Geotechnical modelling, F Deposit, The Pilbara, Western Australia

F Deposit is the western-most deposit of the Mining Area C iron ore mine 100 km northwest of the town of Newman. The mine is located at the hinge of a deposit scale parasitic synclinal fold within the north dipping limb of the Weeli Wolli Anticline. Ore is concentrated within the keel of the syncline such that both the north and south walls of the proposed pit form dip slopes.

PSM was engaged to develop a geotechnical model for the bedrock, including interpretation of 30 section lines covering 16 cored and 400 non-cored boreholes, associated down-hole geophysical as well as geochemical and structural interpretation.

Engineering and technical opportunities included:

• Sparse cored data to characterise the thickness and internal fabric of faults
• Complex faulting resulting in either repetition or loss of strata depending on fault type
• Majority of drill-holes were drilled within the ore-body zone, with less available for characterisation of the final wall area
• Undulations in bedding orientation due to parasitic folding at a wavelength shorter than drill-hole spacing, leading to increased reliance on mapping for overall bedding shape.

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

• Interpretation of sections with sections oriented perpendicular to the fold axis to enable characterisation of the deposit scale parasitic folding of the limb
• Identification of shale marker bands within the stratigraphic sequences
• Interpretation of structural orientations including scale of parasitic folding, fault orientation and thickness, joint propensity, using optical televiewer
• Mapping of surfaces to ground-truth interpreted structures from the drill-hole assessment
• Development of 3D wireframe.

PSM’s geological and structural modelling and slope design allowed structures of significance to geotechnical design and slope stability to be included (bench to multiple bench scale). Previously, the client had developed deposit scale structures with a focus on ore definition. The geotechnical mining solutions present within the PSM approach resulted in improved confidence in the client to develop slope designs.

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