The subdrift caving system, fig.
The bottom-slice caving system of mining begins at the bottom of a hundred-foot block of ground, a floor being excavated under the whole area, leaving pillars of sufficient size to support the ground above.
While the mine workings are small the overlying rocks support themselves of and the full pressure does not come upon the mine Caving p i llars.
The caving-in of the mine, however, is rarely so complete that avenues of escape are not open.
9 illustrates the working of a block of ground by the top-slice caving system.
While abundant warning of the caving-in of the workings is thus given in advance it may happen that men are unexpectedly imprisoned by the closing of the main passage ways.
He knew it, which meant he was counting on her caving at some point.
Although petroleum wells in Russia have not the depth of many of those in the United States, the disturbed character of the strata, with consequent liability to caving, and the occurrence of hard concretions, render drilling a lengthy and expensive Drilling in operation.
In the working of thick deposits the block of ground between two levels is divided into horizontal sections or floors which are worked either from above downward or from of Thick the bottom upward; in the first case the separate floors are worked by one of the caving systems; in the second, generally with the aid of filling.
This is the cheapest of the three caving systems, but is applicable only when the deposit lies between walls of very solid rock, as otherwise wall rock is liable to cave with and become mixed with ore, which adds greatly to the expense of handling.
In the latter case the top-slice caving method will usually be employed for the working of such intervening pillars.
In addition mining operations are subject to interruption and added expense from explosions, mine fires, flooding, and the caving-in of the workings.
It is estimated that the eating and caving of the shore below Little Rock averages 7 64 acres per mile every year (as against 1.99 acres above Little Rock).
The caving in of mine workings results from the excavation of large areas supported upon pillars of insufficient size.