. Steam shovel mining, including a consideration of electric shovels and other power excavators in open-pit mining . rj calculations. The first illustrates the economic limits of stripping in area and Warriner, J. B., Anthracite stripping; T. A. L M. E., Feb., 1917, pp. 33-60. 214 STEAM SHOVEL MINING depth. It is assumed that it has been decided to expend, oncoal recovered from the stripping shown in Fig. 50, an amountper ton equal to the average margin of profit of the colliery, theretmn on the investment being considered to be secured by cer-tain factors or advantages that do not lend themse
. Steam shovel mining, including a consideration of electric shovels and other power excavators in open-pit mining . rj calculations. The first illustrates the economic limits of stripping in area and Warriner, J. B., Anthracite stripping; T. A. L M. E., Feb., 1917, pp. 33-60. 214 STEAM SHOVEL MINING depth. It is assumed that it has been decided to expend, oncoal recovered from the stripping shown in Fig. 50, an amountper ton equal to the average margin of profit of the colliery, theretmn on the investment being considered to be secured by cer-tain factors or advantages that do not lend themselves readilyto calculation in exact figures. Then this cost per ton figureis translated into a ratio of cubic yards of overburden removedper ton of coal uncovered, and amounts for example to 2^ per ton. Then the limits in area and depth are marked outto give this ratio. These limits at first may appear satisfactoryunless the problem is resolved into its component parts, as shownon the figure by the shaded areas. Part A is the lowest compo-nent part which comes within the limits of the ratio, viz., 2% Fig. 50.—Anthracite stripping problem. yd. per ton uncovered, while part B carries 3 cu. yd. and Ccarries 33^^ cu. yd. These then are worked at a loss, regardlessof the fact that parts D and E are operated at a considerableprofit. To justify the removal of B and C areas there must begained some marked advantages not included in the factorsused in setting the 2^^-cu. yd. ratio; otherwise a considerableamount of the coal reserve would be depleted at no profit. The second example, illustrated in Fig. 51, is a crop strippingfor a virgin area where the clay and gravel overburden must eitherbe removed or a chain pillar of coal left below the surface to pre-vent the contamination of the prepared coal. In this case itwould be necessary to leave a 60 ft. chain pillar unless it isstripped. The coal below the chain pillar can be mined ascheaply per ton, for cutting and loading,
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