Assessment says subsidence problems not expected at Rock Creek

An independent assessment of subsidence at Revett Minerals' Troy mine has been completed and indicates that similar issues are not expected to arise at the company's proposed Rock Creek project.

The engineering firm Tetra Tech was contracted by the U.S. Forest Service to conduct an investigation to determine the cause of two sinkholes at the Troy mine. The firm was also asked to consider the potential for similar sinkholes to occur at Rock Creek, said Tom Puchlerz, acting forest supervisor for the Kootenai National Forest.

Key findings of the report include:

* The two sinkholes at the Troy mine were caused by pre-1993 mining activity that did not leave buffer zones of solid rock between the mine workings and East Fault zone.

* Failure in the mine workings propagated upward as "chimney-type" failures through the intensely fractured and deeply weathered rock of the East Fault and resulted in the two sinkholes.

* The Troy mine operating permit did not require buffer zones to prevent mining up against the East Fault and properly sizing and securing mine roads that pass through the fault. Such requirements would have prevented caving and surface subsidence.

* The potential for chimney-type subsidence to also occur at the Rock Creek project is minimal to nonexistent.

* The sinkholes at Troy do not raise new questions or provide information that would lead to major changes in the mitigation for the Rock Creek project.

* The mitigation measures the agencies have imposed at Rock Creek restrict mine development in fault zones and require buffer zones.

* At hard rock room and pillar mines, such as the proposed Rock Creek project, surface subsidence is not an inevitable consequence of mining. Provided that the mine is properly designed, the potential for subsidence to occur is minimal and this view is shared by many experts in the field of subsidence.

* The final environmental impact statement and record of decision properly considered and informed the public of subsidence issues at Rock Creek.

* The approach taken by the final environmental impact statement — that mine planning and subsidence prevention are dynamic processes that should be continually under review and modified as information is collected — is reasonable and proper.