If acceptable pit sand is not obtainable or obtainable only at too great an expense, resort must or may be made to sand crushed from rock. Roughly speaking, it might be said that when the cost of pit sand amounted to 75 cents per cu. yd. it would be well to figure on the cost of manufacturing sand. Of course, special circumstances as hereinafter discussed might be such as to change this figure materially. The cost of pit sand may usually be assumed as being nearly all transportation cost, which depends on length of haul and whether teams or cars are used. The plant charge, including apparatus for screening, elevating and loading, is usually a small item.

The cost of a plant for manufacturing sand also depends on several circumstances. If on account of the character of stone or for any other reason it is necessary to open a quarry and install a complete crushing plant isolated from all other or similar operations, and chargeable entirely to the sand, the cost will be one thing. However, if the stone used in the dam for other purposes (i.e., large stone or for concrete) will crush into suitable sand, the cost for both plant and operation may be very much reduced by manufacturing the sand in connection with other quarrying operations. The quarry for large stone and concrete material naturally is operated on a large scale. Usually there is so much waste stone in connection with quarrying that the stone for sand would be practically a by-product which would require handling only to the crushing machinery, beside which some of the crushing machinery would not have to be duplicated. The attendance might also be very little more if sand was manufactured than if it was not.

An isolated plant to produce say 10 cu. yd. an hour might consist of the following:

One gyratory crusher............................

........ $1,500

One small Blake crusher.................

....................... 700

Two sets 36-in. crushing rolls....................

..... 3,700

Elevators and screen....................

................1,000 to 2,000

Motors......................................

..........................1,000

Compressor and air drills..........

........ 3,000

Belt, shafting, building bins, etc.........

.. .3,000 to 5,000

Cars, track and miscellaneous.................

........ 2,000

Freight, haul, erection..........................

.........3,000 to 4,500

Total.....................................

.$19,000 to 23,400

Call the plant cost $20,000 to $25,000 while the labor cost of quarrying the rock and crushing to sand, including renewals, repairs and power would be 75 cents to $1 per cu. yd. Thus 100,000 cu. yd. of sand would cost between 95 cents and $1.25 per cu. yd., assuming no salvage on the plant. Any reasonable salvage allowance might reduce these figures by 5 cents to 7 cents per cu. yd. However, if the operation can be carried on in connection with other quarrying and crushing operations, using largely stone that would otherwise have to be wasted, the extra expense chargeable to manufacture of sand might easily be reduced to a plant charge of $12,000 and a labor charge of 40 cents to 50 cents per cu. yd., thus making the cost for say 100,000 cu. yd., 50 cents to 60 cents per cu. yd.

A saving of about $2000 in plant cost might be effected by using a pulverator instead of the two sets of crushing rolls, provided the rock is suitable for such a machine. Claims are made in behalf of the machine unqualified by any stipulation as to the hardness of the rock. However, unless improvements have been made quite recently, it seems to be a fact that on the harder rocks the lost time and expense for renewing the wearing parts is too great an item to render the process advisable.

The question of whether the rock will make suitable sand should receive consideration not so much on account of the character of the rock but whether or not the proposed crushing process will reduce that particular rock to sand with grains of suitable size and shape. This can be determined by crushing, by means of the process it is proposed to use, a large enough sample of rock to test fairly the process as well as the product. Thus at the Roosevelt dam where all the sand was reduced from broken stone by crushing rolls, the sand was at first unsatisfactory. The rock used was a hard dolomite limestone and the rolls with the best manipulation possible produced a very flaky sand which did not contain enough fine particles. (It may be mentioned here that the fine rolls cannot be set as close as the size of sand required; capacity of rolls in cu. yd. per hour requires that some thickness of stream be fed and that much of the crushing be done by stone in contact with stone.) This rather coarse flaky sand produced a mortar which would not hold water; it was without consistency or any working qualities. Luckily the remedy was immediately at hand in the shape of a rather soft friable sandstone which was used with the dolomite in about equal parts. The sandstone crushed into better shaped particles and so fine that the result of the mixture was almost ideal.