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consistency and defects such as joints, veins and banding all which may influence the recovery and marketability of blocks. Vertical holes are drilled to test the consistency of the colour and texture of the stone at depth and determine cut-offs. It also helps to identify horizontal or subhorizontal joints which may affect recovery or assist with mining access.
Horizontal or inclined holes should be planned at right angles to the major joint sets to evaluate the spacing. Initial hole spacing in the order of 50m is sufficient in most cases and minimises the risk of sterilising potential blocks with drill holes. A bulk sample of blocks, ranging from two to 20, can be quarried, slabbed and polished for market tests.
The quarry layout is the most important phase of the design of a dimension stone quarry. The layout of the quarry entails the most economical placement of the mining faces and levels. The orientation of the faces and levels will be determined by the orientation and spacing of fractures, cracks, faults, veins and joint sets within the deposit. Other considerations in the solid material include banding, orientation and colour.
Cognisance must also be taken of the defects within the stone to determine the position and sequencing of splitting lines to maximise the recovery and value of saleable blocks. A complicating factor is that the stone often has one or more preferential grain or cleavage directions, referred to as: freeway, second / easy way, tough way, along which it splits preferentially, even when attempting to split oblique to these directions. The flaws in the material however often run oblique to the grain direction, with the result that recovery of saleable blocks is compromised.
Making a start
The initial stage deals with mining of boulders and removal of overburden, without sterilising the underlying deposit.
If done in a properly planned and systematic way, it can aid in the initial cash flows of the project without prejudicing future development. General practice is to start from one end of a hill and advance, but where this is not possible, key cuts can be made in the middle of the formation.
Planning and developing the quarry on one or more levels, with a saw-toothed face shape on each level, allows for sufficient access for all preparation activities for loosening benches without interference from benches being extracted and allows for continuous production and blending of recoveries and qualities.
The sequencing and scheduling of the various operations involve loosening and quarrying a bench and sufficient space between levels. In most dimension stone operations, three benches are required, one being quarried, one being loosened and one already loose. It is also advisable to have one spare face to allow for blending of different recovery / yield areas to stabilise production.
Nicolaas C Steenkamp
Dolerie dimension stone with a pegmatite vein.
12 _ QUARRY SA | MAY / JUNE 2018