John Jenkins, C.E.G., Rajiv Ali, PhD, P.E., and Rick Thrall, PhD, P.E.

Slope movements were observed in March 1999 on the South Hill of Potlatch Corporation’s mill in Lewiston, Idaho. These movements were impacting an operating landfill, a county road and a critical effluent pipeline located near the toe of this landslide. The operating landfill, used for mill generated solid wastes, covers the eastern portion of the 37-acre landslide.

AMEC’s team, using geologic mapping and a system of geotechnical instrumentation characterized the landslide. The landslide involves weak, fine-grained Missoula Flood backwater deposits and an interbed that is part of the Columbia River Basalt Group. The main cause of the movement is high groundwater within the landfill. A temporary emergency stabilization system of six dewatering wells was installed in September 1999 that reduced the rate of landslide movement from 1 inch per month to about 1 inch per year.

The temporary stabilization system was updated with 11 dewatering wells and new inclinometers and piezometers in December 2000 and January 2001. Surface monuments on the slide and the effluent line were established and are being monitored using GPS techniques. Detailed slope stabilization studies were completed to identify permanent stabilization alternates and the associated risks and costs. Alternates included various dewatering schemes, a rock buttress, tiebacks or a combination of these were considered.

A decision matrix was developed with the following criteria:

· Calculated Improvement in Factor of Safety · Calculated Future Effluent Pipe Movement · Capital Cost · Annual Monitoring and Maintenance Costs · Total Cost

Overall long-term stabilization costs are on the order of $8 million. The decision matrix is enabling company management to make informed decisions based on the risks and financial constraints. No final decision has been reached except to maintain and enhance the existing dewatering system while the mill considers financial options for a permanent stabilization system. A program of monitoring and maintenance by Potlatch and AMEC personnel is in place to optimize system operation and to respond to needs as they arise. This case history demonstrates how a landslide can be managed to take the clients needs under consideration when developing stabilization alternatives.