GENERAL CONDUCT OF EXAMINATION.
Complete and exhaustive examination, entailing extensive sampling, a.s.saying, and metallurgical tests, is very expensive and requires time. An unfavorable report usually means to the employer absolute loss of the engineer"s fee and expenses. It becomes then the initial duty of the latter to determine at once, by the general conditions surrounding the property, how far the expenditure for exhaustive examination is warranted. There is usually named a money valuation for the property, and thus a peg is afforded upon which to hang conclusions. Very often collateral factors with a preliminary sampling, or indeed no sampling at all, will determine the whole business.
In fact, it is becoming very common to send younger engineers to report as to whether exhaustive examination by more expensive men is justified.
In the course of such preliminary inspection, the ore-bodies may prove to be too small to insure adequate yield on the price, even a.s.suming continuity in depth and represented value. They may be so difficult to mine as to make costs prohibitive, or they may show strong signs of "petering out." The ore may present visible metallurgical difficulties which make it unprofitable in any event.
A gold ore may contain copper or a.r.s.enic, so as to debar cyanidation, where this process is the only hope of sufficiently moderate costs.
A lead ore may be an amorphous compound with zinc, and successful concentration or smelting without great penalties may be precluded.
A copper ore may carry a great excess of silica and be at the same time unconcentratable, and there may be no base mineral supply available for smelting mixture. The mine may be so small or so isolated that the cost of equipment will never be justified. Some of these conditions may be determined as unsurmountable, a.s.suming a given value for the ore, and may warrant the rejection of the mine at the price set.
It is a disagreeable thing to have a disappointed promoter heap vituperation on an engineer"s head because he did not make an exhaustive examination. Although it is generally desirable to do some sampling to give a.s.surance to both purchaser and vendor of conscientiousness, a little courage of conviction, when this is rightly and adequately grounded, usually brings its own reward.
Supposing, however, that conditions are right and that the mine is worth the price, subject to confirmation of values, the determination of these cannot be undertaken unless time and money are available for the work. As was said, a sampling campaign is expensive, and takes time, and no engineer has the moral right to undertake an examination unless both facilities are afforded. Curtailment is unjust, both to himself and to his employer.
How much time and outlay are required to properly sample a mine is obviously a question of its size, and the character of its ore.
An engineer and one princ.i.p.al a.s.sistant can conduct two sampling parties. In hard rock it may be impossible to take more than five samples a day for each party. But, in average ore, ten samples for each is reasonable work. As the number of samples is dependent upon the footage of openings on the deposit, a rough approximation can be made in advance, and a general idea obtained as to the time required. This period must be insisted upon.
REPORTS.
Reports are to be read by the layman, and their first qualities should be simplicity of terms and definiteness of conclusions.
Reports are usually too long, rather than too short. The essential facts governing the value of a mine can be expressed on one sheet of paper. It is always desirable, however, that the groundwork data and the manner of their determination should be set out with such detail that any other engineer could come to the same conclusion if he accepted the facts as accurately determined. In regard to the detailed form of reports, the writer"s own preference is for a single page summarizing the main factors, and an a.s.say plan, reduced to a longitudinal section where possible. Then there should be added, for purposes of record and for submission to other engineers, a set of appendices going into some details as to the history of the mine, its geology, development, equipment, metallurgy, and management. A list of samples should be given with their location, and the tonnages and values of each separate block. A presentation should be made of the probabilities of extension in depth, together with recommendations for working the mine.
GENERAL SUMMARY.
The bed-rock value which attaches to a mine is the profit to be won from proved ore and in which the price of metal is calculated at some figure between "basic" and "normal." This we may call the "_A_" value. Beyond this there is the speculative value of the mine. If the value of the "probable" ore be represented by _X_, the value of extension of the ore by _Y_, and a higher price for metal than the price above a.s.sumed represented by _Z_, then if the mine be efficiently managed the value of the mine is _A_ + _X_ + _Y_ + _Z_. What actual amounts should be attached to _X, Y, Z_ is a matter of judgment. There is no prescription for good judgment. Good judgment rests upon a proper balancing of evidence.
The amount of risk in _X, Y, Z_ is purely a question of how much these factors are required to represent in money,--in effect, how much more ore must be found, or how many feet the ore must extend in depth; or in convertible terms, what life in years the mine must have, or how high the price of metal must be. In forming an opinion whether these requirements will be realized, _X, Y, Z_ must be balanced in a scale whose measuring standards are the five geological weights and the general industrial outlook. The wise engineer will put before his clients the scale, the weights, and the conclusion arrived at. The shrewd investor will require to know these of his adviser.
CHAPTER VII.
Development of Mines.
ENTRY TO THE MINE; TUNNELS; VERTICAL, INCLINED, AND COMBINED SHAFTS; LOCATION AND NUMBER OF SHAFTS.
Development is conducted for two purposes: first, to search for ore; and second, to open avenues for its extraction. Although both objects are always more or less in view, the first predominates in the early life of mines, the prospecting stage, and the second in its later life, the producing stage. It is proposed to discuss development designed to embrace extended production purposes first, because development during the prospecting stage is governed by the same principles, but is tempered by the greater degree of uncertainty as to the future of the mine, and is, therefore, of a more temporary character.
ENTRY TO THE MINE.
There are four methods of entry: by tunnel, vertical shaft, inclined shaft, or by a combination of the last two, that is, by a shaft initially vertical then turned to an incline. Combined shafts are largely a development of the past few years to meet "deep level"
conditions, and have been rendered possible only by skip-winding. The angle in such shafts (Fig. 2) is now generally made on a parabolic curve, and the speed of winding is then less diminished by the bend.
The engineering problems which present themselves under "entry"
may be divided into those of:--
1. Method.
2. Location.
3. Shape and size.
The resolution of these questions depends upon the:--
a. Degree of dip of the deposit.
b. Output of ore to be provided for.
c. Depth at which the deposit is to be attacked.
d. Boundaries of the property.
e. Surface topography.
f. Cost.
g. Operating efficiency.
h. Prospects of the mine.
[Ill.u.s.tration: Fig. 2.--Showing arrangement of the bend in combined shafts.]
From the point of view of entrance, the cooperation of a majority of these factors permits the division of mines into certain broad cla.s.ses. The type of works demanded for moderate depths (say vertically 2,500 to 3,000 feet) is very different from that required for great depths. To reach great depths, the size of shafts must greatly expand, to provide for extended ventilation, pumping, and winding necessities. Moreover inclined shafts of a degree of flatness possible for moderate depths become too long to be used economically from the surface. The vast majority of metal-mining shafts fall into the first cla.s.s, those of moderate depths. Yet, as time goes on and ore-deposits are exhausted to lower planes, problems of depth will become more common. One thing, however, cannot be too much emphasized, especially on mines to be worked from the outcrop, and that is, that no engineer is warranted, owing to the speculation incidental to extension in depth, in initiating early in the mine"s career shafts of such size or equipment as would be available for great depths. Moreover, the proper location of a shaft so as to work economically extension of the ore-bodies is a matter of no certainty, and therefore shafts of speculative mines are tentative in any event.
Another line of division from an engineering view is brought about by a combination of three of the factors mentioned. This is the cla.s.sification into "outcrop" and "deep-level" mines. The former are those founded upon ore-deposits to be worked from or close to the surface. The latter are mines based upon the extension in depth of ore-bodies from outcrop mines. Such projects are not so common in America, where the law in most districts gives the outcrop owner the right to follow ore beyond his side-lines, as in countries where the boundaries are vertical on all sides. They do, however, arise not alone in the few American sections where the side-lines are vertical boundaries, but in other parts owing to the pitch of ore-bodies through the end lines (Fig. 3). More especially do such problems arise in America in effect, where the ingress questions have to be revised for mines worked out in the upper levels (Fig.
7).
[Ill.u.s.tration: Fig. 3.--Longitudinal section showing "deep level"
project arising from dip of ore-body through end-line.]
If from a standpoint of entrance questions, mines are first cla.s.sified into those whose works are contemplated for moderate depths, and those in which work is contemplated for great depth, further clarity in discussion can be gained by subdivision into the possible cases arising out of the factors of location, dip, topography, and boundaries.
MINES OF MODERATE DEPTHS.
Case I. Deposits where topographic conditions permit the alternatives of shaft or tunnel.
Case II. Vertical or horizontal deposits, the only practical means of attaining which is by a vertical shaft.
Case III. Inclined deposits to be worked from near the surface.
There are in such instances the alternatives of either a vertical or an inclined shaft.
Case IV. Inclined deposits which must be attacked in depth, that is, deep-level projects. There are the alternatives of a compound shaft or of a vertical shaft, and in some cases of an incline from the surface.
MINES TO GREAT DEPTHS.
Case V. Vertical or horizontal deposits, the only way of reaching which is by a vertical shaft.
Case VI. Inclined deposits. In such cases the alternatives are a vertical or a compound shaft.
CASE I.--Although for logical arrangement tunnel entry has been given first place, to save repet.i.tion it is proposed to consider it later. With few exceptions, tunnels are a temporary expedient in the mine, which must sooner or later be opened by a shaft.
CASE II. VERTICAL OR HORIZONTAL DEPOSITS.--These require no discussion as to manner of entry. There is no justifiable alternative to a vertical shaft (Fig. 4).
[Ill.u.s.tration: Fig. 4.--Cross-sections showing entry to vertical or horizontal deposits. Case II.]
[Ill.u.s.tration: Fig. 5.--Cross-section showing alternative shafts to inclined deposit to be worked from surface. Case III.]
CASE III. INCLINED DEPOSITS WHICH ARE INTENDED TO BE WORKED FROM THE OUTCROP, OR FROM NEAR IT (Fig. 5).--The choice of inclined or vertical shaft is dependent upon relative cost of construction, subsequent operation, and the useful life of the shaft, and these matters are largely governed by the degree of dip. a.s.suming a shaft of the same size in either alternative, the comparative cost per foot of sinking is dependent largely on the breaking facilities of the rock under the different directions of attack. In this, the angles of the bedding or joint planes to the direction of the shaft outweigh other factors. The shaft which takes the greatest advantage of such lines of breaking weakness will be the cheapest per foot to sink. In South African experience, where inclined shafts are sunk parallel to the bedding planes of hard quartzites, the cost per foot appears to be in favor of the incline. On the other hand, sinking shafts across tight schists seems to be more advantageous than parallel to the bedding planes, and inclines following the dip cost more per foot than vertical shafts.
An inclined shaft requires more footage to reach a given point of depth, and therefore it would entail a greater total expense than a vertical shaft, a.s.suming they cost the same per foot. The excess amount will be represented by the extra length, and this will depend upon the flatness of the dip. With vertical shafts, however, crosscuts to the deposit are necessary. In a comparative view, therefore, the cost of the crosscuts must be included with that of the vertical shaft, as they would be almost wholly saved in an incline following near the ore.
The factor of useful life for the shaft enters in deciding as to the advisability of vertical shafts on inclined deposits, from the fact that at some depth one of two alternatives has to be chosen.
The vertical shaft, when it reaches a point below the deposit where the crosscuts are too long (_C_, Fig. 5), either becomes useless, or must be turned on an incline at the intersection with the ore (_B_). The first alternative means ultimately a complete loss of the shaft for working purposes. The latter has the disadvantage that the bend interferes slightly with haulage.