Investigations into the quarrying industry in Southeast Queensland (SEQLD) indicate current production of hardrock products is approximately 16 million tonnes per annum, giving a current usage rate of 7.25 tonnes per capita per annum. Based on an increasing population rate of 1.4%, the cumulative demand of hardrock quarry products in SEQLD by the year 2046 is 1.18 billion tonnes. Comparison of this demand with total approved reserves indicate that significant shortfalls in supply are expected, especially in the Central-North Brisbane regions. Underground expansion of suitable quarries in this region could both extend the life of current operations and satisfy the predicted shortfalls in the region.
The Mt Coot-tha quarry, owned and operated by the Brisbane City Council (BCC) has a current mine life of 15-20 years before it exhausts its open cuttable reserves. Its prime location and extensive reserves of quality hornfels rock make it ideally suited to underground expansion, which would see its mine life significantly extended. Such an expansion would incur increased production costs, however this increase can be offset by two key aspects. Firstly, other market suppliers will incur significantly increased transport costs as quarries are forced further from the markets, this will increase the saleable value of hardrock material. Secondly, endorsement and applications of underground space technology as a post mining use of mining voids, will effectively act as a value adding process to the mining cycle.
Geotechnical investigations were made on the hornfels rock to determine the suitability of the rock mass to underground excavations, and to identify design limitations and optimal orientation of these excavations. Uniaxial Compressive Strength (UCS) tests gave the following results for strength and elastic properties of the intact rock mass.