14 



SCIENCE 



[N. S. Vol. XXV. No. 627 



As a matter of fact, in such a case the 

 rigidity of the rock mass distributes a large 

 part of the load out over the rock beyond 

 the walls of the opening. That this rigid- 

 ity may be considerable is illustrated in 

 several cases where areas of hanging as 

 wide as 200 feet or more have no support 

 between walls, and yet have stood up for 

 several years. They are not, however, at 

 maximum depth. 



In such an area a pillar when first cut 

 out may have to carry but little more than 

 its previous load. As the hanging wall 

 slowly bends the pillar must take up more 

 and more of the extra weight. This is not 

 applied uniformly. As the rock between 

 pillars and walls bends downward the tend- 

 ency is to concentrate the load at the edge 

 or face of the pillar, or wall, much as a 

 beam does when supported in like manner. 

 The outer parts of the pillar may thus be- 

 come overloaded and here it will fail. 



It does so by the splitting off of pieces of 

 rock much as may sometimes be observed 

 with a specimen in the testing machine, 

 though on a much greater scale. These 

 pieces break from the base as well as the 

 top, and, as a rule, like any hard rock 

 under a crushing load, the pillar fails sud- 

 denly. Small pieces of rock may fly to a 

 considerable distance, and such occurrences 

 have undoubtedly given rise to the above- 

 mentioned exaggerated impression of the 

 compressive stress to which the rock is sub- 

 jected in the lowest levels. 



The hanging rock mass moves, of course, 

 when the pillar crushes, and the vibration 

 due to the sudden though slight displace- 

 ment is often conveyed to the surface. The 

 result is a miniature but perfectly genuine 

 earthquake which may be felt over a dis- 

 tance several times that of the pillar from 

 the surface. With the cnishing of the 

 pillar and the movement of the hanging 

 a readjustment of the weight takes place. 



and the process begins over again. Instead 

 of the process being repeated exactly it is 

 possible for the hanging to break in such a 

 manner that the arching effect may protect 

 this pillar, and place the load on others. 

 Eventually, at great depths the hanging 

 and foot must come together, and in one 

 mine the final steps in the process came so 

 rapidly as to completely wreck it. 



The pressure normal to the plane of the 

 lode is not the only action which may ap- 

 pear. The pillars are not, as a rule, sepa- 

 rate from either foot or hanging. They 

 are parts of the same rock mass, and it is 

 not possible for the hanging to slide over 

 the pillar. In consequence the readjust- 

 ments which take place when a pillar fails 

 as above described sometimes put an enor- 

 mous longitudinal thrust on the foot, and 

 in places its surface portion has buckled up 

 under such stress. Also, at points where 

 shaft pillars have been weak, shafts have 

 been pinched and twisted under the same 

 conditions so as to interfere with their 

 operation. 



Experience seems to have shown that at 

 the great depths recently reached it is use- 

 less to expect to hold up the hanging rock 

 mass for a long time by any scheme of 

 pillars unless far too much of the lode is 

 left in place, and that the only feasible 

 method is to cut away the entire lode and 

 permit the hanging to cave as rapidly as it 

 will to the point where the broken rock fills 

 again the whole space, and redistributes 

 the weight over the footwall. Following 

 this plan, cutting out the lode, or 'stop- 

 ing,' begins at the point furthest from the 

 shaft, and progresses toward it. With a 

 wide shaft pillar, or with the shaft in the 

 footwall, and with some such general meth- 

 od which avoids concentration of pressure 

 where it can do harm, there seems no rea- 

 son to anticipate serious difSculties due to 

 crushing for a further depth at least as 

 great as that already attained. 



