A pillar created during mining of the 1100 orebody at Mount Isa Mine has been observed in the field and studied, using finite element and boundary element stress analyses. Monitoring showed that, in the early stages of mining, a fault transecting the pillar limited its longitudinal load bearing capacity; at later stages, substantial shear along the fault and yielding in adjacent rock were observed. The essential features of this behaviour have been reproduced in numerical analyses which allow yielding in both fault and the rock mass. General implications of this study for future mining elsewhere in the orebody are outlined.
Un pilier cree au cours de l'exploitation minière de l'amas de minerai 1100 à Mount Isa Mine a ete observe in situ et a fait l'objet d'une etude utilisant les analyses de resistance d'element limite et d'element frontier. Le contrôle a fait ressortir qu'au cours de la première periode d'exploitation, une faille recoupant le pilier transversalement avait limite sa capacite de charge longitudinale; à un stade ulterieur, un cisaillement substantiel dans le sens de la faille et un flechissement de la roche adjacente ont ete observes. Les caracteristiques essentielles de ce comportement ont ete traduites en analyses numeriques qui tiennent compte du flechissement à la fois de la faille et de la masse rocheuse. Les consequences generales de cette etude pour l'exploitation future dans le corps mineralise sont indiquees.
Ein im Laufe des Abbaues gebildeter Pfeiler im Erzlager 1100 der Mount Isa Gruben wurde vor Ort beobachtet und mit Hilfe der Finite Element und Boundary Element Methoden untersucht. Die Beobachtung zeigte, daß eine den Pfei1er durchsetzende Störung dessen Langslast-Tragfahigkeit zu Beginn des Abbaues begrenzte, und daß spater bedeutende Schubkrafte entlang dieses Sprunges und Zusammenbrueche im umliegenden Gebirge eintraten. Die wichtigsten Kennzeichen dieses Verhaltens wurden in numerischen Analysen reproduziert, die das plastische Fließen sowohl der Störung als auch der Gebirgsmasse beruecksichtigten. Die allgemeine Bedeutung dieser Untersuchung fuer den zukuenftigen Abbau an anderen Orten des Erzlagers wird skizziert.
The 1100 orebody at Mount Isa Mine, Queensland, Australia, is a massive copper orebody which is being mined using open stope methods along with the placement of cemented rock fill to enable complete extraction of the ore. In the course of mining, details of which are given in Section 2, substantial transverse pillars are created. The presence of several major faults which pass through the orebody has been found to affect the behaviour of some of the transverse pillars, and will affect others as mining proceeds. Results of laboratory testing of rock and fault samples from the orebody are also given in Section 2. Because of the implications of fault movement for the mining strategy being employed, the development of stresses within the P519 transverse pillar, and movements on the T53 fault which transects it, were carefully monitored. These field observations are outlined in Section 3. Numerical stress analyses have now been performed in an attempt to understand the observed behaviour of P519 pillar and to elucidate the factors which govern the mechanics of a faulted pillar in the orebody. The boundary element and finite element models which have been used are described in Section 4 and, in Section 5, the results obtained are presented and discussed. Finally, in Section 6, the results of this study are assessed. The relative merits of the two stress analysis methods for analysing this problem are discussed, and the implications of the results for future mining elsewhere in the 1100 orebody are outlined.
The 1100 orebody at Mount Isa is being mined by a series of primary open stopes on an irregular grid, with pillar recovery taking place - between cemented rock fill (Alexander and Fabjanczyk, 1981). Figure 1 shows the location of the P519 pillar in relation to other stoping and Figure 2 the layout of primary: stopes and pillars in a cross-section of the stoping block immediately to the north.
The P519 pillar is located in a large siliceous and dolomitic breccia mass with an irregular upper, and uniform eastern boundary formed by a shale sequence. dipping at 650 to the west (Mathias & Clark, 1975). The main structure in the pillar is the T53 fault which cuts the pillar as shown in Figures 2 and 3. The fault consists of several branches up to 3 m thick with graphite and quartz infilling.
The monitoring of the P519 pillar was carried out as part of a comprehensive program covering the whole orebody.
The rock properties used in the numerical analyses were derived from extensive laboratory tests on samples from throughout the orebody and from further determination of elastic constants using bi-axial compression tests of overcored rock containing the CSIRO soft inclusion cell, as obtained in the stress measurement program.
