The following table will indicate an hypothetical extreme case,--not infrequently met. In it a vertical shaft 1,500 feet in depth is taken as cutting the deposit at the depth of 750 feet, the most favored position so far as aggregate length of crosscuts is concerned. The cost of crosscutting is taken at $20 per foot and that of sinking the vertical shaft at $75 per foot. The incline is a.s.sumed for two cases at $75 and $100 per foot respectively. The stoping height upon the ore between levels is counted at 125 feet.
Dip of | Depth of | Length of |No. of Crosscuts| Total Length Deposit from | Vertical | Incline | Required from | of Crosscuts, Horizontal | Shaft | Required | V Shaft | 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 |Cost Vertical| Total Cost | Cost of Incline|Cost of Incline Crosscuts $20| Shaft $75 | of Vertical | $75 per Foot | $100 per Foot per Foot | per Foot |and Crosscuts| | -------------|-------------|-------------|----------------|--------------- $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.
[Ill.u.s.tration: 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 Gra.s.s Valley is an ill.u.s.tration 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 princ.i.p.al 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 pa.s.sed 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.
[Ill.u.s.tration: 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.
In most situations of this sort, and in all of the second type (where the outcrop is outside the property), actual choice usually lies between combined shafts (_C D F_) and entire vertical shafts (_H G_). The difference between a combined shaft and a direct vertical shaft can be reduced to a comparison of the combined shaft below the point of intersection (_D_) with that portion of a vertical shaft which would cover the same horizon. The question then becomes identical with that of inclined _versus_ verticals, as stated in Case III, with the offsetting disadvantage of the bend in the combined shaft. If it is desired to reach production at the earliest date, the lower section of a simple vertical shaft must have crosscuts to reach the ore lying above the horizon of its intersection (_E_).
If production does not press, the ore above the intersection (_EB_) can be worked by rises from the horizon of intersection (_E_).
In the use of rises, however, there follow the difficulties of ventilation and lowering the ore down to the shaft, which brings expenses to much the same thing as operating through crosscuts.
The advantages of combined over simple vertical shafts are earlier production, saving of either rises or crosscuts, and the ultimate utility of the shaft to any depth. The disadvantages are the cost of the extra length of the inclined section, slower winding, and greater wear and tear within the inclined section and especially around the bend. All these factors are of variable import, depending upon the dip. On very steep dips,--over 70,--the net result is in favor of the simple vertical shaft. On other dips it is in favor of the combined shaft.
CASES V AND VI. MINES TO BE WORKED TO GREAT DEPTHS,--OVER 3,000 FEET.--In Case V, with vertical or horizontal deposits, there is obviously no desirable alternative to vertical shafts.
In Case VI, with inclined deposits, there are the alternatives of a combined or of a simple vertical shaft. A vertical shaft in locations (_H_, Fig. 7) such as would not necessitate extension in depth by an incline, would, as in Case IV, compel either crosscuts to the ore or inclines up from the horizon of intersection (_E_).
Apart from delay in coming to production and the consequent loss of interest on capital, the ventilation problems with this arrangement would be appalling. Moreover, the combined shaft, entering the deposit near its shallowest point, offers the possibility of a separate haulage system on the inclined and on the vertical sections, and such separate haulage is usually advisable at great depths. In such instances, the output to be handled is large, for no mine of small output is likely to be contemplated at such depth. Several moderate-sized inclines from the horizon of intersection have been suggested (_EF_, _DG_, _CH_, Fig. 8) to feed a large primary shaft (_AB_), which thus becomes the trunk road. This program would cheapen lateral haulage underground, as mechanical traction can be used in the main level, (_EC_), and horizontal haulage costs can be reduced on the lower levels. Moreover, separate winding engines on the two sections increase the capacity, for the effect is that of two trains instead of one running on a single track.
SHAFT LOCATION.--Although the prime purpose in locating a shaft is obviously to gain access to the largest volume of ore within the shortest haulage distance, other conditions also enter, such as the character of the surface and the rock to be intersected, the time involved before reaching production, and capital cost.
As shafts must bear two relations to a deposit,--one as to the dip and the other as to the strike,--they may be considered from these aspects. Vertical shafts must be on the hanging-wall side of the outcrop if the deposit dips at all. In any event, the shaft should be far enough away to be out of the reach of creeps. An inclined shaft may be sunk either on the vein, in which case a pillar of ore must be left to support the shaft; or, instead, it may be sunk a short distance in the footwall, and where necessary the excavation above can be supported by filling. Following the ore has the advantage of prospecting in sinking, and in many cases the softness of the ground in the region of the vein warrants this procedure. It has, however, the disadvantage that a pillar of ore is locked up until the shaft is ready for abandonment. Moreover, as veins or lodes are seldom of even dip, an inclined shaft, to have value as a prospecting opening, or to take advantage of breaking possibilities in the lode, will usually be crooked, and an incline irregular in detail adds greatly to the cost of winding and maintenance.
These twin disadvantages usually warrant a straight incline in the footwall. Inclines are not necessarily of the same dip throughout, but for reasonably economical haulage change of angle must take place gradually.
[Ill.u.s.tration: Fig. 8.--Longitudinal section showing shaft arrangement proposed for very deep inclined deposits.]
In the case of deep-level projects on inclined deposits, demanding combined or vertical shafts, the first desideratum is to locate the vertical section as far from the outcrop as possible, and thus secure the most ore above the horizon of intersection. This, however, as stated before, would involve the cost of crosscuts or rises and would cause delay in production, together with the acc.u.mulation of capital charges. How important the increment of interest on capital may become during the period of opening the mine may be demonstrated by a concrete case. For instance, the capital of a company or the cost of the property is, say, $1,000,000, and where opening the mine for production requires four years, the aggregate sum of acc.u.mulated compound interest at 5% (and most operators want more from a mining investment) would be $216,000. Under such circ.u.mstances, if a year or two can be saved in getting to production by entering the property at a higher horizon, the difference in acc.u.mulated interest will more than repay the infinitesimal extra cost of winding through a combined shaft of somewhat increased length in the inclined section.
The unknown character of the ore in depth is always a sound reason for reaching it as quickly and as cheaply as possible. In result, such shafts are usually best located when the vertical section enters the upper portion of the deposit.
The objective in location with regard to the strike of the ore-bodies is obviously to have an equal length of lateral ore-haul in every direction from the shaft. It is easier to specify than to achieve this, for in all speculative deposits ore-shoots are found to pursue curious vagaries as they go down. Ore-bodies do not reoccur with the same locus as in the upper levels, and generally the chances to go wrong are more numerous than those to go right.
NUMBER OF SHAFTS.--The problem of whether the mine is to be opened by one or by two shafts of course influences location. In metal mines under Cases II and III (outcrop properties) the ore output requirements are seldom beyond the capacity of one shaft. Ventilation and escape-ways are usually easily managed through the old stopes.
Under such circ.u.mstances, the conditions warranting a second shaft are the length of underground haul and isolation of ore-bodies or veins. Lateral haulage underground is necessarily disintegrated by the various levels, and usually has to be done by hand. By shortening this distance of tramming and by consolidation of the material from all levels at the surface, where mechanical haulage can be installed, a second shaft is often justified. There is therefore an economic limitation to the radius of a single shaft, regardless of the ability of the shaft to handle the total output.
Other questions also often arise which are of equal importance to haulage costs. Separate ore-shoots or ore-bodies or parallel deposits necessitate, if worked from one shaft, constant levels through unpayable ground and extra haul as well, or ore-bodies may dip away from the original shaft along the strike of the deposit and a long haulage through dead levels must follow. For instance, levels and crosscuts cost roughly one-quarter as much per foot as shafts. Therefore four levels in barren ground, to reach a parallel vein or isolated ore-body 1,000 feet away, would pay for a shaft 1,000 feet deep. At a depth of 1,000 feet, at least six levels might be necessary. The tramming of ore by hand through such a distance would cost about double the amount to hoist it through a shaft and transport it mechanically to the dressing plant at surface. The aggregate cost and operation of barren levels therefore soon pays for a second shaft. If two or more shafts are in question, they must obviously be set so as to best divide the work.
Under Cases IV, V, and VI,--that is, deep-level projects,--ventilation and escape become most important considerations. Even where the volume of ore is within the capacity of a single shaft, another usually becomes a necessity for these reasons. Their location is affected not only by the locus of the ore, but, as said, by the time required to reach it. Where two shafts are to be sunk to inclined deposits, it is usual to set one so as to intersect the deposit at a lower point than the other. Production can be started from the shallower, before the second is entirely ready. The ore above the horizon of intersection of the deeper shaft is thus accessible from the shallower shaft, and the difficulty of long rises or crosscuts from that deepest shaft does not arise.
CHAPTER VIII.
Development of Mines (_Continued_).
SHAPE AND SIZE OF SHAFTS; SPEED OF SINKING; TUNNELS.
SHAPE OF SHAFTS.--Shafts may be round or rectangular.[*] Round vertical shafts are largely applied to coal-mines, and some engineers have advocated their usefulness to the mining of the metals under discussion. Their great advantages lie in their structural strength, in the large amount of free s.p.a.ce for ventilation, and in the fact that if walled with stone, brick, concrete, or steel, they can be made water-tight so as to prevent inflow from water-bearing strata, even when under great pressure. The round walled shafts have a longer life than timbered shafts. All these advantages pertain much more to mining coal or iron than metals, for unsound, wet ground is often the accompaniment of coal-measures, and seldom troubles metal-mines.
Ventilation requirements are also much greater in coal-mines. From a metal-miner"s standpoint, round shafts are comparatively much more expensive than the rectangular timbered type.[**] For a larger area must be excavated for the same useful s.p.a.ce, and if support is needed, satisfactory walling, which of necessity must be brick, stone, concrete, or steel, cannot be cheaply accomplished under the conditions prevailing in most metal regions. Although such shafts would have a longer life, the duration of timbered shafts is sufficient for most metal mines. It follows that, as timber is the cheapest and all things considered the most advantageous means of shaft support for the comparatively temporary character of metal mines, to get the strains applied to the timbers in the best manner, and to use the minimum amount of it consistent with security, and to lose the least working s.p.a.ce, the shaft must be constructed on rectangular lines.
[Footnote *: Octagonal shafts were sunk in Mexico in former times.
At each face of the octagon was a whim run by mules, and hauling leather buckets.]
[Footnote **: The economic situation is rapidly arising in a number of localities that steel beams can be usefully used instead of timber. The same arguments apply to this type of support that apply to timber.]
The variations in timbered shaft design arise from the possible arrangement of compartments. Many combinations can be imagined, of which Figures 9, 10, 11, 12, 13, and 14 are examples.
[Ill.u.s.tration: FIG. 9. FIG. 10. FIG. 11. FIG. 12. FIG. 13. FIG.
14.]
The arrangement of compartments shown in Figures 9, 10, 11, and 13 gives the greatest strength. It permits timbering to the best advantage, and avoids the danger underground involved in crossing one compartment to reach another. It is therefore generally adopted.
Any other arrangement would obviously be impossible in inclined or combined shafts.
SIZE OF SHAFTS.--In considering the size of shafts to be installed, many factors are involved. They are in the main:--
_a_. Amount of ore to be handled.
_b_. Winding plant.
_c_. Vehicle of transport.
_d_. Depth.
_e_. Number of men to be worked underground.
_f_. Amount of water.
_g_. Ventilation.
_h_. Character of the ground.
_i_. Capital outlay.
_j_. Operating expense.
It is not to be a.s.sumed that these factors have been stated in the order of relative importance. More or less emphasis will be attached to particular factors by different engineers, and under different circ.u.mstances. It is not possible to suggest any arbitrary standard for calculating their relative weight, and they are so interdependent as to preclude separate discussion. The usual result is a compromise between the demands of all.
Certain factors, however, dictate a minimum position, which may be considered as a datum from which to start consideration.