Granite is a hard brittle stone, principally because of its high quartz mineral content. It is difficult to drill through and even harder to split without causing unwanted fractures. In the old days drilling was done by hand by either a crew of two (1 man jack) or a crew of three (2 man jack). In either case some poor soul had to hold the drill then rotate it a quarter turn in the hole after every blow of the sledge hammer. With a 2 man jack, 2 men had sledge hammers and successively rained down blows on the drill. It took strength, nerves of steel and a great deal of trust. Needless to say, accidents were commonplace. Steam machine drilling came into the quarries in the 1870s and in turn was replaced by pneumatic (compressed air) drilling in the 1890s. Despite its massive appearance granite does have planes of weakness in it due to the preferred orientation of swarms of microscopic short cracks. The ‘Rift’ plane is parallel to the orientation where most of these micro-cracks occur, whereas the ‘Hardway’ is at 90 degrees to that orientation. Where possible granite is split along the Rift by using the ‘Plug and Feather’ method. First shallow (4-6 inch) closely spaced (6 to 12 inch apart) holes are drilled, all in a straight line. Steel shims (feathers) are placed in the holes with the shims aligned to the line of holes. Steel wedges or plugs are placed between the feathers in each hole and these are tapped in succession along the line until the granite breaks cleanly on a plane corresponding to the line of holes.
In deeper quarries compressive forces tend to bind the rock in place. Often a ‘key block’ must be removed from a particular sheet (also referred to as a layer or bench) before other blocks can more easily be extracted. The image on the lower right shows such a key block having been cut and removed.
