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PLANS, SPECIFICATIONS, AND COSTS.
if at the block corners, might be sufficiently near together; but for less depth the strata might be too weak to bridge the whole width of a city block, even though the usual coal pillars were left in place. Therefore it would be better, in the latter case, to install pillars in the center of each block also, in which case the pillars should each be about one-half as large as if located at the block corners only.
In locating artificial pillars intermediate between school properties, advantage should be taken of those places already flushed by the coal companies. It will be found that considerable filling of this sort has been done.
The question of the size and consequent cost of artificial piers is approached by us with much hesitation and caution, because, although we have secured considerable valuable information on the subject through the underground investigations we have been making, we know that in the last analysis the size of these piers must be estimated from the results of the testing we have done at the Fritz engineering laboratory at Lehigh University. In view of the great magnitude and importance of the question at stake, we realize that these tests, which are the first of the kind that have ever been made, are very meager and do not constitute a sufficient foundation upon which to base final conclusions.
Therefore we have decided to use a factor of safety of two. In other words, the size of the pillars we have recommended is twice as large as the tests indicate might be necessary. These tests are subject to check by further tests and other data that may be procured later, either through the working out in practice of the recommendations herein made, or otherwise, but which have not been and are not now available to us.
The size of artificial pillars should be determined by the local conditions found at the exact spot chosen upon inspection by the engineer in charge of the work. Subject to the above observations, the horizontal areas of artificial piers of flushed material confined in mine rooms are indicated in detail by the table following.