Control of weak roof strata in the iron ore mines of Lorraine

Abstract

I. PRELIMINARY NOTES I. 1. Statement of the problem TrIE iron ore mines of the Lorraine, in which a fiat-lying deposit is worked by the room and pillar method, have supported the roof in their mine roadways by roof bolts for about 10 years. This type of support has proved up to the present to be perfectly suited, both to the mining conditions, and to the class of ground to be supported, even when a mine section is locally under pressure. In the latter case, it is advisable to make clear that roof bolting is truly effective only provided that it has been carried out according to the regulations (systematic roof bolting of adequate density and length and set as soon as the headings are opened out) and is finished by wire netting when necessary. The mine operators have, however, now decided to develop, to an increasing degree, workings in new beds (siliceous beds) which are generally at greater depth and have a much less rigid roof. We therefore find roadway roofs which, though apparently sound after a first scaling followed by roof bolting, tend later to deteriorate slowly even in the absence of any pressure due to the overburden. Advances in the development of the mining equipment (blasting and loading) make it possible and perhaps economical to carry out greater advances per round when driving the headings. Blasting around uncharged large-diameter shot holes can, for example, raise the advance per round from 2 m to 3 m and even to 3.5 m. Before this method is generally adopted it is desirable to assess whether long advances have no serious consequences on the standing capacity of the roof strata. Finally, a number of mines have started, by way of experiment, to replace the roof bolts of the conventional type by rods made of indented steel and fixed in holes filled with cement mortar. Such rods, which are joined solidly to the ground over their whole length, seem to have good supporting qualities. It was therefore necessary to try to find an objective method for the evaluation of their effectiveness. In brief, we can state the problem to be solved as follows: In presence of a weak roof, what are the causes connected with the mining operations which may accelerate or delay the process of deterioration? In particular, what are the influences of the blasting method and of the advance achieved for each round ?