Mathematical principles of electrothermal enlargement of boreholes

Abstract

Drtvlng vertical or steeply sloping mine workings in permafrost is well known to be one of the least mechanized and inefieleientoperatlons in the underground working of aUuvtal deposits and the fabrication of underground engineering structures, and It leads to marked Impairment of the economic Indices.With existing methods (drilling and steam-heating of predrilled holes) the rate of drivage of vertical workings tn frozen rocks for communication with the surface Is only 2.5-5.0 m/day. It ts therefore evident that new, more efficient methods must be found.At the Leningrad Mining Institute an electrothermal (ETM) machine for borehote dtiUfng has been constructed: it is the first practical specimen in the world [1]. It can give a tenfold Increase tn the rate of driving workings.The physical basis of the machine Is the discovery that frozen rock Is softened by a powerful high-frequency electromagnetic field [2], the energy of which is absorbed by the rock. After absorption of a sufficient quantity of energy the rock becomes suttable for mechanical working by rock-breaking cutter bits.Inasmuch as during widening of a borehole there are two surfaces accessible for working, it was possible to design a comparatively simple ETM cutting head with efficient combined action on the frozen rock -softening by a uhf eleetromagnetie field at the stdes of the hole and mechanical rock cutting tn a cavity perpendicular to its axis. A diagram of the cutting head Is shown tn Fig. 1 (the horizontal arrows denote the direction of the flux of electromagnetic energy, and the vertical arrows denote the direction of the mechanical forces). From this figure we see that the machtne makes use of a two-stage system of electromagnetic radiators and rock-cutting elements. We will show that this type of construction is more efficient than a one-stage machine for driving large boreholes (800 mm or more in diameter). The physical necessity of making two-stage (or tn principle even multistage) ETM cutting heads is evident from the exponential attenuation of an electromagnetic field In the rock.In this article we will attempt to calculate some parameters for such a machine. In the first place, we were Interested In the Influence of the electromagnetic field on the choice of optimum driUing conditions. Distribution of Energy Sources in the Rock. The electromagnetic field crea~ed by the ETM emitter must possess the same symmetry as the hole, i.e., it depends only on the coordinates p and z. Because the mechanical action Is exerted on a layer with a thickness which Is comparable with the original dimensions of the borehole, we can assume that within this layer the field depends practically only on p.In this case, Maxwell's equatiom for an electromagnetic field which changes with time as exp iwt are [3] as follow s:~' dp (pH~)-----ikeEz,l d dE = + ik~tH~, T" dp (pE~)=--ikpHz, Hp-----0. dp E(0. E @. E z) is the electric field, H(0, Hir Hz) is. the magnetic field strength, ~ Is the dielectric constant of G. V. Plekhanov Leningrad Mining Institute.

Electrothermal breaking of frozen rocks (Principal problems and research results)

“…Effects discovered included weakening of frozen rocks in PRAE [4] and interaction of PRAE with frozen rock, manifested in extension of the zone of intensive penetration of electromagnetic energy to the interface between media [5].…”