When rock is broken up by the cutting head of a dynamic plow, a part of the impact energy is expended on the generation of elastic waves which are attenuated as they travel through the rock. The energy carried by the elastic wave is lost unproductively in the rock, and the magnitude of the loss depends mainly on the strength parameters of the rock, together with its fracture parameters and the total energy of the impact.in bench tests of a powerful dynamic plow on a coal-cement block, we made special investigations in order to estimate the amount of energy expended on generation of an elastic wave for various impact energy values. For this purpose, in the coal-ce~nent block at various distances from the cutting edge of the head of the plow, we measured the intensity of the elastic wave emitted at impact.The stresses in the wave were rectxded with specially developed three-component temometric gages fixed in the block. Each gage was a cube, 60x 60x 60 mm in size, made of strong cement. In the cube was embedded a shaped steel p.late, to which wire strain gages were glued in three planes and carefttlly insulated. Before being fixed in the block, each cube was calibrated in a press. The signals from tlm strain gages were amplified by a UTSI-VT-12 rensometric amplifier (transmission band 0-7 kI-Iz) and recorded on photographic paper by a K-10fi oscillograph.The fracture parameters of the rock at the time of the measurements were as follows: the thickness of the plowed slice varied between 15 and 20 cmi the angle of attack of the blade was 10"; and the total impact energy for one blade varied between 200 and 700 kg-rn. The resistance of the coal-cement block to fracture Ok) was found with the aid of an SDM dynamometer drill, and averaged 240 kglcm. The scheme of the measurements is t ~,~Y ~ ~=2o cm l~s;l-VZ-19 ~=={KsI~

In impulsive rock breaking, the question of the best shape of the active force pulse on the bit is very ~mpor-rant. It is involved in the design of powerful impulse planes.The problem of the optimum relation between the duration of the active force pulse and the natural period of the bit-rock system was discussed by Dokukin [1]. The optimum ratio is considered to be that corresponding to the maximum dynamicity coefficient 15. The dynamicity coefficient is defined as the ratio of the static force to the amplitude of a force pulse of finite duration with an equivalent action [2]. Optimization of the dynamicity coefficient involves trying to get the maximum breaking effect per unit force pulse amplitude. The system bit-rock is regarded as a linear one-mass vibrating system with rigidity (quasielasticity) k and damping H.The frequency properties of such a system depend on two constants: the natural frequency u) 0 = 2rr f0 = 4 k/M and the reduced damping factor g = H/2M w0, where M is the mass of the bit. According to determinations made in tests of impulse planes on a coal--cement block [1], g = 0.6-0.7.If the active force pulse is a half-period of a sine wave, with duration At, the dynamicity coefficient of the bit-rock system depends on the ratio of the duration of the active force pulse to the natural period of the system (see Fig. 1). Let us consider the curve for g = 0.6 in more detail. For small values of the argument, for 0 _< 2At/T 0 = 2f0At _< 0.7 the curve 8 =/3 (2At/T 0) coincides with a straight line making an angle of 45" to theaxis. In this case the dynamicity coefficient is equal to the ratio of twice the force pulse duration to the natural period, B = 2At/To = 2:oAt.The static force equivalent to the pulse is To the right of the value At = 0.35T 0 or 2At/T 0 = 0.7, the curve for the dynamicity coefficient begins to flatten out, and the static force equivalent to the pulse p becomes less than 4f0 p. In this range the active energy for fracture of the rock is not fully utilized, although the coefficient of dynamicity continues to increase with the pulse duration up to 2At/T 0 = 2. Finally, for pulse durations greater than T 0, the impulsive action becomes equivalent to a static value which is equal to the force pulse amplitude.The right-hand boundary of the region of complete utilization of the energy on fracture, AEop t = 0.35T 0' is the optimum duration, because it permits fracture with a dynamicity coefficient 15 which is maximal forthe region in which a fall in static force equivalent to the pulse is still not observed. The optimum duration depends on the natural frequency of the bit-rock system, which is in turn governed by the strength properties of the rock, the A. A. Skochinskii Institute of Mining, Moscow.

To determine the tendency of a coal seam to bunts or other dynamic phenomena -caving, sudden spills of coal, shock bumps -use is made of its seismic activity (noise) [1][2][3][4] and the nominal strength index, determined from the depth of pene~ation of a pointed object into the seam under the effect of a standard force [5,6]. It is assumed that the noise, i.e., the number of fissures arising per unit time during working of the seam reflects the capacity of the seam to undergo brittle fracture. The greater the seismic activity the more readily will the seam fracture and the sooner will it lose stability. Therefore periods of high seismic activity indicate a heightening of the danger of dynamic phenomena. Although very primitive, such a viewpoint is useful because it agrees closely with practical experience: dynamic phenomena are actually manifested during higher seismic activity; this enables one to make a practical seismic prediction of the burst proneness of a seam from variations of the noise [1][2][3][4]. As regards the second parameter (the nominal strength index), it is assumed that it reflects mainly the natural strength of the coal seam: the greater the depth of penetration of the point, the weaker the seam and the more likelihood of the occurrence of dynamic phenomena [5]. This parameter is used for making local forecasts of the likelihood of a burst, the aim being to determine in good time the tendency of coal seams or their individual sectors to exhibit dynamic phenomena [6].It was of interest to determine to what extent both these parameters, which characterize from different angles the strength of a coal seam and its tendency to fracture, depend on rock pressure and to what extent they are mutually correlated. In 1967 a great number of seismic measurements were made by the procedure described in [4] in the steep burst-prone Devyatka seam (sector No. 78, Yunkom colliery, 716 level), accompanied simultaneously by strength measurements with an instrument designed by G. N. Feit [5]. Furthermore, in this sector episodic observations of the pressure curve were made using a hydraulic transducer designed by L. 13. Famin [7]. In the 10 months of these observations the face advance was 400 m.The strength determinations were performed in the lower part of the working-in the cross-hole of the haulage road and in the first and second benches. * The measurement lines were located in the gate ends of the face, in the middle part, and in the foot of the corresponding benches. The measurements were made once per day: in the first and second benches the measurement sites were located at 2 m intervals along the strike of the seam. In the cross-hole of the haulage road the measurements were less regular owing to the nonuniform advance of the latter. Measurements were performed at the surface of the working face; at each measurement point we made five measurements of the depth of penetration and then found the mean value. This Value, l, is one of the correlation parameters.The seismic measurements were made [1-4] with a ...

These concepts could be used to describe other forms of destructLon of rocks around workLegs in the presence of geologLcal faults, such as squeezing-away of coal and roof inrushes in thick units. LITERATURECITED

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