MINING CONDITIONS UNDEE CITY OF SCEANTON, PA.
64
places under this city, and also of other devices we were cognizant of or have ourselves originated, but the supporting values of these various devices, as indicated in the tables of tests presented, have convinced us that the method of flushing material into the mines by means of water is the best and only well-tried method.
The method we have originated and experimented upon to some meager extent—that of providing roof support by simultaneously blasting the floor and the roof of the coal mine to form a permanent pillar from the debris resulting from the blasting—is an economical one, and has its advantages for certain special localities where the overburden is light. Still, the amount of compression to which the pillars of this sort would be liable is considerable and the extent of the possible disrupting effect of the heavy blasting upon the strata is unknown. Consequently, much more extended and practical experiments or tests of the method must be made before its value and importance as a roof supporting device can be established.
Therefore, the only method we feel like recommending, after a careful study of the conditions, is the flushing method.
As before mentioned, the best protection for the surface from caves would require the complete filling of the mine openings with culm, sand, or other form of material, and even then there would be more or less subsidence if pillars were removed and filled-in material were substituted. The excessive cost of completely filling the openings, and the tremendous magnitude of the project makes this plan prohibitive.
In our opinion reasonable protection can be afforded by the introduction of artificial pillars of flushed material, reinforcing the natural coal pillars to the extent of relieving them of one-third of the overburden. Such artificial pillars to be located about as follows, reference being had particularly to the plans shown in Plates 1 to 24, which are part of this report.
Owing to the great importance of substantial support for school properties, we would suggest that in each bed mined under any school, for at least 50 feet outside the lot limits, or rather for a distance equal to one-half the depth of a bed below the surface, the openings be flushed full of the best available material. The school properties—being located in all parts of the city, and over widely varying mining conditions—would afford good starting points from which to space the additional artificial pillars necessary for the protection of the surface elsewhere.
Following out the above scheme in systematic order, we would recommend the installation of artificial pillars of flushed sand or flushed culm at each street intersection where the city blocks are of the usual size, about 5 acres, the present coal pillars to remain. In the case of all coal beds of greater depth than 150 feet, such pillars,