Fault factor for the stability graph method of open-stope design

The lack of a factor to account properly for the effects of faults on open- stope stability is a current shortcoming of the stability graph method. In this method the rock-mass quality, Q, is used together with a stress factor , a rock defect factor and a gravity factor to define the stability number, N'. The Q system does not specifically include a factor that accounts for the presence of a faults, yet nearby faults frequently affect the stability of open stopes. In many mines that practise open-stope mining stope walls assessed as stable by reference to a conventional stability graph often cav e when faults are present. Sloughage of stope walls is a major concern in m ines because of ore dilution. A procedure for calculating fault factors has been developed for incorporat ion into the stability graph method. The procedure employs numerical stress modelling to take account of geometrical relations between faults and stop es, fault characteristics and in-situ stresses. It is shown that faults can increase sloughage from stope walls by their ability to increase the size of the zone of critical low stress in stope walls. Faults tend to have the greatest influence on stope stability when the angle between fault and stop e is about 20-30 degrees; faults that are nearly perpendicular to the stope wall have little effect. The procedure is applied to two case histories fr om Canadian underground mines.