Eggers, MJ & Casparis, DL 2007, 'Pit Slope Design in Pilbara Iron Deposits - Deposit A West Angelas, Western Australia', in Y Potvin (ed.), Slope Stability 2007: Proceedings of the 2007 International Symposium on Rock Slope Stability in Open Pit Mining and Civil Engineering, Australian Centre for Geomechanics, Perth, pp. 463-476, https://doi.org/10.36487/ACG_repo/708_31
(https://papers.acg.uwa.edu.au/p/708_31_Eggers/)
Abstract: The Deposit A pit design at West Angelas, Western Australia is characterised by a long, narrow shape which 
will ultimately be some six kilometres in length and up to 200 metres deep. This paper summarises 
geotechnical investigations undertaken to date for pit slope design, describes the notable elements of the 
structural and rock mass models that characterise the deposit and discusses their impacts on slope stability 
and design.  
The deposit comprises three primary synclines which are asymmetrical in shape, north-verging and open to 
isoclinal in attitude. The southern limbs are short, steeply dipping to overturned and contain tight second-
order parasitic folds while the northern limbs are longer, planar and moderately dipping. Thrust faults are 
interpreted in some areas of the southern limbs to accommodate a repetition in sequence although there is 
some evidence this may also be due to recumbent folding. 
A major objective of the geotechnical studies was to investigate any differences in structural and rock mass 
conditions between the northern and southern limbs of the primary synclines. Several controls were 
investigated including lithostratigraphy, weathering and alteration related to near-surface and 
mineralisation effects and structural controls such as folding and faulting. A “halo” of oxidised and altered 
Banded Iron Formation (BIF) surrounds the mineralised zone, while there are some differences between the 
relatively clay-rich and clay-poor zones of the BIF units.  
The main influence on slope stability and design is structure, in particular planar sliding along low strength 
shale bands where bedding dips out of the slope. The key elements are the position, shape, orientation and 
condition of the shale bands controlled by lithostratigraphy and folding. Rock mass failure and combination 
structure and rock mass mechanisms also require checking, especially within the poorer altered BIF rock 
mass adjacent to the mineralised zone.