Both the source, and the manner of distribution of the power are matters of increasing importance with increase in magnitude of the work. In addition to mere magnitude there may be many individual factors which should be considered in arriving at the best selection. The various conditions are here discussed and supplemented by some information gathered from certain examples of large dam construction. The questions of source and distribution are so inseparable that they will be discussed together.
For a very small dam, the mere yardage involved may indicate that it should be handled by one or two hand derricks or by derricks operated by a team of horses. If the size of the work indicated this method it would also mean that the dam would be a simple structure of rubble masonry comprised largely of one-man stone, spalls and mortar hand-laid; or if some natural deposit of concrete aggregate is available, a structure of hand-mixed concrete into which might be put any suitable one-man stone that could be readily gathered and brought to the job. The natural power for a dam considerably larger, but still in the class of small dams, would naturally be steam even at rather a high cost of fuel. This would require separate units of boiler and engine for each derrick, crusher and mixer, boiler to run steam drills in the quarry, etc. Such dams as these are so small that the various operations are not and need not be sharply differentiated. The work consists in bringing the materials to the site by teams in the simplest way possible and of putting them into a dam by the use of the most inexpensive and most easily transported plant.
When we get above the limit in size for which such simple procedure is advisable we encounter more and more conditions which should be more and more closely scrutinized. Steam used in small units or distributed over portions of the work from one central boiler station soon becomes out of the question. Recourse must be had to a central power plant and distribution by compressed air or electricity. In some locations or under certain conditions it may be that hydro-electric power can be economically generated for construction purposes. A casual consideration of conditions may answer the question or at least indicate that a detailed study of the question should be made.
The conditions favoring hydro-electric generation are:
High cost of fuel. This may be owing to scarcity in the immediate vicinity; distance from a fuel supply with consequent high freight charges, or distance from a railroad involving expensive wagon haul.
An available quantity of water in combination with a stream slope such that the necessary channel for conveying the water from the stream to the wheel would be within bounds as to expense.
The possibility of diverting the water from the stream with an inexpensive dam.
Wood at best is an expensive fuel. Even if nothing is paid for the wood it must be cut, gathered and brought to the boiler. The cutting alone would make it equivalent to coal at $3 per ton. The haul to the boiler must usually be largely over the poorest roads or no roads at all. If the job is of some length and magnitude and the wood thin and scattering, it must soon be brought considerable distances. Finally the wood is usually green and of low fuel value.
After obtaining prices of coal or fuel oil delivered at the nearest point on the railroad, figure on the cost of haul, if any, to the work. Use say the previously mentioned basis of 25 cents per ton mile. Remember that if fuel oil is used only about one-half the tonnage must be hauled; i.e., six 42-gal. barrels of oil weigh about 1 ton and when used to generate steam have a fuel value equal to 1 1/2 to 2 tons of average coal. While the use of fuel oil involves an initial outlay for a storage tank, this tank need not be very costly. In operation the fuel oil can be handled and the boilers fired more economically than with coal because the use of coal involves expensive ash disposal, etc. Internal combustion engines and gas producer plants, however economical of fuel and eminently worthy of consideration in permanent plants, are so costly to install that they are usually outside the range of discussion for temporary plants for construction purposes.
If it is necessary to haul the fuel because the work is not accessible by rail, it may be cheaper to build the power station at the railroad and transmit electric power to the work. The determining factors are the cost of hauling the total amount of fuel required, and the cost of a transmission line including necessary transformers, etc. Should the completed project contemplate the generation of electric power and its transmission to a market located (as is almost necessarily the case) on a railroad, it may be advisable to anticipate somewhat the construction of the transmission line even at the expense of interest. In this event, power would be sent back from a steam plant at the railroad to the work during the construction period.
In figuring upon the relative merits of steam and hydro-electric generation, the determining consideration may very likely be the length of time that the plant will be in service. A hydro-electric installation might require a much larger initial outlay than an equal steam installation but its operation and maintenance would be much less; while the much lower first cost of the steam installation is offset by its greater labor and fuel costs. Thus a steam plant might be much cheaper for a two-year job and much more expensive for a four-year job. In estimating the plant charge for each method make proper allowances for interest and depreciation. These items will usually be heavily against the hydro-electric installation, as in addition to higher first cost and larger item of interest, the depreciation on items of diversion dam and channel for conveying the water must generally be taken as 100 per cent.
In connection with the depreciation of the hydro-electric plant, study should be made as to whether the plant may not have a value after the construction period as a permanent plant if continued in operation at the same place. It may indeed be planned as a desirable permanent feature of the project for which the dam is being constructed, in which case an agreement might be embodied in the construction contract. Or, as was the case in one prominent instance, the installation might be made by the owners of the project and power sold to the construction company at a stipulated price. A still further possibility is that power might be economically purchased from some adjacent company engaged in the commercial production and distribution of power. A large electric light plant, for instance, with an installation equal to several times its day load might welcome such an addition to its day output as this would involve practically no additional expense except for fuel. A company in such a position might make a price for power sufficiently attractive to warrant the construction of the transmission line.