Geotechnical design, Kempfield silver deposit, NSW, Australia

The Kempfield volcanic-hosted massive sulphide silver deposit is located in the eastern Lachlan Fold Belt, 25 km south of Bathurst. The deposit is bounded to the east by the regional Copperhania Thrust, a north-south oriented, steeply west-dipping fault with west-over-east reverse dip-slip movement sense.  This structure strongly influences the deposit scale faulting and shearing. This was the first geotechnical investigation and geotechnically based design for the site

PSM was engaged to train site staff in geotechnical logging and collating and analysing available drilling and mapping data to develop a geotechnical model. PSM then carried out slope stability analyses and a feasibility level pit slope design.

The site had complex small-to-medium scale faulting. Engineering and technical opportunities included:

• Air-borne geophysics indicated the presence of structural lineaments but due to poor surface exposures, dip-and-dip direction of the structures could not be ascertained
• Two main lithological groups had been logged but were strongly overprinted by metamorphic alteration and deep weathering profile
• Significant RC data was available with assays for resource estimations, but with sparse cored data, particularly oriented core data to characterise the orientation, continuity, thickness and internal fabric of faults.

PSM’s tailored approach included:

• Review of core photographs and orientation data to identify and characterise fault structures of potential influence to slope stability
• Successful assessment of geochemical data co-relating the surface expression of lineaments with changes in chemistry in RC holes at depth to identify the position at depth and therefore the dip and dip direction of two lineaments
• Detailed assessment of oriented core data refined the expected orientation of four other lineaments across the site
• Development of a rock mass model based on alteration types recognised
• Development of pit slope design based on kinematic and rock mass strength performance assessments and slope experience.

Recognising the significance of structure to slope stability of the proposed pit, we used our pioneering geochemistry approach to delineate lithological and structural boundaries. The understanding of the structural setting, the geochemical assessment and the detailed site-specific assessment of oriented core increased the confidence in the geotechnical model. This allowed development of slope design appropriate to site conditions.

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