Determination of the strength, brittleness, and fracturing of in-situ coal
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OF T H E
AND FRACTURING I.
G.
STRENGTH,
OF I N - S I T U
Ishchuk
and
V.
BRITTLENESS,
COAL
G. K a n o n i k i n
UDC622.23.02
Correct evahlation of the strength, brittleness and fissured state of in-situ coal is important for establishing its resistance to working and solving other practicaI problems. At present, the strength properties are assessed by summarized (average) characteristics obtained by on-face methods. These indices not only take into account the strength of the coal, but also its fissuration, stress pattern, etc. Such methods are determination of resistance of coals to ploughing by a DKS instrument [1J, of the resistance to cutting by drilling holes with a SDM instrument [2], and of the resistance to fracture on the basis of the specific water absorption of the seam as an index [3, 4]. In order to solve a number of mining problems, it is essential to differentiate the strength properties of the in-situ coal; an individual definition must be made of its fissuration, its resistance to fracture, etc. It is difficult to do this with the a b o v e - m e n t i o n e d methods. The fissured state of coal is evaluated by two methods [5-7] which involve determination of the distance b e tween the cracks and calculation of the number of cracks per unit length or volume. Neither of these methods is accurate, nor do they take account of the whole system of intersecting cracks in the seam (inherent and tectonic cracks and those resulting from rock pressure) or the enlargement of the cracks in time. There are no methods for determining the brittleness of in-situ coal, nor its resistance to blasting. The Skochinskii Mining Institute and UkrNIIGidrougol' (Ukrainian Coal Research Institute) have evolved and checked a method for determining the brittleness, fissured state, and resistance of coal to fracture under field c o n ditions at various seams in the Donets coalfield, the method being based on the filtration properties of the solid. Essentially it is as follows. P, k g / c m ~ 8
C
/)
t, s e c
Fig. 1. Pressure P of water injected into the hole versus time t.
A 45 m m hole (or shothole) is drilled in a production or development face, of sufficient length to intersect the sector investigated. At 1 m from the end, the hole is sealed off with a hydraulic plug. A hand, portable, or stationary pump is used to inject water through the sealedoff surface. The pump must provide a water pressure sufficient to fracture the seam. Water fills the sealed cavity of the hole and then begins to work into the cracks in the seam. As soon as the cracks adjacent to the hole have been filled, the pressure of the injected water increases sufficiently to open the closed cracks and form new cracks. Injection continues as a destruction process, so that hydraulic fracturing of the seam occurs and there is a reduction in i n j e c t i o n pressure. Figure 1 is a plot of the pressure P of the infused water versus t i m e t. It has been established experimentally that the pressures at which the coal begins to break up (maximum
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