ABSTRACT
This paper presents a case study where Cenozoic age detritals and an ancient buried landslide have
been modelled in the final walls of an iron ore pit. Orebody 25 is located in Western Australia’s Pilbara region
and is an operating pit where a redesign of the final wall cutback was required. The existing slope design
utilised a simplistic geological model of mixed conglomeratic, calcretic and clay –dominated detritals overlying
deformed bedrock sequences. This simplicity resulted in a conservative design that was not reflective of the
slopes performance to that point. Further field investigation was carried out with the results assessed in
conjunction with historic drilling, mapping and laboratory testing. A detailed geotechnical model was developed
including three conglomerate and two clay dominated units and later-stage calcrete overprinting. The clay units
were of significant importance to the stability assessment and resulting slope design due to the thickness and
poor strength conditions indicated by the laboratory and field testing. However, the drilling investigation shed
only limited understanding as to the origin of the clays and thereby how they should be treated as a component
in the stability analysis. A comparison of the ratio of various geochemical elements measured by assay was
made between the clays and 1) shale bedrock and 2) clays of known detrital origin from other nearby sites. This
indicated the clays had a strong affinity with weathered bedrock. This and other textural features resulted in the
interpretation that the clay units were relic debris preserved following ancient landslides in bedrock shale. Using
laboratory testing data and calibration of results for a landslide origin, stability analyses were run, resulting in a
steeper slope design being implemented. Excavation of the final cutback is now in progress with the model
confirmed as appropriate.