(Specially written for FIRE AND WATER).

WHAT was once a great primeval lake at the entrance to the South Platte gorge, forty-eight miles south of Denver, Colo., is about to become a huge reservoir, with a capacity of between 35,000,000,000 to 65,000,000,000 gallons—enough to guarantee the citizens of Denver against water famine for at least three years, no matter what contingency may arise The dam to be built, across the valley will be 210 feet high, the highest in the world, and will cost $500.000. The structure will be 6 700 feet above the level of the sea, and 1,500 feet above that of Denver itself.

The dam will impound the waters of the mountain streams, the South Platte and the Goose creek. Its height may be raised twenty two feet higher,if necessary. In size it will be hardly twenty-five feet from side to side at the base, and for some fifty feet upwards its frontage will not exceed fifty feet. It will then gradually slope outwards on the south side; on the north side it will be flanked by an enormous mass of granite and will spread out to a width of 500 feet. Immediately above it will rise the entrance to a gorge, whose walls, almost perpendicular, rise to a height of nearly 500 feet. The basin will be irregular in shape, with its greatest width from the south side of the dam to the roadway leading into the basin—a distance of about one and one-quarter mile. This artificial lake will extend seven miles up the Platte, with many side canal-like shoots up the gulches, the largest of which will be two and one-half miles long and 300 or 400 feet wide.

The dam will be built almost parallel with the left e of the valley above, and will be hidden from view in the valley itself by a rounded breast of granite, 470 feet thick, and with a shield-face of some 1,130 steel plates, each ten feet by five and three-eighths of an inch thick, all coated with asphalt, and all dipped on the spot in a specially built dipping plant. These will be securely riveted to huge steel beams, deeply and firmly imbedded in the surrounding granite rock, and extending from the very bottom of the dam to the top. The anchorages to the side walls and bottom of the canyon will be extra heavy steel beams, bolted each two inches by twelve-inch split bolts to the face of the granite, in which will be drilled holes one foot deep and into which heavy sledge hammers will drive the bolts—these bolts being further secured by cement, which will also cover all ‘he interstices of the anchorages. The steel facing will be backed by two feet of cement of the very highest grade, behind which will be fifteen feet of hand-laid granite blocks. Backing all this up will be broken granite boulders, which giant powder may force down from the top and sides of the gorge, or, as is more likely, they may be dropped from cars run out upon a bridge which is now being built across the gorge at a height of 200 feet. The impact of these blocks falling one upon the other from such a height will not only shatter them, but will pack them tighter than any human power could do.

For the first 100 feet of height, the dam will measure 240 feet from heel to toe, and for the total height of 210 feet it will be more than (500 feet thick. An additional strengthening will be accorded the dam by a backing of granite walls at-the angle of the gorge. Thus, if by &ny possible chance the top of the structure should ever give way, the enormous mass of material could not be carried down the gorge, but would add its weight to that of the lower part.

Three outlets have been prepared for the surplus waters. One—the most important is through a great tunnel cut through the 470 feet of granite,forming the north buttress. For the first half of its course this tunnel is six feet wide and seven feet high; for the second, eight feet wide by nine feet high— large enough to carry the normal flow of the Platte river and Goose creek, with space for any flood waters that may come. In the centre of this tunnel a lateral gallery, seven feet high, twelve feet wide, and forty-five feet long IUIH been blasted at right angles on each side of the main bore, providing space for two twenty-two-ton valves laid in cement, to be operated by hydraulic pressure through lead-lined steel pipes laid in solid cement, and carried to an engine house on the top of the mountain.

In addition to being laid in cement, these valves are braced in position by steel truss beams, imbedded three and one-half feet top and bottom in solid granite and concrete. A larger valve still is a four-part one of eighteen-ton weight, which is imbedded in the granite at the entrance to the tunnel—to be operated also by hydraulic pressure, and ordinarily to be kept closed—the others being closed only to prevent back flow from a 110-foot weir. So perfect is the mechanism of these valves that one man can open or close them in a few moments.

Just beyond the two smaller valves is a second tunnel, starting from the 110-foot level of the reservoir, piercing the granite side of the rock at an angle of forty-five degrees. In height this tunnel, which will be the main waste waterway is six feet; in width, seven feet. Through it will flow the surplus water of the reservoir—thus supplying the quantity necessary for the fulfilment of the requirements of the irrigation law, and at the same time regulating the flow of the South Platte past the main pumping station.

The force of the water as it issues from this tunnel will furnish t housands of horse power for the operation of an electric plant which will be installed there. It is estimated that the water will strike the canyon’s granite wall with a force of 15,000 horse power in itself. An eighteen-ton four-part valve, the exact counterpart of that which covers the entrance to the main tunnel, and, like it, operated by hydraulic power from the enable above will close the entrance to this tunnel. A natural spill way,probably the outlet of the great prehistoric lake already alluded to, is located in a depression lie tween two masses of granite some 200 yards north of the dam, at the 200-foot level. It will be used only at flood time for relief purposes, and the water from it will fall sheer 225 feet down to thecanvon beneath.

The illustrations accompanying this article are as follows: (1) The canyon. The face of the dam will come about where the break in the flume allows the escape of the water. The 100-foot level reaches vithin a very short distance of the top of the picture. 2) Site of the dam The compressor house for the peration of air-drills and other machinery is in the ipper left corner. The 200-foot level is about fifty Bet above the tops of the smokestacks. Owing to the tarrowness and crookedness of the canyon, it is tactically impossible to obtain a satisfactory view >f the canyon to thefull height of the dam. (3) Lookng from the temporary diverting dam (4) The emporarv dam as it now appears, with the lower hirty five feet of false work for the bridge which is o beused in the construction of the dam for dump-


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