Furthermore, an adit will obviate the installation and use of hoisting machinery, and thus there may be maintained a greater efficiency in the operating expense of the mine than would be possible with a shaft.
Again, it is a simpler and cheaper matter to maintain a mining tunnel in working shape than it is a shaft, particularly in bad ground. By the settling or "working" of the ground, a shaft may be thrown perhaps but slightly out of alignment and annoying interferences will be experienced in hoisting, especially when rapid and uninterrupted hoisting is necessary to maintain the desired output. While the same amount of disturbance does take place in an adit, it is an easy matter to readjust track grades while continuing regular haulage operations.
The timbers, in the case of either a shaft or an adit, will require occasional renewal, but the expense of such repairs is less in adits than in shafts or inclines, while the delay to other operations of mining, in the case of the adit, will be inappreciable.
Topography has been referred to above, but it must be again briefly mentioned. There are some places in which ore bodies extend to, or exist at, such depths that adits could not be projected to get beneath enough of the ore to warrant their construction. An adit mine is not a practicable thing in a flat country like Nevada or the Rand, but in the rough country of the San Juan it is the customary kind of a mine. In the very early days of Comstock Lode mining, shafts were sunk by each of the hundreds of companies. Before a great while, the advantages that would accrue from having a deep "tunnel" became evident, and the famous Sutro Tunnel, with its historic, checkered career, was driven. Although it loomed up like a gigantic undertaking for that period, the immense prospective or future value of it could not be denied.
The following relative advantages of the several types of mine mouths are in addition to those already given and are worth consideration:
With an incline, the value of a tabular deposit is determined as work progresses; the course and dip of the body will be known at all depths along the incline; the body may be explored from the incline in both directions, simultaneously, with a resulting doubling of the development and production; all, or nearly all, the material removed is "vein stuff" and its value may repay the sinking expenses; there is no losing of the ore body unless a geological fault is met.
With a shaft, more rapid hoisting is possible than with an incline; the timbering labor is less than in the case of an incline, but greater than in the case of an adit; with ground containing ore bodies in irregular masses and at no uniform intervals, vertically or horizontally, stations and levels may be started wherever desirable; the crosscuts which are usually necessary to reach the bodies may disclose otherwise unknown bodies.
Mills and Shaft House of Daly West Mine, Park City, Utah.
With a vein adit, the vein is prospected as work advances; the ore removed may pay its own way, as it were; the drainage is automatic; ore is transportable from the mine by haulage rather than by hoisting; the ore in place is above the level and will handle itself to the outgoing passage by gravity.