Failure mechanisms of orebodies under shear loading

Experience at the Geomechanics Research Centre in studying three orebodies loaded with major far-field stresses oblique to strike over the last ten years showed several characteristic problems that differentiate them from orebodies having major the far-field stresses perpendicular to strike. Orebodies loaded with major far field stresses oblique to strike are subjected to both compressive and shear stresses. These orebodies suffer from unusual frequency of seismic activities and high dilution levels at locations where such activities and dilution levels are least expected during mining. Despite these major problems associated with mining orebodies under shear loading, little information exist in the literature on any detailed studies to understand and mitigate the problem.;A two-step approach was adopted in the study as follows: (i) Literature review a) A critical review of the literature on established mining case histories where seismicity and dilution problems were encountered because such orebodies were under shear loading: Quirke Mine (Maybee, 2000), Lac Shortt Mine (Falmagne, 2001) and Campbell Red Lake Mine F2-Zone (Suorineni and Kaiser, 2006). b) Review of known mining case histories where severe seismicity and dilution problems were encountered but could not be adequately explained by conventional knowledge: Vale Copper Cliff North Mine 120 orebody (Morrison and Galbraith, 1993). c) The empirical pillar design charts: Maybee (2000), Lunder and Pakalnis (1997), and Hedley and Grant (1972) (ii) Two- and three-dimensional stress analyses numerical modeling using Phase2D and Map3D codes.;The stress analyses approach was validated against the case histories and empirical pillar design charts. Room-and-pillar mining-type rib pillars with width to height ratios between 0.5 and 2.5, and sill pillars were analyzed. When ore lenses are not continuous but have offsets in between them, the spatial arrangement of the ore lenses dictate different geometries of offsets. Offsets are considered as pillars and analyzed. Stability of open stopes under shear loading was briefly investigated with respect to dilution.;The following are the main conclusions from the research: (i) Failure of rib pillars in inclined stress fields initiates at diagonally opposite comers depending on the orebody inclination relative to the major far field principal stress. (ii) Rib pillars subjected to combined compression and shear loading suffer from reduced confinement compared to pillars loaded in pure compression. The loss of confinement results in the pillars being more brittle and weak. (iii) The behaviour of offsets in discontinuous orebodies under shear loading depends on the spatial arrangement of the orebodies. Offsets in coplanar ore lenses behaved in a similar manner to the rib pillars. Offsets in en-echelon type ore lenses with and without overlap are similar. (iv) Examination of stress paths at the offsets indicate that mining results in high stress build-up at these locations making them potentially burst-prone at some mining stage. (v) For uniform continuous tabular orebodies under shear loading damage is either in the hanging wall or footwall depending on the mining direction relative to inclination of the major far field principal stress. (vi) The existing empirical pillar design formulae and charts are not applicable when dealing with orebodies under shear loading since they do not account for the loss in confinement in these pillars. Shear loaded pillars will be under designed by these methods. (vii) Shear loaded pillars get progressively weaker as the angle between the major far field stress and orebody progressively increases from 15°up to about 45° in room-and-pillar mining. For dips greater 45° other mining methods are employed that create sill pillars. Sill pillar stability decreases as the included angle between orebody and the major far field stress decreases from 90° with the weakest pillar occurring again at 45°. (viii) The effect of shear loading is more pronounced for k- and pillar width to height-ratios> 1. (a) The effect of k-ratio on pillar stability is sensitive to the angle between the major far field principal stress and the orebody.;The objectives of this thesis are to: (i) Review case histories of documented mining problems that have been unexplained and the circumstances under which they occurred to see if they fit the phenomenon of shear loading. (ii) Develop fundamental knowledge on the behaviour of orebodies under shear loading. (iii) Establish procedures for proper simulation of orebodies under shear loading; and (iv) Develop a means for simulating the behaviour of offsets between various spatial arrangements of ore lenses under shear loading.;It is recommended that in designing pillars under shear loading, the existing empirical pillar equations and design charts should not be used. The charts developed in this thesis are suggested for use in these situations.