomembrane with an integral drain layer as well as a 300-mm roller. The ditches were lined with a geocomposite lining system with a high grade geosynthetic material

All minimum physical and mechanical specifications were followed according to the Geosynthetic Institute Standard (GRM GM-17) except for asperity height, which was bumped up to 15 mils.

Outcome

The cap system utilized geosynthetic materials, along with a well-planned installation methodology, to provide an innovative long-term solution for reducing water infiltration and the generation of ARD water. The entire 65-acre Ruby Waste Rock Repository cap and drainage layer was installed in under 60 days without installation difficulties or discrepancies in materials or installation.

Project feedback shows that success factors include the hiring of a prime contractor with excellent experience working with large earth moving projects and an extensive CQA plan. Installation of Phase 1 and Phase 2 was completed in 2002 at an estimated cost of $17.5 million. Phase 3 was completed a year later for $3.5 million. The client saved up to $487,500 in material and labor costs by not having to purchase and install a separate geonet drain layer thanks to the AGRU integrated structured geomembrane.