In the present paper, correlation coefficients between the numbers of outbursts which occurred in various collieries in IshikariCoal Field, Hokkaido, Japan from 1926 to 1970 and 13 types of geological characteristic values of each colliery are shown. According to the consideration of calculated correlation coefficients, it was confirmed that all characteristics which are of geological complex structure have a high correlation with the frequencyof outburst incidence.
Next blasting experiments were made ona small scale to study the back effect of blasting which is considered one of the important causes of outburst incidence. In theexperiments, the blasting conditions were selected to simulate asite where the geological structure has a crossed and complicated formation, with a fragile and weak coal seam located at the back of the charged hole tip.
As a result of the experiment, weobtained quantitative relations between the charged explosive volume and back effect and also between the charge hole length andback effect.
For example, when the charged explosive volume (
L) and the length of charged hole (
l) are increasedrespectively, the expansion rate of back material (η) which is one of the important measurement of the back effect was observed to increase in proportion to
L 0.35-0.4 and
l1.9-2.4We are of the opinion that thesefacts indicate qualitatively that both over-charging and long length hole blasting may give rise to gas outburst at coal mines.
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