Groundwater, Pore Pressure and Wall Slope Stability – a model for quantifying pore pressures in current and future mines.

Abstract

The Hamersley Province, located approximately 1200 km north of Perth, Western Australia forms part of the southern Pilbara craton, an extensive area of Band Iron Formations (BIF). The area has a high economic significance due to several enrichment stages of the country rock (BIF) resulting in several large high-grade iron ore deposits. Mount Whaleback near Newman and Mount Tom Price are the largest deposits, where reserves have been estimated at 1400 Mt and 900 Mt respectively. These ore bodies have been quantified as being high grade resources at approximately 64 % iron, with a high lump to fines ratio, and low impurities. The Mount Tom Price ore body is a hematite-rich ore, associated with a variety of shale and some dolomitic units (MacLeod et al., 1963, MacLeod, 1966, Taylor et al., 2001, Morris, 1980).

The local hydrogeology of the Mount Tom Price area involves two main aquifer systems. The Dales Gorge member of the Brockman Iron Formation with contributions from the upper mineralised section of Footwall zone make up the main semi confined aquifer within the area. The underlying low permeability Mount McRae Shale and Mount Sylvia Shale lithologies separate a secondary aquifer which is located within the Wittenoom Formation. A dewatering program within Mount Tom Price has been ongoing since installation in 1994.

Within the open pit mining industry, pits depths are increasingly being deepened as the easily accessible surface ore has been removed. This involves excavating pit walls below the existing groundwater table, which can lead to instabilities within pit walls. Added to this is the timing and economic considerations which need to be accounted for in a working mine. As dewatering and depressurisation are pivotal to the extraction of ore resources below the groundwater table, there can often be considerable time pressures to maintain planned mine developments (Hall, 2003).

The South East Prongs pit, located within the Mount Tom Price mine, holds some of the most valued low impurity, high grade hematite ore. Structurally the South East Prongs is unique as the deposit lies in the base of a steeply dipping double plunging syncline, intersected by the Southern Batter Fault which runs parallel in strike to the Turner Syncline.

The current pit floor of South East Prongs is located at 600 mRL. The long term development plan for the western end of this pit includes a further 30 m of excavation to a final depth of 570 mRL. This currently poses a number of stability issues that require resolution before any development can be undertaken.

A conceptual understanding of flow dynamics within structurally complex wall rock environment has been generated through the utilisation of finite element numerical modelling. The complex structural setting within the northern wall of the South East Prongs has shown to interact with high conductivity lithologies to promote preferential flow of groundwater from the underling Wittenoom Formation aquifer. Recharge to the semi confined DG aquifer occurs as groundwater travels up shear zones within the South East Prongs Fault Zone before migrating along Brunos Band.

An investigation into alternative methods of depressurisation has been recommended to ensure the ongoing management of pore water pressures within the northern pit wall during planned pit cut backs. Limiting recharge from the WF to the pit through stated preferential flow paths has been identified as a potential issue when the remaining DG aquifer is removed. Maintaining the proposed dewatering buffer will be difficult to achieve using the current system.

The ability to design optimal pit shells for access and ore recovery as well as an effective dewatering and depressurisation system relies heavily on the a sound geological model. Further to this, time allocations to ensure forward planning deadlines are met can be significantly interrupted if adjustments to initial plans are required.