Deformation rebound in a blast-damaged cutback rock mass slope atOkTedi Mine was monitored over three years. Geology and structure is complex. A bull-nose shaped, up to 150m horizontally wide, slice of slope was cutback over a crest length and height of 500m and 200 m, respectively. Monitored rebound was proportional to horizontal cutback distance. Purpose, strategy and accuracy of monitoring and the interpretation approach to distinguish global pit slope displacements from the local component solely attributable to rebound of slope cutback area are discussed. Simple elastic numerical modelling did not adequately account for the observed displacements. Observed response is likely better explained in terms of non-elastic dilation and shear sliding along geological structures in the rock mass than just simple elastic slope rebound. Effective rock mass modulus required to match monitored rebound is presented. Backanalysis using more sophisticated numerical tools such as UDEC would provide better insight to factors impacting the monitored rebound.

1.1 Mine Background

Ok Tedi is a major open pit copper-gold mine in the southern highlands near the western border of Papua New Guinea (PNG) with Indonesian Papua. The current pit is 3000m by 2000m in plan and 700m deep; final depth will be 900m by 2013. Major and intermediate principal stresses are interpreted to be subhorizontal and 2.5 and 1.5 times the vertical overburden stress. The cutback area is located on the east slope. Figures 1 and 2 show early and completed stages of cutback, respectively.

1.2 Pit geology

Geology and structure is complex. Figure 3 shows pit layout and plan geology. Figure 4 presents an eastwest geological section, looking north, for the cutback slope. Geology comprises a marine sedimentary rock sequence that was uplifted, faulted and folded during the regional orogeny of the New Guinea highlands.

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