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

Synopsis The lack of a factor to account properly for the effects of faults on open-stops stability is a current shortcoming of the stability graph me thod. In this method the rock-mass quality, Q, is used together with a stre ss factor, a rock defect factor and a gravity factor to define the stabilit y number, N'. The Q system does not specifically include a factor that acco unts for the presence of faults, yet nearby faults frequently affect the st ability of open stopes. In many mines that practise open-stope mining stope walls assessed as stable by reference to a conventional stability graph of ten cave when faults are present. Sloughage of stope walls is a major conce rn in mines 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.