MINING CONDITIONS UNDER CITY OP SCRANTON, PA.
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Table 6.—Approximate cost per foot of coal-bed thickness of artificial mine pillar of confined flushed culm or flushed sand required under various permissible compressions to sustain one-third of the overburden of one city block of five acres, at various depths.
Ultimate uniform compres- sion permitted.	Cost.					
	Depth, 25 feet.		Depth, 50 feet.		Depth, 100 feet.	
	Culm.	Sand.	Culm.	Sand.	Culm.	Sand.
Per cent. 3 5 10 3 5 10	$286 176 70	$266 150 60	$572 352 140	$532 300 120	$1,144 704 280	$1,064 600 240
	Depth, 200 feet.		Depth, 400 feet.		Depth, 800 feet.	
	«$2,016 1,408 560	$2,128 1,200 480	(<־) (») SI,120	$4,256 2,400 960	(a) (•־) (״)	«$8,070 4,300 1,920
« Filled.
The approximate cost per foot of bed thickness for each acre of complete flushing under schools and elsewhere, to take the place of pillars, if removed, would be:
For culm below level of river........................... $405
For sand above or below river........................... 1,615
If, in the case of the coal beds 150 feet or more deep, the board of control concludes to be satisfied to relieve the present pillars under the schools of about one-third of the burden they now sustain, the approximate cost of so doing, per foot of bed thickness for each acre thus protected, will be about one-fifth of the cost shown in Table 6, for the same depth, material, and settlement. For example, to relieve the present pillars of the Clark bed, which is about 200 feet deep under the central city and hill sections, of one-third of the weight they now sustain, allowing 5 per cent as ultimate permissible settlement for the coal-bed roof, would cost per foot-acre of coal bed, about $280 for culm flushing, if the piers are below the river level; or $240 for sand flushing, whether above or below the river; and since this coal bed is about 7 feet thick, the total cost of such piers would be about $1,960 and $1,680 per acre, respectively.
The above table is estimated on the supposition that the pillars of flushed culm would be installed only at points that are in coal beds below the level of the river (so the necessity of pumping culm to ail elevation above the river would be avoided) and that piers of flushed sand would be installed in coal beds at locations that are above the level of the river, or at any location where such sand pillars would be convenient and not more costly than culm pillars.
It will be noted that there is no large difference in cost between culm or sand. This, of course, is on account of the greater efficiency