Rock-fill Dams.—Hydraulic mining on the Pacific slope developed the rock-fill dam, and it has been utilized for irrigation works. Orig inally this darn consisted of a dump of loose rock in a canyon or other rock-bound site, tak ing its natural slope down stream, the upper slope being hand-laid and steeper and faced with redwood or other planking and calked.
Later a sheet of iaetal was substituted for the planking, and the construction was varied by setting the metal sheet vertically in the heart of the dam. The metal sheet was coated and some times enclosed in asphaltum concrete. Such dams had an independent spillway.
These dams have been carried to consider able heights, several exceeding 100 feet, the highest being the Lower Ottay, near San Diego, Cal., 161 feet above rock and 130 feet above natural surface. It is 565 feet long, 20 feet wide at the top and 282 feet at the base. Set tlement has occurred in most of these dams, causing apprehension, but no serious disaster has occurred.
A darn of this character was undertaken in Gonse-Peck canyon on the south fork of the South Platte, some 50 miles southwesterly from Denver. In order to avoid settlement, the rock was dropped into place from a height of about 100 feet, as experience in the spoil-banks of the Chicago Drainage Canal had shown such con solidation from this cause that blasting was re sorted to in opening up a roadway across such a fill. The downstream slope was to be flattened and the hand-laid stone up stream was to be filled with lean concrete, and faced with a metal sheet. When this darn had reached a height of 50 to 60 feet it was overtopped by an unprece dented flood and away. The experiment was valuable in showing that a mound of gran ite rock thus consolidated was nearly water tight, under a head of 50 feet of water. (The facing was not carried up). This experience suggested that a filling of finer material gradat ing to adobe at the face would have made a successful structure without the intervention of wall or metal sheet. The site was later occu pied by a high masonry dam.
The Castlewood Darn (Colorado) is a rock fill dam 600 feet long and 92 feet high. It is 8 feet wide at the crest, and 225 feet at the base, and has a facing of rubble laid in cement on both slopes.
The Pecos Valley Dam is a rock-fill, 1,686 feet long and 52 feet high. It is 14 feet wide at the top and 115 feet at the bottom, with an earth backing for 162 feet on the water slope. The earth fill is faced with riprap one foot thick.
The Morena Dam (California) is 520 feet long, with r( maximum height of 267 feet. The water slope 9 horizontal to 10 vertical, of masonry behind which was placed loose rock by hand 54 feet wide at the base and 16 feet at the crest. The loose rock-fill was dumped behind this, forming a total base of 357 feet. The water face was finished with reinforced concrete slabs 12 inches thick.
Experience in this class of construction also suggests the flatter slope down stream and the water-tight skin at the upper face, rather than a core or intercept in the heart of the dam. Dams which depend on a timber or metal sheet for tightness cannot be regarded as permanent structures, and a lack of care in maintenance invites destruction. They are, however, useful expedients in the early development of remote regions.
Metal Dams.— Metal dams are often pro posed. and a few examples have been executed in steel. Steel is perhaps more perishable than timber, and its failure from growing weakness and lack of care is likely to be more complete and disastrous. It is adapted to much greater heights, but is to be regarded as an expedient for a few years until something better is justified.
Prominent examples of the steel dam are: The Ash Fork Dam (Arizona), 184 feet long and 46 feet high. .It is of open front design, the deck being supported on 24 triangular bents set 8 feet between centres on a concrete foundation. The water face is of steel plates which are riveted to 20-inch I-beams which form the upstream members of the bents. The deck plates are of an inch thick, and are curved to a radius of feet (laterally), the concave side being up stream. The deck makes an angle of 45 degrees with the horizontal.
1-7!. The Redridge Darn (Michigan) hi- the ,:same general construction as the Ash Fork Darn. it is 464 feet long and 74 feet high. The face Members of the bents vary from 15-inch I-beams -,on the low bents to 24-inch I-beams on the high ones.