Belikov [1] and Vlasov et al. [2] report successful thawing of frozen loose rocks in the high-frequency electric field of a capacitor with single-side electrode positioning.With single-sided electrode placement, even in a homogeneous material there is a markedly inhomogeneons electric field, the intensity of which is maximal near the surface of the electrodes and fails off progressively with increasing distance from them [3]. Clearly, the relative power dissipated in the material will fall off even more rapidly [3, 4]. A few centimeters from the surface of the electrodes the heating will be insufficient for effective high-frequency electrothermal thawing of frozen rock.Besides this shortcoming which limits the use of the field of a capacitor with single-sided electrode positioning for thawing frozen rocks, there is an equally serious drawback: owing to the necessity of thawing the whole volume of frozen rock to be extracted with this electrode configuration, the energy consumption of the process will be high [2]. * Thus, the high energy consumption on the one hand, and the impossibility of intensifying the process of thawing owing to the slow heating rate on the other, make it inefficient to use the field of a capacitor with singlesided electrode configuration for high-frequency electrothermy of frozen rocks. To make use of high-frequency energy, it is thus necessary to secure a great increase in the relative power conveyed into the frozen rock so as to intensify the process of thawing it; thawing of the frozen rock must be carried out not over the whole bulk of the rock to be extracted, but only along the peripheries of individual pieces, which are thin in comparison with the volumes of the strata. Meeting this condition will enable us to reduce greatly the cost of breaking frozen rocks; the action of high-frequency currents transformed to heat inside the rock itself should be combined with mechanical action which secures penetration of the working tool and final separation of a definite volume of the thawed zone from the solid rock.To conform with the above requirements, the working tool (working capacitor) must possess such an electrode configuration as to ensure greater power injection into the rock and heating of the frozen rock in a thin zone.Thus, the working capacitor must be constructed so that it has reasonable mechanical strength and can be introduced without hindrance into the thawed zone.These requirements can in principle be satisfied by the field created by a capacitor with plane parallel electrodes (Fig. 1).The method of high-frequency electrothermal fracture of frozen rocks (the V~TM method), developed at the Leningrad Mining Institute, combines the operations of thawing in the high-frequency field of a plane-parallel capacitor and of separation of the contoured thawed layer of pieces of frozen rock from the mass. In essence, it is as follows.* In this case, the high energy consumption of high-frequency thawing is partly due to dissipation of energy by the edges and internal surfaces of the elec...
The electrothermal (ETM) method of breaking frozen rock has recently been receiving increasing publicity [1]. At the Leningrad Mining Institute the first experimental ETM units have already been built and tested.An experimental hand-held ETM instrument working at high frequency [2] was successfully tested in [1969][1970] in driving inclined shafts in artificially frozen quicksands during construction of the "Moskovskaya" and "Zvezdnaya" stations of the Lenmetrostroi (Leningrad subway). The rate of tunneling was found to be 2-5 times greater than that of the MO-10 pneumatic drill.The first practical ETM machine in the world was constructed in 1970-1972: it utilized ultrahigh-frequency (uht) energy to cut large-diameter (800 ram) rising boreholes in permafrost strata consisting of loose rocks. Tests on the experimental model ETM machine revealed that for a weight of 650 kg and a power consumption of up to 15 kW it can cut boreholes at up to 10 m/h to practically any required depth [3].The design of the first ETM machines of a new type has resulted from the solution of a group of research problems involving primarily studies of the electrical characteristics of frozen rocks and the behavior of frozen rocks in powerful rapidly alternating electromagnetic fields (PRAE).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].Investigatiom of frozen rocks in weak and strong electromagnetic fields enabled us to clarify the mechanism of the weakening action of PRA E on them and the nature of the interaction between the PRA E and the frozen rock with allowance for the distribution of the electromagnetic field created by the radiators. Ultimately this made it possible to give, in very general terms, scientifically based recommendations for the design of radiators for any type of ETM machines. At the problem Laboratory for Breaking of Frozen Rocks by Powerful Rapidly Alternating ElectromagneticFields of the G. V. Plekhanov Leningrad State University, current research is directed towards the creation of a scientific basis for designing a new class of mining and earth-shifting machines with gYM "business ends."The main problems in this research were as follows.1. Investigation and development of efficient means of injecting PRAE energy into the frozen rock(radiators).2. Investigation and development of the best schemes for breaking frozen rock by a combination of electromagnetic (at given wavelength) and mechanical energy to create efficient ETM units.3. Investigation of the relation between the physical state of frozen rock and the rate of injection of electromagnetic energy into it, with the aid of finding the conditions for minimum energy consumption for the breaking action of PRAE and mechanical action on frozen rock. 4. Development of reliable, economical, powerful uhf power systems capable of stable operation under heavy inpact and vibration loads.G...
The energy consumed in fragmentation of a solid body is expended on elastic and plastic deformation as well as on potential energy of deformation (i.e., on increasing the potential energy of the molecules situated on the newly-formed surface) [1]. The absolute values of the two first components of the energy expenditure are determined by the fragmented volume of the body, while the potential energy of deformation is determined by the area of the newly-formed surface due to fracture. To determine the total energy expended on these types of deformation, we can use an equation developed by P. A. Rebinder [2], A=qva V4-qs ~S, (1) where A is the total work of deformation, qv is the work done in elastic and plastic deformation per unit fragmented volume, qs is the work done to form unit area of new surface, AV is the fragmented volume, and AS is the area of the newly-formed surface. Khanukaev and Dolgov [3] and the present authors [4] give methods for determining the expenditure on potential energy of deformation qsAS, as well as the value of qs, which is known as the specific surface energy.We will give a method O f determining experimentally the energy expended on elastic and plastic deformation in the fragmentation of rocks by blasting. The apparatus consists of a calorimetric system including a bomb calorimeter which secures discharge of the gaseous explosion products to the atmosphere directly after completion of the explosive reaction of the charge. The rock specimen is placed inside the bomb. Calorimetric measurements are made to find the energy QI absorbed by the calorimetric system when the rock specimen is fragmented by the explosion of the charge, together with the energy Qz of explosion of a charge of equal weight but without fragmentation of the specimen. For this purpose the latter is placed some distance from the explosive charge. Figure 1 shows experimental temperature rise curves of the calorimetric system for various conditions of blasting diabase specimens. The temperature rise curve for explosion of the charge without fragmentation of the specimen clearly represents the energy transferred to the calorimetric system simply from the gaseous products from the moment of the explosion to their release to the atmosphere. This energy can be taken as approximately constant for a given calorimetric system and explosive charge, whether or not the rock specimen is fragmented by the explosion. This inference is based on the following assumptions: I) the time of interaction of the gaseous explosion products with the calorimetric system remains constant throughout all the experiments; II) as implied by the work of A. F. Belyaev and others, there is negligible heat exchange between the gaseous explosion products and the specimen.Transfer of heat energy of the gaseous explosion products to the calorimetric system has a different time dependence from transfer of energy expended on elastic and plastic deformation during fragmentation of the specimen and converted to heat energy. TMs is clearly seen from Fig. 1, w...
In view of developments in the theory of blasting and in applied problems concerning contlnuous methods of mining, and owing to the continuing increase in the size of large-scale blasts, it is necessary to study the laws of propagation of seismic waves in rocks, and to develop experimental methods of investigation based on transducers (TD) from various measurement systems.At present, in experimental fleld research to study the seismic effect from blasting, wide use is made of a method, based on inductive transducers registering the particle velocity [1][2][3], which enables us to detect waves with the following maximum parameters: displacemsnt, i0 mm; velocity, 20 m/sec; acceleration, 104; stress, 3 9 10 s kg/cma; frequency, 20 kHz.These par~-~ters of the movements of the medium during blasts in ledge rocks are characterlstic of zones 20-30 times ROz (where ROz is the radius of the charge). Measurements closer to the center of the blast do not give usable results owing to destruction of the transducers [4]. Strengthening the transducer also fails to solve the problem, because the limited frequency response of the method cannot ~n principle permit measurements at distances less than 20ROz: The reduction in the rise time as we approach the center of the blast leads to a rapid increase in the errors of measurement owing to the need to correct the signals. Correction of the rise times in the velocity traces is also necessary if small charges are used.Thus, theuse of the induction method to measure seismic waves is methodologically Justified only in research on long-period processes occurring in large-scale blasts in the zone ou=slde 20ROz.However, to get a complete representation of the wave pattern in the medium, it is necessary to have sufficient information on the par--~ters of the blast wave at any point in space, primarily in the zone of fracture. If we knew the parameters of the seismic waves in the short-range zone, we could reliably assess the pheno-~-a occurring in the medium during various stages of its fracture, and this would serve as a basis for creating technological methods of controlling the blast energy [5].Successful experiments to measure the parameters of seismic blast waves in the shortrange zone require a correct choice of the quantity to be measured. One must be guided not only by experimental feasibility but also by the acquisltlon of the necessary information.Analysis of methods of measurement of various parameters of the stress field reveals that the most informative and universal characteristics are the motion parameters of the medium [4], among whlchj from the viewpoint of experimentation under heavy dynamic loads, the acceleration is preferable [6].Acceleratlon transducers --acceleromaters (Fig. 1) --consist of a frame 1, which is an inertial element, =he deformation of which is proportional to the force acting on it, an inertial mass 2 which creates this force in proportion to the acceleration to be measured, and a plezoelement 3. In the most popular accelerometers the plezoelements are ...
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