A theme of high-speed excavation of the rock entry in the coal mines has been much discussed for the purpose of reducing production cost.
The Robbins Tunnel Boring Machine, which has driven a circular 26-foot, 9-inch tunnel through shale at Oahe Dam in South Dakota, U.S.A., at a rate of 12 feet an hour without benefit of explosives, was introduced to Japan in 1956.
Although the remarkably high efficiency of this machine had been proved in the Oahe Dam project, we are rather sceptical whether similar results would be expected in Japan as well, we take into account a considerable difference in the rock formation and its physical characteristics in U.S.A. and Japan.
We, therefore, experimented the application of this machine to the rocks collected from various local mines (mostly from our own mines) and also to the sample shale from Pittsburgh (similar to the shale at Oahe Dam which proved to be suitable for economic cutting in U.S.A.), as well. as the sample graywacke from New York which proved unsuitable for economic cutting, particularly in view of its drillability.
In case of rotary drilling by means of such a specific machine as Robbins Tunnel Boring Machine in which the rotation speed as well as shape of bit and thrust have been standardized, the drilling rate of rocks depends mainly on the physical characteristics of rocks, especially abrasiveness and indentation hardness.
We have examined physical characteristics of these rock samples, in cooperation with Mining Division of Resources Research Institute, Agency of Industrial Science and Technology of the Japanese Government. We have sorted these rocks into two different groups, namely:
(1) rocks suitable for economic cutting
(2) rocks unsuitable for economic cutting.
This classification was based on the above test by us and also on the result of operation of Robbins Tunnel Boring Machine in several projects in U.S.A.
Our conclusion is that the Robbins Tunnel Boring Machine is not suitable for economic cutting of Tertiary sandstone and conglomerate which mainly constitute the rock entry of coal mines of our company.
However, with the improvement of the cutting mechanism and the material of bits, this machine may become applicable in future to more kinds of rocks and to wider extent even in Japan.