ABSTRACT

An exploratory study has been conducted on seismicity and the state of stress induced during open stoping in the 3000 Orebody of the Deep Copper Mine, Mount Isa. Inversion of seismic records has been used to locate sources of seismic events, and seismic events and related static deformation have been correlated with rock structure, mining layout and excavation sequence. Fault plane solutions for slip events are consistent with the orientations of structural features which transgress the orebody. Further, for each type of slip plane, a simple Coulomb friction relation between shear and normal stresses is sufficient to describe the condition at slip. The stress paths leading to slip for virtually all seismic events involved a reduction in the state of fault normal stress from the initial condition.

RESUME

Une etude preparatoire a ete conduite sur la seismicite et l'etat des contraintes produites lors de I'exploitation du reservoir minier 3000 de la mine Deep Copper, Mount Isa. L'inversion des mesures seismiques a ete utilisee pour localiser les sources (hypocentres) des evenements seismiques, et les evenements seismiques et la deformation statique induite ont ete correles avec la structure de la roche, I'amenagement de la mine et Ie plan d'exploitation. Les solutions de plan de failles des evenements de glissements sont compatibles avec I' orientation des caracteristiques structurales qui traversent Ie reservoir minier. De plus, pour chaque type de plan de glissement, une simple relation de friction de Coulomb entre les efforts tranchants et normaux suffit à decrire la condition de glissement. La trajectoire des efforts conduisants au glissement pour presque tous les evenements seismiques comprend une reduction de l'etat des contraintes normales de faille par rapport à leur condition initiale.

ZUSAMMENFASSUNG

In der Erzlagerstatte 3000 der Deep Copper Mine am Mount Isa wurde wahrend einer Oeffnungsphase eine exploratorische Studie zur Seismik und zum Spannungszustand durchgefuehrt. Dabei wurde eine Inversion der seismischen Aufzeichnungen benutzt, um Quellen seismischer Ereignisse zu lokalisieren. Seismische Ereignisse und entsprechende statische Deformationen wurden mit der Gesteinsstruktur, der Minenauslegung und der Abbaufolge in Beziehung gebracht. Die Lösungsansatze in den Faltebenen wurden fuer Abgleitereignisse als konsistent mit der Orientierung der Strukturmerkmale gefunden, welche den Erzkörper durchziehen. Weiter wurde fuer jeden Typ von Gleitebenen festgestellt, dass eine einfache Coulomb'sche Reibungsbeziehung zwischen Scher- und Normalspannungen genuegt, um die Gleitbedingungen zu beschreiben. Die zur Abgleitung fuehrende Spannungumlagerung brachte bei praktisch allen seismischen Ereignissen eine Erniedrigung des Normalspannungszustandes in den Falten im Vergleich zum Ausgangszustand.

INTRODUCTION

Mines whose ore reserves extend to depth must excavate ore safely, reliably and economically from deep workings. At the Mount Isa Mine, Queensland, Australia, the Deep Copper Mine involves current and planned extraction in the 3000 Orebody at depths exceeding 1500 metres. An economic rate of production can be provided by an open stoping mass mining method, in which several stopes are mined simultaneously. Large scale deformation involving slip on discontinuities in the orebody and the host rock mass is a ground control issue to be taken into account in extraction planning and stope and pillar design. Static and dynamic slip on planes of weakness provides the potential for large deformations and failures in stopes, pillars and service excavations. Two developments provide the opportunity for advances in mining science and practice for mass mining of deep orebodies. The first is the development of reliable hardware for recording of mine seismic events and of scientifically sound methods for source location, seismic moment and seismic energy analysis. The second is ready availability of methods for computational analysis of discontinuum behavior of rock masses. For sparsely jointed rock, both the boundary element method and the distinct element method of analysis can account explicitly for slip on planes of weakness, providing a mechanically appropriate model for discontinuous deformation of the rock mass. Two- and three dimensional versions of the methods can be used, in conjunction with seismic monitoring, to map rock mass displacement and degradation, in detail sufficient to predict mining conditions ahead of the advance of stoping. There are two modes of impulsive failure of a rock mass which cause seismicity. In the first mode, under a small perturbation of the equilibrium condition, a highly stressed pillar or rock near an excavation surface may crush or rupture explosively, causing local damage in the adjacent excavation (Cook, 1967). A body wave with a high P component radiates from the source. For the second mode, a suitably oriented plane of weakness may transgress a zone of stress concentration. If the condition for slip on the plane is satisfied, unstable displacement causes a seismic event similar in many ways to a natural earthquake (McGarr, 1971).

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