| Dip of Deposit from Horizontal | Depth of Vertical Shaft | Length of Incline Required | No. of Crosscuts Required from V Shaft | Total Length of Crosscuts, Feet |
|---|---|---|---|---|
| 80° | 1,500 | 1,522 | 11 | 859 |
| 70° | 1,500 | 1,595 | 12 | 1,911 |
| 60° | 1,500 | 1,732 | 13 | 3,247 |
| 50° | 1,500 | 1,058 | 15 | 5,389 |
| 40° | 1,500 | 2,334 | 18 | 8,038 |
| 30° | 1,500 | 3,000 | 23 | 16,237 |
| Cost of Crosscuts $20 per Foot | Cost Vertical Shaft $75 per Foot | Total Cost of Vertical and Crosscuts | Cost of Incline $75 per Foot | Cost of Incline $100 per Foot |
| $17,180 | $112,500 | $129,680 | $114,150 | $152,200 |
| 38,220 | 112,500 | 150,720 | 118,625 | 159,500 |
| 64,940 | 112,500 | 177,440 | 129,900 | 172,230 |
| 107,780 | 112,500 | 220,280 | 114,850 | 195,800 |
| 178,760 | 112,500 | 291,260 | 175,050 | 233,400 |
| 324,740 | 112,500 | 437,240 | 225,000 | 300,000 |
From the above examples it will be seen that the cost of crosscuts put at ordinary level intervals rapidly outruns the extra expense of increased length of inclines. If, however, the conditions are such that crosscuts from a vertical shaft are not necessary at so frequent intervals, then in proportion to the decrease the advantages sway to the vertical shaft. Most situations wherein the crosscuts can be avoided arise in mines worked out in the upper levels and fall under Case IV, that of deep-level projects.
There can be no doubt that vertical shafts are cheaper to operate than inclines: the length of haul from a given depth is less; much higher rope speed is possible, and thus the haulage hours are less for the same output; the wear and tear on ropes, tracks, or guides is not so great, and pumping is more economical where the Cornish order of pump is used. On the other hand, with a vertical shaft must be included the cost of operating crosscuts. On mines where the volume of ore does not warrant mechanical haulage, the cost of tramming through the extra distance involved is an expense which outweighs any extra operating outlay in the inclined shaft itself. Even with mechanical haulage in crosscuts, it is doubtful if there is anything in favor of the vertical shaft on this score.
| Fig. 6.—Cross-section showing auxiliary vertical outlet. |
In deposits of very flat dips, under 30°, the case arises where the length of incline is so great that the saving on haulage through direct lift warrants a vertical shaft as an auxiliary outlet in addition to the incline (Fig. 6). In such a combination the crosscut question is eliminated. The mine is worked above and below the intersection by incline, and the vertical shaft becomes simply a more economical exit and an alternative to secure increased output. The North Star mine at Grass Valley is an illustration in point. Such a positive instance borders again on Case IV, deep-level projects.
In conclusion, it is the writer's belief that where mines are to be worked from near the surface, coincidentally with sinking, and where, therefore, crosscuts from a vertical shaft would need to be installed frequently, inclines are warranted in all dips under 75° and over 30°. Beyond 75° the best alternative is often undeterminable. In the range under 30° and over 15°, although inclines are primarily necessary for actual delivery of ore from levels, they can often be justifiably supplemented by a vertical shaft as a relief to a long haul. In dips of less than 15°, as in those over 75°, the advantages again trend strongly in favor of the vertical shaft. There arise, however, in mountainous countries, topographic conditions such as the dip of deposits into the mountain, which preclude any alternative on an incline at any angled dip.
Case IV. Inclined Deposits which must be attacked in Depth (Fig. 7).—There are two principal conditions in which such properties exist: first, mines being operated, or having been previously worked, whose method of entry must be revised; second, those whose ore-bodies to be attacked do not outcrop within the property.
The first situation may occur in mines of inadequate shaft capacity or wrong location; in mines abandoned and resurrected; in mines where a vertical shaft has reached its limit of useful extensions, having passed the place of economical crosscutting; or in mines in flat deposits with inclines whose haul has become too long to be economical. Three alternatives present themselves in such cases: a new incline from the surface (A B F, Fig. 7), or a vertical shaft combined with incline extension (C D F), or a simple vertical shaft (H G). A comparison can be first made between the simple incline and the combined shaft. The construction of an incline from the surface to the ore-body will be more costly than a combined shaft, for until the horizon of the ore is reached (at D) no crosscuts are required in the vertical section, while the incline must be of greater length to reach the same horizon. The case arises, however, where inclines can be sunk through old stopes, and thus more cheaply constructed than vertical shafts through solid rock; and also the case of mountainous topographic conditions mentioned above.
| Fig. 7.—Cross-section of inclined deposit which must be attacked in depth. |
From an operating point of view, the bend in combined shafts (at D) gives rise to a good deal of wear and tear on ropes and gear. The possible speed of winding through a combined shaft is, however, greater than a simple incline, for although haulage speed through the incline section (D F) and around the bend of the combined shaft is about the same as throughout a simple incline (A F), the speed can be accelerated in the vertical portion (D C) above that feasible did the incline extend to the surface. There is therefore an advantage in this regard in the combined shaft. The net advantages of the combined over the inclined shaft depend on the comparative length of the two alternative routes from the intersection (D) to the surface. Certainly it is not advisable to sink a combined shaft to cut a deposit at 300 feet in depth if a simple incline can be had to the surface. On the other hand, a combined shaft cutting the deposit at 1,000 feet will be more advisable than a simple incline 2,000 feet long to reach the same point. The matter is one for direct calculation in each special case. In general, there are few instances of really deep-level projects where a complete incline from the surface is warranted.