ROCK DESTRUCTION
A basic method for destroying rock is mechanically induced stress, such as drills and erosion jets. A turbine drill for example, uses a turbine to rotate a diamond faced cutter wheel. The power output, p, can be calculated using the equation p=2 πNT where N is the rotational speed of the turbine and T is the torque on the cutter wheel. As the rotary speed increases, the torque decreases. The maximum power output of a turbine drill is when the rotary speed is between 5,000 and 10,000 rev/min.
Erosion jets can be classified as low speed or high speed. The low speed jets use abrasive particles, such as sand to erode rock at 10 to 200 m/s and the high speed jets use water at 200 to 1,000 m/s. The high speed jets are used for the hardest rock and require pressures up to 5,000 kg/cm²!
http://www.lakemac.infohunt.nsw.gov.au/lmphotos/lak010/lak01044.htm
In underground, non coal mining, the excavation of rock is done largely by conventional percussive drills. These break the rock by indentation. The drilling machine applies a force to the drill bit, giving it a velocity which hits the rock and causes it to break. The equation for the peak stress induced by the drill bit on the rock is as follows: σ = ρ cv, where σ is the maximum stress induced, and ρ, c, and v are the density, compressional wave length, and velocity of the drill bit, respectively.
In coal mining, a common machine used to drill is a shearer, a large machine with a rotating drum on each end which does work on the coal by cutting it and loading it at the same time on to a face conveyer. It is normal for these shearers to have up to 1 megawatt of power!