ABSTRACTA series of analyses of possible mining excavationsassociated with development of the copper-nickeldeposits of northeastern Minnesota is presented. Themajority of these analyses were conducted using aboundary element method but some cases were also analyzedusing the finite element method. Comparable resultswere obtained using the two methods but theboundary element method required less data preparationtime and computer run time. Results of the analyses,as well as indicating the capabilities of geotechnicalnumerical models, provide some insights into behavioraround multiple excavations in jointed rock.INTRODUCTIONCopper-nickel mineralization in northeasternMinnesota occurs to varying degrees along the basalcontact of the Duluth Complex. Exploration in thisarea has been active since 1947 and two potential minesites have been identified. Of these, the MINNAMAXsite, which is located at the eastern end of theMesabi Range, is most promising and consideration isbeing given to a combined open pit and undergroundmining operation. This paper examines two possibleunderground mining methods for the MINNAMAX orebodies,using numerical geotechnical models to provide someguidelines for design in a situation where there isno previous mining experience.Mineralization at the MINNAMAX site is extensivebut low grade, the reserves typically assaying lessthan 1% combined metal. The limits of the potentialorebodies are defined below by the basal contact andabove by a grade cut-off which results in an irregularthickness and some potentially minable lenses abovethe main mass (Figure 1). Mining methods must thereforebe selected that are capable of handling orebodiesvarying from 15m to 120m in thickness, with possiblemultilevel excavation in some cases. Given thesevariable conditions, coupled with a high rock massstrength and relatively high initial field stresses,caving methods appear to be inappropriate. Preliminarydesign has been based on open stoping methods for primary extraction, with stopes 34m by 134m in plan and separatedby 12m pillars.(1)

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