Auburn Dam will be a thin, double-curvature concrete arch dam about 685 ft high. With a crest length of about 4000 ft, it will be the world's longest single-arch dam. The site is located on the North Fork of the American River about 40 miles northeast of Sacramento, Calif. Construction is scheduled to get under way in 1970.
In planning for the design and construction of Auburn Dam (see artist's conception), the foundation material, with its various discontinuities, is being investigated and the structural characteristics of the rock mass evaluated. The investigation and testing programs will aid in defining the pertinent geologic features and their individual and cumulative influence on the foundation. The work includes geologic surface mapping, surface trenching, core drilling, geophysics, tunneling, excavating shafts and raises, performing in-situ tests, and conducting laboratory tests on core and block samples.
Competent amphibolite is the predominant material in the foundation, but significant bodies of chloritie, tacose, and metasedimentary rocks, a strongly developed jointing pattern, faults, and shear zones also affect the deformability of the rock mass. A comprehensive rock mechanics program, closely coordinated with geologic investigations as well as the engineering studies, therefore is required.
The rock mechanics work is a combination of many analytical studies and experimental investigations. The experimental work includes laboratory tests on core samples from holes drilled for geological studies of the entire foundation and in-situ tests performed in selected areas. The laboratory testing program evaluates the general properties of all important rock types and aids in correlating many of the key structural features encountered within the foundation. To derive maximum benefit from the field tests, they are conducted in materials which represent a [Artist's conception of Auburn Darn, California.], significant portion of the foundation or in zones where special considerations are warranted.
Auburn damsite is located on the west flank of the central Sierra Nevada. The American River has cut a 1000-ft-deep canyon through a peneplain capped by Tertiary volcanic and fluvial sediments into metamorphosed Paleozoic and Mesozoic volcanic and marine sedimentary formations intruded by ultrabasic and granitic plutons during Nevadan orogeny in late Mesozoic. The foundation at the damsite consists mainly of a steeply inclined sequence of amphibolitic foliated metamorphic rock with minor metagabbroic, metasedimentary, and meta-ultrabasic bodies. The latter are represented chiefly by talcose and chloritic schists with lesser amounts of serpentine. Fine-grained dioritic dikes are also present, and locally prominent. Some faults and shear zones in both abutments and in the channel section, as well as low-dipping and steep joint sets and cleavage along the metamorphic structure, form a pattern of discontinuities in the rock mass.
The distribution of the various lithologic types and pattern of discontinuities are being established by a comprehensive program of geologic exploration involving detailed mapping of extensive surface trenches and underground workings and core drilling. This program is providing a basis, along with engineering design considerations for all phases of the foundation testing program. Fig. I shows a geologic model1 constructed by the project geological staff to aid in the selection of the in-situ test sites and special core sampling areas for laboratory testing.
