53
METHODS POE ROOF SUPPORT.
cipal refuse material of the mine. In all the mining methods adopted under the city of Scranton, all the material of the coal beds that is not coal is laid aside as refuse and stowed away in the chambers, either on one side or on both sides of the mine tracks. In thin beds where it is necessary to remove some of the roof rock or take up some of the floor of the mine in order to obtain height sufficient for the mules and the men to travel along the roads, much mine refuse is produced, which is also stowed in the chambers. In beds less than 4 feet thick many chambers are filled with mine refuse or gob, as it is more familiarly termed, from floor to roof. In places this gob is merely thrown in carelessly, or is shoveled in; in other localities it is packed as tightly as possible by hand. It frequently happens that the whole chamber from pillar to pillar is packed full of mine refuse. Where there is much interstratified fire clay or bone in the coal bed there will be larger quantities of the gob, and the thinner the bed the greater will be the quantity of mine rock raised or taken down for roads. Consequently, this stowage of gob in the rooms of the mines under the city of Scranton becomes an important item when the support of the surface over these coal beds is considered. As in the case of the gob piers and the cogs, the supporting value of stowxed gob depends upon the compressibility of the material of which it is composed.
CONCRETE AND MASONRY PIERS.
The above methods of artificial support already mentioned are comparatively cheap to install. Some other methods of support used in the city of Scranton are concrete and sandstone piers. In these the material for construction has been introduced from the surface (through bore holes in the case of concrete or through shafts from the surface in the case of blocks of sandstone), and in the mine has been wrought into substantial piers by the use of cement. (See PI. 29 B.) Though these forms of piers are much more substantial than those previously mentioned, they are also much more costly; consequently fewer of them have been installed.
IRON PROPS.
In one place, namely, in the Big bed under the Central High School building in this city, a number of iron props or posts have been installed as additional roof support. There has been no subsidence of pillars in the locality where they have been used; therefore the supporting value of the props has not been tested and their efficiency is largely a matter of opinion. In any case they are costly to install, and it is entirely probable that much more efficient means might be used for the same purpose at less expenditure.