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tions are largely granite, but on the west auxiliary work to prevent leakage or unside this deposit is covered with a mass dermining at the bottom or where the of loose material, such as volcanic ash, ends of the embankment were set into to varying depths. By damming the the canyon sides. The varying volume gorge, this would be under water. of water in the river and the great dif
An examination of the proposed site ference in the current at flood height and surroundings convinced the engi- were factors in the problem. One of the neers that the only practical method of mountain water courses having a channel creating a reservoir was to make an of rapid descent, and as stated carrying earth fill embankment protected at each down yearly much debris in the food end with a reinforcement of rock, also season, the flow of the Bigwood as measured for a series of years at the site of be called a "back bone" of steel but this
GENERAL VIEW OF THE BIGWOOD EMBANKMENT. LOOKING UPSTREAM. The south and the north enclosures, the earth and rock formation, the rock-filled toes, and the enormous area filled
with earth are shown. At the left can be seen the rock deposits used in making a part of the fill.
a the dam ranges from two hundred cubic is the only material in it that is not rock, feet per second in the dry season to 6,000 sand, gravel and ordinary earth. cubic feet in May and June, when it is In creating the “Magic Dam,” as it has filled with the waters from the melting been called, some very interesting presnow of the mountains. The work must liminary work had to be performed. The be strong enough to resist the enormous engineers found they could economize force of the food currents also planned time, labor and material by first preparto divert the surplus water in an emer- ing the sides of the gorge where they gency and thus aid in relieving the.pres- joined the barrier. Chambers were dug sure.
into each side to a distance of forty feet, This is why the embankment forming all of the loose material such as sand, the reservoir, when completed will be by loose stone and fissured rock being far the largest yet planned in this coun- cleared away, so that the ends of the try and the greatest in the world for re- dam would rest against the solid rock. straining and conserving a river. The A curious feature of this work was that figures give an idea of its dimensions. the material removed was used for At the center it is no less than 789 feet building up the “toes” of the two barriers in thickness, rising to a maximum height which inclose the embankment-known of 140 feet above the natural bottom of as the up stream and down stream dams. the river, while the length of the main · Much of the composition of these dams dam is 623 feet. To this, however, is was taken from the excavation in the added an earth dike 2119 feet long ex- canyon sides and the balance needed tending from the west side, forming a from quarries. The type of the dams is barrier to hold back the overflow of the the rock fill and they were constructed flood where it has but little current. from each side by dumping the rock and Simply speaking, the work is an enor- finer “spoil” from cars on tram roads mous fill. Compared with the great reaching to the excavations. Portable Roosevelt dam its base is nearly five derricks placed in position the larger times as great in width. It has what might rocks.
rocks. By the plan an opening was left
for the river channel and the work was river flow and the Bigwood was barred but slightly interfered with during high in July, 1909, as stated. water. In July, 1909, the dams were so With the ends of the embankment far advanced that advantage was taken completed the river channel between of the low stage of the river to close them became a pond which drained them.
under the down stream barrier as an The first work done on the project, aperture had been made for letting it however, was the building of the diver- escape. Operations were now begun in sion tunnel, cut through the west wall building the "back bone” already reof the gorge, a distance of 591 feet, and ferred to. A trench thirty feet wide and lined with concrete. This tunnel is large ten feet deep was dug along the axis of enough to convey half of the river flow the embankment connecting with the exat food and was built first in order to cavations made in the side walls of the divert the river current at high water gorge.
Into the trench for its entire and minimize the pressure on the uncom- length, was driven a row of steel sheet pleted works. It will be utilized per
It will be utilized per- piling, down to solid rock, the top of the manently to serve the irrigating system. piling extending to a height of several The method of receiving and distributing feet above the top of the trench and ten the water is by means of an intake tower. feet upstream from the axis. This work This is composed of concrete, is one hun- was done to prevent any possible seepdred and thirty-seven feet above the age of water through the embankments foundation of the dam, and is designed and is also intended as a reinforcement in octagon form, to offer the least re- to the earth work. sistance to water pressure.
The water The system employed is notable for its from the reservoir enters the tower economy, and capacity, contrasted with through several openings to minimize what it accomplishes. The material is the pressure within, and is thus carried readily taken out by steam shovels movinto the tunnel. The water reaches the ing on tracks. Such is their capacity tunnel through two water gates, five feet that the shovels excavate 90,000 cubic in diameter, which are operated by hand. yards a month with their automatic fillThis unique water distribution is con- ing and emptying buckets. The spoil is nected with the embankment by a steel dumped by the buckets into tram cars on bridge of two spans each one hundred railway tracks running to the material and fifty-three feet in length. When the pits, then hauled to the edge of the fill engineers were ready to close the dams, where it is to be deposited or at som this tunnel carried away most of the point up stream from the place it is to fill.
. Then it is forced into the embankment by hydraulic jets—literally washed into position as the gold of the placer mines is extracted by the stream of water. One advantage of this method is that the filling is done more compactly and firmly than it could be formed by throwing in the material and grading it with the shovel or otherwise. As fast as the dump cars are emptied through their hopper bottoms, a stream of water is turned on the pile from the nearest pipe connection thus forcing it into the fill without the necessity of any human labor in grading or distribution. The drainage is such that the water leaves the newly made formation in a few hours.
IN FLOOD TIME. As to the formation of the dams or
Looking northward, toward the dam, showing the rockborders of the work, the down stream
filled toes. structure was composed of large rock on the face, its inner slope being of smaller material worked into the crevices between the rock. In making the upstream face the finer filling material forms the exterior, the rock work thus being protected from the eddies and currents, and not directly exposed to the river. Thus in flood season the currents and eddies cannot work into the filling between the rocks beneath. In addition to the diversion mentioned, further protection against flood currents is an emergency
BEGINNING OF WORK UPON THE TOWER FOR RECEIV
ING AND DISTRIBUTING THE CONTENTS OF spillway on the embankment. This is to
THE RESERVOIR. be four hundred feet long and extend to a depth of four feet below the crest. A the river at the highest point recorded by concrete weir has also been constructed, instruments. In all about six hundred 1,600 feet in length to carry away sur- miles of distributing canal reaching the plus water. These works were only built part of Idaho described, will be served after studying the river stages for a by this reservoir which will cover an period of years and getting the depth of area of 3,300 acres.
AN AERIAL TROLLEY
På short stretch of track in one
N aerial trolley car, driven by a
rm foundation in the earth and are
central poles, so as to equalize the pull
which drives it along the track. It is a The car has a length of fifty feet, is radical departure from the type of prosix feet wide and seven-and-a-half feet peller in use on aeroplanes, having two high. It is of a torpedo or cigar shape, huge fan-shaped blades, which, the inwith a propeller at one end, and is built ventor claims, add greatly to the force of of angle steel and aluminum.
the thrust. The construction of the fanIt is suspended on an overhead rail of shaped blades is unique. They are of iron, three-and-a-half inches broad, and sheet iron, supported by frame work of this is scientifically trussed with iron seamless steel tubing. Each of these rods which are kept taut by a system of tubes is welded into a disk of aluminum, turn buckles. Trolleys above and below an inch in thickness and six inches in di! hold the car firmly to the rail. The track ameter, and the driving shaft runs is suspended from twelve-inch wooden through this series of fifteen aluminum posts with iron bars, and these posts rest disks like a core.