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TESTS OE ROOF-SUPPORTING DEVICES.
are being extracted in a surface bed and the space formerly occupied both by the chambers and the pillars is being completely filled with surplus rock taken from a lower bed.
BLASTING ROOF AND FLOOR IN THINNER BEDS.
It is a well-known fact that loose rock occupies one and two-thirds to two times the volume of the same weight of solid rock. In other words, if a cubic yard of solid rock be broken to pieces the pieces will occupy a space of If to 2 cubic yards.
We have conceived the idea of taking advantage of this fact for the purpose of cheaply producing an adequate roof support for certain classes of coal beds under the city. So far as we know, this method, in its entirety, has never been used before in any coal-mining district, and the suggestion is here made for the first time.
The process would be applicable to beds less than 6 feet thick and so situated that the shock of heavy blasting would not produce ruptures of the measures supporting adjacent coal beds. It consists simply in blowing up the floor of the mine to a depth equal to the thickness of the bed, and blowing down the roof of the mine directly over, to a height equal to the thickness of the bed. This would produce a total thickness of loose rock equal to three times the thickness of the coal bed. The rock would be well packed together and have great supporting power, and the process would be comparatively inexpensive.
This method might be adopted throughout the Dunmore and other thin seams, by blasting down the roof and raising the floor in the abandoned rooms or the roadways between the gob piles in the chambers. Wherever it is applied the effort should be to completely fill the whole width of the chamber or roadway from pillar to pillar, so that the loose rock will be confined between the pillars, thus greatly increasing its resistance to compression. The value and supporting power of this method of roof support and of other methods will be referred to below.
GOB PIERS AND TIMBER COGS.
There are localities under the city of Scranton where it has been deemed advisable in the past to build gob piers for the support of the roof overlying certain surface beds. The Lehigh University tests mentioned in this report show that this method of support lacks the merit of strength. The piers are very compressible, and as they have been built heretofore their supporting value is small. Their efficiency would be greatly increased, as shown by the differences between test No. 1 and No. 2 (pp. 83-84), by building them in circular form and carefully filling all the voids between the larger pieces with smaller particles of shovel stuff.