MINING CONDITIONS TJNDEK CITY OF SCRANTON, PA.
58
CONCRETE PIERS.
During the progress of this investigation many persons have called our attention to concrete piers as suitable for firm roof supports. This material is so costly, however, that we have hesitated to recommend it except perhaps in special cases where very valuable surface property requires unyielding support and the expense of such support is no object.
We have tested the strength of one sample of concrete composed of about the cheapest good materials available for this locality, i. e., cement, sand, and gravel. The results of this test are shown in Table 3, test 15, and Table 4, test 14 (pp. 55,59). It will be noted that such concrete is firm and comparatively unyielding up to a certain maximum strength sufficient to cause about 3 per cent of compression and cracking, beyond which a much less weight will crush it to powder. This latter characteristic is an exceptionally bad one for a mine roof support, because such piers are liable, under the excessive stress due to a general squeeze, to collapse quickly, and thus permit the sort of caving that resembles a small though severe local earthquake in its suddenness and excessively damaging effect on the surface improvements.
DRY FILLING WITH BROKEN STONE OR SAND.
For the construction of isolated and more substantial low-cost piers in the surface beds under the city, to take the place of the gob piers for the purpose of the local support of valuable surface improvements where general flushing is not convenient, we strongly recommend the filling with broken stone or sand, or a combination of these materials, through bore holes from the surface.
Filling of this sort should be spread out by hand in the inside of the mine so that it completely fills the whole width of the space from pillar to pillar, and thus, being more or less confined has its supporting power increased. If broken stone is used the relative proportion of the voids should be ascertained and an amount of sand, coal ashes, or other fine material added sufficient to fill these voids.
By this method one bore hole 4 to 6 inches in diameter would be necessary for each pier, and inasmuch as these materials, if flushed in by water, are much more closely packed and have from ten to fifteen times the compressive strength of dry filling, it would always be wise, if water is handy or is not too costly, to flush the material in, using in some cases the city water, or the water from the gutters during the storm seasons for the purpose. The more sand there is in the mixture the better it will be, for our tests have shown that clear river sand is the strongest and least yielding material available, if it is flushed into and confined in the limits of a mine chamber; moreover it is about the cheapest material obtainable for this purpose in Scranton.