(also thermal boring or, in some cases, fusion piercing), a method of drilling that involves the use of a jet-piercing drill or plasma drill. It was developed in the USA in the late 1940’s and has been used in the USSR since the mid-1950’s. The Soviet scientists A. V. Brichkin, R. P. Kaplunov, I. P. Goldaev, A. P. Dmitriev, and A. V. Iagupov made great contributions to the study of the physical principles of jet piercing and the development of equipment for this technique.
During jet piercing, a solid medium, such as rock, concrete, or ice, is destroyed by spalling and fusion; in spalling, small hard particles (1–20 mm) separate from the surface of the face, which is heated to a temperature of 300°–600°C. Destruction results from thermal stresses induced by uneven heating of the surface layer of the medium. The spalling process is characteristic of granites, sandstones, and ore-free and ferriferous quartzites. During the fusion process, the heated medium is converted from the solid state into a liquid—the melt. The products of destruction are removed from the hole by a gas jet; concrete, ice, and certain rocks, such as shale, basalt, and gabbro, are destroyed during the melting process.
The use of jet piercing is expedient only in rocks susceptible to thermal spalling. This susceptibility is determined by certain of the thermal, elastic, and strength properties of the rocks, which are known as the jet-piercing criteria. Holes are usually drilled with maximum linear speed and minimum permissible diameter, the latter determined by the diameter of the jet-piercing drill. The net speed of piercing in rocks susceptible to spalling is 4–25 m/hr. An advantage of jet piercing is the possibility of widening the hole to 300–500 mm at any desired point. This is accomplished by moving the jet-piercing drill through a section of a previously drilled hole at a rate of 10–20 m/hr, usually from bottom to top.
Jet piercing is used only in opencut mining because of the presence in the gas jets of highly toxic and noxious gases, such as carbon monoxide and nitrogen oxides. The development of industrial plasma drills using water vapor as the plasma-forming gas, which ensures operation without the emission of harmful gases, raises the possibility of using jet piercing even in underground mining.
Improvements in jet piercing can be achieved by using a combination of various physical effects (mechanical, ultrasonic, and so on) with thermal effects. This makes possible an increase in the thermodynamic parameters of the gas jets and a reduction in the spalling temperature.
REFERENCESOgnevoe burenie vzryvnykh skvazhin. Moscow, 1962.
Iagupov, A. V. Teplovoe razrushenie gornykh porod i ognevoe burenie. Moscow, 1972.
Dmitriev, A. P., S. A. Goncharov, and G. A. Ianchenko. Termoelektrofizicheskoe razrushenie gornykh porod, part 2. Moscow, 1975.
K. I. NAUMOV and G. A. IANCHENKO