Masonry Magazine June 1997 Page. 34
Drive-in Quarry Systems
Many of the best ideas developed in an industry appear so ridiculously simple that one cannot understand why no one had thought of it previously. The concept of the "Drive In" quarrying system is such an idea.
The drive-in quarry consists of a series of multi-level terraces, called "benches". Access to all of the benches is done via ramps cut from the natural stone beds. The slope of the ramps is quite steep, usually between 40 and 45%. The bench height can vary anywhere from 5 to 8 meters, with the optimum being about 5 1/2 meters.
Separation of the Stone Masses
The granite in a quarry is normally removed in large sections, called "loaves". The size of the loaf will vary depending on the material type and quarry dimensions. The vertical dimension is governed by the bench height, which is usually between 5 and 8 meters. The depth, or distance back from the exposed face of the loaf is usually about 6 meters. The length of the loaf varies widely from quarry to quarry, and can be anywhere from 10 to 50 meters.
The first step in separation of the granite loaf is to free the ends of the loaf. This is accomplished by "burning", diamond abrasive cable saws, or high-pressure water cutting. The most frequently used method is to burn the stone away with a cutting tip using fuel oil and compressed air to support an intense flame.
Once the two ends of the loaf are cut free, the balance of the stone separation is done with explosives. The two planes that remain are separated in one effort, commonly called "L- Blasting". To facilitate the separation, the stone is perforated with holes which both define the separation plane and accommodate the loading of the explosive.
The drilling of the holes is accomplished with a percussive carbide tipped bit under hydraulic force.
The amount of explosive required to free a loaf is calculated based on the mass of the loaf that is to be freed. Charges are loaded in the horizontal holes by tying a detonation cord to a charge and pushing it to the full depth of the hole. Additional charges are pushed into the holes as required to achieve the correct amount of explosive in a uniform pattern. The vertical holes are loaded from the top with the charges tied to the detonation cord to maintain vertical spacing. Once the charges are loaded, the detonation cords are linked by a "trunk line" detonation cord. The trunk lines for both the vertical and horizontal hole charges are fitted with a blasting cap on the same end of the loaf. Ignition of the blasting caps is done electrically. After the explosives are detonated, a post blast inspection is performed to ensure that all charges were detonated and no live charges remain within the loaf. The entire process of cutting the ends, drilling the vertical and horizontal holes, loading the charges, and blasting the loaf free takes less than two weeks to complete.
Modular Sizing
The usable blocks taken from a quarry constitute a small percentage of the total amount extracted. The usable portion, or "yield", is typically between 10 and 40%, but in extreme cases it can be less than 5% of the total extraction.
Many building projects require sizes that will produce unacceptable amounts of waste if the standard block size is used. The quarry foremen are furnished with "stock lists" for all projects and will identify these atypical sized pieces. Benches will be specifically laid out and drilled to produce optimum sized blocks for use in all areas of a project.
Material Grading
The last process to occur at the quarry site is the grading of the material. This will usually be done by a very experienced individual with keen eyesight for color. The blocks will be wetted to intensify the color, after which they will be inspected for color variations, background color tones, inclusions, lines, cracks, seams, or any other property that would be unacceptable in a given product line.
Those of us who work in the stone industry appreciate the inherent variations that occur in this natural material, yet the very unique feature about a stone that we find attractive may offend our customer. In nearly every natural granite type, the clearest blocks with the most consistent color and least amount of veining will be considered the highest grade and command the highest price.
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34 MASONRY-MAY/JUNE, 1997