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HOW THE CONCRETE IS SET.
The south entrance to the diversion tunnel of the great irrigation project in southern Idaho.

ID A HO'S HUGE “MAGIC DAM

By

DAY ALLEN WILLEY

AST volcanic eruptions have located in four sections of the state, the resulted in a very large area of largest tract aggregating 55,000 acres. southern Idaho being covered To conserve the water necessary to with a deposit of what is insure permanent and adequate irriga

known as volcanic ash. This tion it was necessary to form a barrier material is known for its richness when across a gorge through which the main properly irrigated, as is shown in the river flows. The formation at this point Hood River valley in Oregon, lying be- is of such a character that it would be tween the extinct volcanoes of Mount impossible to erect a masonry dam. The Baker and Mount Hood. Here orchards distance from the nearest railway transproduce some of the finest fruit grown in portation, twenty-five miles, also entered America, especially apples.

into the problem. The natural course of The portion of Idaho covered with the river is over a bottom which is comvolcanic ash caused plans to be consid- posed of sand, some clay and strewn ered with a view to irrigating a part of with bolders and rock fragments which it from what are known as the Bigwood have fallen from the canyon sides, and River and Littlewood River which rise have been washed down by flood curin the ranges of the Smoke and Saw- rents. The depth of this material has tooth Mountains. The water shed serv- been found to be as great as sixty feet ing these rivers is estimated as sufficient lying above the foundation of hard rock. to irrigate fully 150,000 acres which are On each side of the canyon the forma

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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 difAn 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 flood end with a reinforcement of rock, also season, the flow of the Bigwood as meas

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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 tbe rock deposits used in making a part of the fill.

ured for a series of years at the site of be called a "back bone" of steel but this 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 flood 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. 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.

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THE EXCAVATION WORK IN THE HILLSIDE, WHERE, LATER. ROCK WAS SET INTO ROCK,

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THE CONCRETE APPROACHES TO THE NORTH PORTAL OF THE DIVERSION TUNNEL. UNDER

CONSTRUCTION.

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 flood 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 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 some this tunnel carried away most of the point up stream from the place it is to fill.

filled toes.

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 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 RECEIY

ING AND DISTRIBUTING THE CONTENTS OF spillway on the embankment. This is to 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.

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THE RESERVOIR.

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