Jakuaey 4, 1907] 



SCIENCE 



13 



SOME PROBLEMS CONNECTED WITH DEEP 



MINING IN TSE LAKE SUPERIOR 



COPPER DISTRICT 



The copper mines of the Lake Superior 

 district are essentially low grade. Their 

 profitable operation is made possible by the 

 great extent of the lodes, their comparative 

 uniformity of character and the investment 

 of great sums of money to maintain opera- 

 tions on a vast scale over a long period of 

 years. 



With but one important exception, the 

 lodes are the vesicular tops of ancient lava 

 flows which subsequent to solidification 

 have had the cavities wholly or partly filled 

 by the deposition of various minerals, 

 among which is native copper. They dip 

 at angles varying from 38° to 70°. 



The modern shafts through which the 

 rock is hoisted are either inclined, follow- 

 ing the plane of the lode, or are vertical. 

 The inclined shafts are of dimensions such 

 as to provide for two railroads of approxi- 

 mately standard gauge on which run the 

 'skips' which are operated in balance. In 

 addition there is room for the ladder way 

 and air pipes, usually placed at one side. 

 The vertical shafts have compartments pro- 

 viding usually for 'cages' and pipe and 

 ladder way. Several of the inclined shafts 

 are over 5,000 feet long. One has a length 

 of 8,100 feet. Of the vertical shafts the 

 three deepest are, respectively, about 5,200, 

 5,000 and 4,900 feet deep. 



Long before such depths were actually 

 reached there arose the question as to a 

 possible limit set by the ultimate crushing 

 strength of the rock which is penetrated. 

 Manifestly, mining can not go to a depth 

 such that the weight on walls of drifts and 

 stopes will exceed the ultimate strength of 

 the material of which they are composed. 

 There is a widespread impression that the 

 lake mines are approaching such a limit. 

 There are current statements to the effect 

 that pieces of rock occasionally snap off 



the rock faces because of the great strain, 

 and are violently projected as if propelled 

 by an explosive. 



In this connection a few figures vvill be 

 of interest. The average density of the 

 rock of the copper-bearing series is not far 

 from 2.87, that is, a cubic foot weighs about 

 179.3 pounds. Therefore, a horizontal 

 square foot of area at 5,000 feet from the 

 surface has above it a column of rock 

 weighing 448 tons. The ultimate crushing 

 strength of the average rock is not well 

 known, but since it is mostly trap this may 

 be safely assumed as at least 1,200 tons per 

 square foot. If, therefore, the square foot 

 above defined carries the entire column 

 above it, it is loaded to much less than half 

 the crushing strength and only at nearly 

 three times the assumed depth will the load 

 reach its crushing limit. At a dip of 38°, 

 the pressure normal to the plane of the 

 lode at 5,000 feet from surface is only 354 

 tons per square foot. It is in this direction 

 that the crushing forces are mostly called 

 into play. As the dip increases this nor- 

 mal pressure of course diminishes. At 52° 

 it is 278 tons, and at 70° it is 152 tons per 

 square foot. 



However, the matter does not end here. 

 The removal of large portions of the copper 

 zone leaves considerable areas of the roof 

 or hanging wall to be supported by the 

 pillars which are left for the purpose, or 

 by the walls of the opening, or by both. 

 The weight on pillars and walls is thus in- 

 creased and may easily approach the crush- 

 ing limit. Take, for example, a long pillar 

 50 feet wide having on either side an open 

 space of 150 feet. Suppose it in a lode 

 dipping 38°. Allowing for neither rigidity 

 nor arching, and supposing the weight on 

 the pillar evenly distributed, at 5,000 feet 

 deep it would be subjected to a pressure of 

 1,239 tons per square foot, a pressure under 

 which it would fail. 



