Two and three-dimensional stress analyses of an idealised geometry of the 1100 orebody at Mount Isa, Australia have been undertaken by the boundary integral method (B.I.M.). Stress analyses were conducted for both the cases of transverse pillars present and removed. The transverse pillars were reduced in stiffness to approximately account for geological weaknesses present in the pillars. All of the transverse pillars were found to be under approximately constant biaxial compression for at least 80% of their respective volumes. The extraction of the transverse pillars resulted in almost doubling the circumferential stresses in the crown, floor and abutment areas. There was found to be close agreement between two and three dimensional analyses for the case of 50% extraction of the orebody. The advantage of the B.I.M. in requiring only discretization of the boundaries is clearly illustrated for this example as being considerable, compared with numerical methods requiring discretization throughout the volume, such as, the finite element and finite difference methods.
The 1100 orebody is the major producing copper orebody at the Mr. Isa Mine in Queensland, Australia. The orebody has a known strike length of 2,500 metres containing ore reserves in excess of 100 million tonnes. The mineralized zones is generally 300 metres in width, with an overall bedded trend dipping between 450 and 65 o westwards and is truncated at the base by fault implaced basement rocks¹. The height of the mineralization is variable and in some areas exceeds 300 metres. A typical cross section is shown in Figure 1. The ore rocks consist of silicious and dolomitic breccias within a pymitic shale sequence and have unconfined compressive strengths ranging from 150 to 250 MPa. The ore zone is transversed by several major fault sets which are illustrated in plan in Figure 2.
