Dimension stone (Salisbury area granites)
Mining (continued)

Key cut in white granite dimension stone quarry, Salisbury area, NC. Rock near the surface is stained by weathering and does not represent to color of the actual rock product.

 


Quarrying takes a keen eye to determine the "grain of the rock." Quarrying begins by opening a 'keyhole' cut (left). 

The keyhole cut begins by exposing the rock's surface. An initial key block may be broken free by drilling a hole in the center to the depth of the vertical cuts. An explosive charge in this hole frees the block with or without damage to the block. Alternatively the block may be wedged free or forced free with an air bladder. The image to the left shows part of a new quarry are being developed around the key cut.

Cutting the stone -- After the key cut is made dimension stone blocks at this quarry are extracted using a combination of horizontal and vertical drill holes and jet burning.  Dimension stone benches are defined by the following sequential steps: 

  1. Define the back of the block by drilling closely spaced 20-foot-deep drill holes;

  2. A slot drill is used to drill 20-foot-long vertical, 32mm diameter drill holes that are spaced eight inches apart;

  3. Jet channeling (also known as flame-cutting) is used to burn both sides of the block; and

  4. A slot drill is used to drill vertical holes to create five-foot-thick slabs.

Jet channeling (flame-cutting) consists of disintegrating the rock with intense heat from the combustion of fuel oil and oxygen. The thermal shock causes the rock to spall and the fragments are blown out of the channel.  Jet channeling is being replaced by other methods.

Slot drill positioned for horizontal hole drilling (left). Jet channeling in progress (right) .