Slope design, Southern Ridge Cutback, The Pilbara, Western Australia

Southern Ridge is a cutback excavated into the slope of the original Mt Tom Price at Tom Price Iron Ore Mine. The mine is located on the outskirts of Tom Price town. The ore is contained within the Dales Gorge Member of the Hamersley Group, a unit widely regarded as a premium source of iron ore. Extension of the cutback proposed a final wall 400 m high, the highest for the Pilbara. The water table is about 70 m above the proposed pit floor.

PSM was engaged to prepare a slope design for the cutback, train staff in data collection methods and make depressurisation recommendations.

Due to the existing cutback configuration only one berm was accessible, albeit a wide one, for positioning the drill-rig for the investigation. This constrained the orientation that holes could be drilled.

Engineering and technical opportunities included:

• The lower most 70 m of the proposed cutback slope would be located below the water table coinciding with a thick shale unit where seepage issues had been problematic at other pits within the mine
• The proposed cutback was intersected by two splays of a major fault, the Southern Batter Fault
• Those dolerite dykes intersecting the cutback that could be mapped exhibited shearing along the contacts and were offset by small scale faults
• Bedding dips shallowly out of the wall, however, multiple-bench scale joints were mapped dipping back into the wall, but with a potentially unfavourable spacing
• Substantial historic data was available for assessment in addition to the new data collected.

Following a rigorous assessment and modelling, PSM’s tailored approach included:

• Use of a down-hole televiewer to obtain structural data for all holes, regardless of how access confined the borehole orientation
• Detailed assessment of logging data to assign rock mass strengths appropriate to the structural conditions
• Recognition of poorer rock mass conditions in a halo surrounding the Southern Batter fault splays, leading to lower rock mass strengths being assigned to these zones
• Detailed review of shale marker bands and dolerite contacts
• Recognition of slippage resulting from regional fold development and inputting this into adopted defect shear strengths.

Design of a cutback of this scale required detailed understanding of the relationships between strata, structure and rock mass and how these interact as potential failure mechanisms.  PSM’s familiarity with the site assisted in developing a geotechnical model that accurately reflected the variability in rock mass conditions due to alteration resulting from large scale faulting. PSM’s extensive experience with Pilbara geology and large scale structurally controlled slopes contributed to recommendation of a robust slope design based on a combination of kinematic, limit equilibrium and finite element analysis.