Inferred Weak Rock Mass Classification for Stope Design

Abstract

Empirical design methods are commonly used for rock mechanics evaluations. An appropriate method of rock mass classification is required to use these empirical methods. There are limitations for rock mass classification methods when access to the ore zone is restricted.

The Cameco Corporation Eagle Point Mine in northern Saskatchewan, Canada, uses the longhole open stope mining method for the recovery of uranium ore. The Modified Dilution graph is used for the prediction of stope hanging wall dilution. The mine currently uses a rock mass classification based on an estimate of the alteration and strength of a rock mass from geological drift mapping. Since this method is highly subjective, point load testing of diamond drill hole core was completed to attempt to correlate the alteration and strength of different rock types to remove the user subjectivity. The results of the testing indicated a general trend of decreasing rock strength with increasing alteration, albeit with considerable scatter.

A repeatable, standardized method of evaluating the stope geometry and inferred rock mass classification for reconciliation purposes was developed. The standardized stope evaluation method removes significant subjectivity currently involved in estimates of stope geometries and the magnitude of dilution. A new lithology based method for interpreting the mine specific geological alteration and strength classification system was developed based on several sources of rock mass classification observations. This resulted in a correlation linking individual rock mass property descriptions between different classification systems for an improved estimate of the Q’ classification value. This improved method of estimating the rock classification Q’ value, as well as conventional techniques for linking classification systems, was used in a stope reconciliation process to predict open stope dilution.

Twenty-seven stope reconciliation case histories were documented and used to compare predicted and measured dilution, based on three different approaches for estimating rock mass classification values. The results showed a minor improvement in dilution prediction using the approach developed in this study. The systematic stope reconciliation and rock mass classification approach did highlight areas in the weak pegmatoidal rocks where improved rock classification estimates should be investigated.