Jet Piercing

6.4-1. Principles of Operation. The jet-piercing process is a patented thermal process which depends upon a characteristic of the rock which is termed spallability. In its simplest terms, spalling is considered as decrepitation resulting from differential expansion of rock crystals due to thermally induced stresses. The number of individual properties influencing the spalling characteristics of rock are so numerous and their interaction so complex that the only truly reliable way to ascertain the spallability of rocks under thermal shock conditions is to expose them to these conditions. However, laboratory and field work have determined the principal thermal, physical, and mechanical properties of rock which affect spallability (10, 18). The approximate relationship between spallability and these characteristics is as follows: [ ] where To is the critical temperature at which, it is believed, a rock becomes elastic or sufficiently plastic to prevent further spalling action. Thermal diffusivity takes into account the amount of heat used in saturating the medium itself during the migration of a heat front. The rate and degree of dimensional changes which influence the spalling tendency of the rock will depend upon the characteristic of the heat gradient in the rock. The greater the spalling tendency, the higher the piercing rate. Those properties most favorable to a high degree of spallability are: 1. Large linear thermal expansion at temperatures below 700°C. 2. High thermal diffusivity at temperatures below 400°C. 3. Equigranular interlocking structure with little or no fine-grained clay or mica alteration products. 4. No significant amounts of soft, low-melting, or elastic mineral or materials which yield after thermal decomposition.