Norwest has been developing reclamation and closure designs for mine pits, overburden dumps and coal refuse impoundments at a coal mine in Washington State. Mining was discontinued at one of the mine pits due to geotechnical concerns following a pit side-wall failure in November 2006. As a result of cessation of mining operations in the pit, it was necessary to develop a new reclamation plan for this mine pit. Backfilling of the pit was not a viable option. The plan needed to address geotechnical and hydrologic stability and acid mine drainage (AMD) concerns raised by the regulatory agencies as well as demonstrate that the proposed pit lake would be ecologically viable and meet the water quality requirements for both the pit lake and receiving stream.

 

Norwest developed a reclamation plan for this pit that provided hydrologic and geotechnical stability and ensured adequate blending of isolated pockets of potentially acid-forming material with alkaline spoils to mitigate AMD and meet post-mining water quality criteria for mine drainage. A principal feature of the reclamation plan was the establishment of a permanent pit lake with a surface area of approximately 217 acres. This included a buttress design to meet minimum stability criteria for the Smith footwall closure configuration. The geotechnical characteristics of the Smith footwall slopes were defined by a combination of inputs including laboratory testing, geological models and back analysis of slope failures. The post-mining topography and drainage was developed using a geomorphic drainage design where feasible and both a terrace and down drain approach for some the remaining areas and a geomorphic drainage design for other portions of the post mining surface. The plans included the removal of the sediment ponds for final reclamation and establishing a lake inlet channel and constructed wetland at the location of the former sediment ponds. The majority of run-off that flows into the lake is routed through a the constructed wetland. The wetland habitat was added as a “polishing” feature for the lake inflow even though the modeling performed in support of the probable hydrologic consequences (PHC) assessment predicted that the lake water quality would be suitable for meeting the needs of aquatic environments, and meeting applicable discharge limits without the need for wetland treatment.  The lake outlet structure design included rock weirs and rootwads to provide stable gradient control while providing the necessary fish passage and habitat.

 

Norwest developed the detailed permitting and design plans within schedule and budget. Approvals were obtained from State and Federal agencies so that implementation of the plan could proceed according to schedule. The elements of the plan address all aspects of permit approval and regulatory requirements. Implementation of the Plan will allow the client to successfully meet the reclamation and post-mining water quality requirements.