Authors: McInnes, D; Haberfield, C; de Graaf, PJH; Colley, C

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McInnes, D, Haberfield, C, de Graaf, PJH & Colley, C 2013, 'Mine design for below water table clay detritals mining: Marandoo Mine, Western Australia', in PM Dight (ed.), Slope Stability 2013: Proceedings of the 2013 International Symposium on Slope Stability in Open Pit Mining and Civil Engineering, Australian Centre for Geomechanics, Perth, pp. 1111-1124,

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Geotechnical investigations were carried out as part of the Feasibility Study Extension (FSE) and Trial Mining program for the development of below the water table (BWT) expansion at Rio Tinto Iron Ore’s (RTIO’s) Marandoo Mine. The Marandoo Mine site is located in the Pilbara region of Western Australia, 1,500 kilometres north of Perth. Previous studies have indicated that a 40 m thick, at approximately 40 m depth, layer of Tertiary clay, will present significant challenges relating to slope and waste dump design as well as operational challenges (trafficability and handleability). Due to uncertainty over the confidence of parameters derived from triple tube sampling and testing, conservative design parameters were adopted for the previous studies. It was recognised that significant upside could be realised through improved material characterisation techniques. The main objectives of the investigation and testing program were to: 1) characterise the physical properties and variability of the clay; and 2) obtain design parameters for the slopes for mining of the pit. This was carried out with conventional drilling, sampling and logging procedures. Hyperspectral scanning of diamond core was undertaken, which provided useful information on the vertical distribution of clay species through the detrital stratigraphy. Following review of the historic sample test results, it was hypothesised that sampling and testing methods had contributed to pre-softening of samples. Subsequent testing focused on Shelby tube and in situ testing; both resulted in significant improvement in measured shear strength parameters over triple tube sampling. A self-boring pressuremeter was used in two locations to carry out in situ testing of the clay at various depths. The results of the pressuremeter testing have indicated that the permeability of the clay is significantly lower than assumed in previous studies. An assessment of the effect of the structure of the clay on the stability of the slopes will be an objective of further studies. Key outcomes so far have emphasised the significant impact of sampling method, preservation, timely testing on derived material properties, and recognition of the limitations of various techniques in characterising heavily over consolidated clays. The material characterisation has also confirmed the critical importance of water management in mining in relation to preserving clay strength. This work has enabled detailed planning to be undertaken for studies covering trafficability, handleabilty, waste dump and slope performance; with field trials scheduled in the mine plan.

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