Authors: Bruning, T; Webster, S; Teet, R

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Bruning, T, Webster, S & Teet, R 2022, 'The Equotip hardness method for spatial geotechnical assessment of a Northparkes Mine block cave', in Y Potvin (ed.), Caving 2022: Proceedings of the Fifth International Conference on Block and Sublevel Caving, Australian Centre for Geomechanics, Perth, pp. 1279-1292,

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One of the biggest challenges in characterising a rock mass during block cave feasibility is to determine the spatial distribution of intact rock strength. Traditional methods such as uniaxial compression and the less accurate point load test require the selection and destruction of drillcore specimens. This is not always possible due to the large amount of core required and the expense of testing. Due to the low number of specimens usually sent for test work, the spatial distribution of rock strength is normally characterised by a few data points applied to geological rock units. This homogenises the rock mass and omits any variation in rock strength within each large-scale unit. To enable more accurate numerical modelling and geotechnical assessment, the Equotip hardness tester has been implemented onsite at CMOC-Northparkes. The method consists of a spring-loaded impact device which strikes the specimen and records the rebound velocity. The measure is then converted to Leeb hardness. At Northparkes Mine, the sampling of diamond drillcore at half metre intervals using this method provides significantly more data than traditional test work. The hardness values for rocks are then converted to uniaxial compressive strength (UCS) via site calibrated relationships. This then creates large spatially oriented datasets for use in geotechnical assessment. This paper highlights the recent Equotip logging of the MJH block cave prospect. The logging procedure, calibration and analysis methodology is presented which shows the quantitative and spatial strength distribution of the deposit. It was also found that the Equotip logging method could identify and delineate weakness zones within the deposit due to geological contacts and other structural features. This led to the ability to characterise the thickness and shape of these zones for future use in numerical modelling. Overall, the Equotip core logging method developed and implemented onsite, provides larger more spatially relevant datasets than UCS testing alone. The ability to develop and calibrate relationships to other physical measures of the rock enables more accurate assessment along with future potential in geometallurgical studies. This, combined with higher accuracy compared to point load testing, has cemented the process in geotechnical core logging at Northparkes Mine.

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