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Jet Piercing
(also thermal boring or, in some cases, fusion piercing), a
method of drilling that involves the use ofa jet-piercing drill or
plasma drill. It was developed in the USA in the late 1940s and has
been usedin the USSR since the mid-1950s. 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 andfusion; in spalling, small hard
particles (120 mm) separate from the surface of the face, which
isheated to a temperature of 300600C. Destruction results from
thermal stresses induced byuneven heating of the surface layer of
the medium. The spalling process is characteristic ofgranites,
sandstones, and ore-free and ferriferous quartzites. During the
fusion process, the heatedmedium is converted from the solid state
into a liquidthe melt. The products of destruction areremoved from
the hole by a gas jet; concrete, ice, and certain rocks, such as
shale, basalt, andgabbro, are destroyed during the melting
process.
The use of jet piercing is expedient only in rocks susceptible
to thermal spalling. This susceptibilityis determined by certain of
the thermal, elastic, and strength properties of the rocks, which
areknown as the jet-piercing criteria. Holes are usually drilled
with maximum linear speed andminimum permissible diameter, the
latter determined by the diameter of the jet-piercing drill. Thenet
speed of piercing in rocks susceptible to spalling is 425 m/hr. An
advantage of jet piercing isthe possibility of widening the hole to
300500 mm at any desired point. This is accomplished bymoving the
jet-piercing drill through a section of a previously drilled hole
at a rate of 1020 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 toxicand noxious gases, such as
carbon monoxide and nitrogen oxides. The development of
industrialplasma drills using water vapor as the plasma-forming
gas, which ensures operation without theemission 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
thethermodynamic parameters of the gas jets and a reduction in the
spalling temperatur
