BUSINESS
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