K.J. Rankine

Chapter 18 Geotechnical aspects of hydraulic filling of underground mine stopes in Australia

Abstract Mining for minerals creates large voids underground which have to be backfilled to provide regional stability for subsequent mining operations. Hydraulic fill is one of the most popular backfill materials used to fill these underground voids, and is generally placed in the form of a slurry at water contents in the range of 30–45%. Porous brick barricades are placed across horizontal access drives to retain the hydraulic fill, allowing free drainage and enabling the hydraulic fill to settle under its self-weight. Several accidents have been reported worldwide, where the porous brick barricades have failed, resulting in an inrush of the hydraulic fill slurry into the drives claiming lives of miners and causing heavy economic losses. A recent example of this is the barricade failure at Bronzewing Mine in Western Australia, where three miners were killed. The problems associated with the hydraulic filling of mine stopes in Australia, with special references to the numerical modelling and laboratory studies carried out to improve the current state-of-the-art in hydraulic filling of underground mines are discussed in this chapter. At James Cook University, extensive laboratory tests were carried out on more than 25 different hydraulic fills obtained from five different mines, in an attempt to fully understand their behaviour and to develop a geotechnical database for the mines. The data from laboratory tests compare well with the in situ measurements. Tests were also carried out on two types of porous barricade bricks that are commonly used in Australia, to quantify their permeability and load-deformation characteristics. Fast Lagrangian Analysis of Continua (FLAC) and Fast Lagrangian Analysis of Continua in 3-dimensions (FLAC3D) were used to numerically model the drainage through the hydraulic fills and the stress developments within the fill. Due to arching mechanism, the vertical normal stresses within the stopes are significantly reduced. Typical parameters for hydraulic fills and porous barricade bricks are given in this chapter and these would be very valuable in numerical modelling exercises when no other data are available.

A Numerical Analysis of the Arching Mechanism in Paste Fill During a Complete Mining Sequence

ABSTRACT A newly developed FLAC3D finite difference model, was used to study the arching mechanism in a paste filled stope at BHP Cannington Mine. Arching was investigated through a full mining sequence and with respect to various base to height and width to depth aspect ratios. An in-depth understanding of the arching effect within the stope and through the mining sequence will enable more accurate predictions of vertical stresses within a stope and thus backfill strength requirements. At present the cement which is added to the paste backfill constitutes approximately 20% of total mining costs at Cannington, thus a reduction in the cement content, even if small, will result in significant cost benefits.