ABSTRACT: The Montana Tunnels Mine was mined out to a depth of 900 feet at the end of 1999. The first signs of instability were recognized in 1996 along the upper slope area of the northwest sector when the pit was mined out to a depth of approximately 500 feet. To extend the life of mining operations, it has been proposed to extend and steepen this highwall to a depth of 1000 feet. Slope stability analyses were carried out using the UDEC software program to assess the stability and deformation of the proposed highwall design. Back analysis of the 1996 instability along the upper slope section and highwall deformation to date has indicated reasonable agreement with the observed mode of failure and displacement monitoring results, The results of detailed analyses of the proposed highwall design have indicated very limited additional deformation, further development of tension cracks is unlikely and large-scale instability is not expected to occur.

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