ROM FROM OWYHEE DAM' Owyhee Project, Oregon-Idaho By J. L. Savage, Chief Designing Engineer, Denver Office, Bureau of Reclamation an engineering standpoint the Owyhee Dam, to be constructed on the Owyhee project in eastern Oregon, is the most outstanding dam undertaken to date by the Bureau of Reclamation. With a height of 405 feet at maximum section and a total height of 520 feet above the lowest concrete in the foundation cut-off, this dam is likely to stand as the highest dam in the world until the great Boulder Canyon Dam is constructed. The Owyhee Dam will be of the concrete archgravity type, with about three-fourths of the water. load carried to the abutments by arch action and onefourth carried to the base by gravity cantilever action. The radius of the upstream face of the dam at the top will be 500 feet and both faces will be concentric about a common center. The top thickness will be 30 feet and the bottom thickness at maximum section will be 265 feet. The upstream face will be vertical for the top 75 feet and below this will have a batter of 0.05 to 1. The downstream face will be generally on a slope of 0.626 to 1. The accompanying drawing shows the general plan, elevation, and sections of the dam in addition to the principal sections through the diversion and spillway tunnel and other general information. PRELIMINARY INVESTIGATION The preliminary investigation of the "Hole in the Ground" reservoir site and the Owyhee Dam site included five different geological examinations by three eminent geologists and the careful consideration of three eminent consulting engineers. A total of nearly $100,000 was expended for the preliminary investigations, including the cost of test drilling the dam site. Test holes to the number of 73 and to a total length of 7,800 feet (nearly 11⁄2 miles) were drilled to disclose the character of the formation rock on which the dam will be constructed. dam where water depths are great. The upper end of the basin where water depths are small is in practically tight formations of conglomerate, sandstone, shale, and tuff. The portion near the mouth of Dry Creek is in Columbia River basalt, which constitutes the only portion where any considerable leakage might occur. However, the geological opinions indicate that no serious leakage is to be expected through this formation. owing to the great distance (several miles) to an outlet with various possibilities for stoppage along the way. GEOLOGY OF DAM SITE The test holes show the foundation material to consist of sand, gravel, cobbles, and bowlders to a maximum depth of 60 feet, below which rhyolite is found of the same character as that exposed in the canyon walls. The rhyolite extends to a depth of 170 to 215 feet below the water surface and is bedded on pitchstone agglomerate. Tuff of unknown depth is found below the agglomerate. All of the geological and engineering opinions have agreed that the foundation rock is suitable for the construction of the dam. The most serious flaw in the dam site is the presence of a shattered zone or fault crossing the dam site at about the center of the river canyon. This shattered zone was first disclosed by the core drilling at the dam site and later by an open test pit located about 1,450 feet downstream from the dam site, where the fault leaves the river canyon. The shattered rock is confined between two seams of clay gouge spaced about 10 feet apart. These seams are described by the geologists as indicating movement along fault lines. The rock between these clay-gouge seams is broken rhyolite composed of fragments generally under 6 inches in their longest dimension and occasionally reaching 14 inches in length. Although minutely shattered the fragments retain their original relative position and have not been rotated. This shattered zone is believed to extend through the stratum of rhyolite to agglomerate or tuff and it is anticipated that considerable leakage would occur through this broken rock if left in place. The geological opinions have indicated that the shattered zone probably stops at the less brittle formation of agglomerate below the rhyolite and in any case at the tuff. It is therefore anticipated that the fault zone cut 1 Detail specifications for the construction of Owyhee Dam will be found in the appendix, p. 138. off will extend entirely through the rhyolite and that it will terminate on either the agglomerate or the tuff. PRELIMINARY DESIGNS AND ESTIMATES Before adopting the arch-gravity type of dam careful preliminary designs and estimates were prepared for Owyhee Dam Site five different alternative types, including a light arch section, an intermediate arch section, a heavy arch section (arch-gravity section), a straight gravity, and a slightly curved gravity dam. Based on these studies the heavy arch or "arch-gravity section" was adopted. This type is in reality a section which if straight and not subject to uplift pressures would figure safe as a gravity dam with the resultant line of pressure passing through the downstream one-third point and with a sliding factor of 0.65 or less at all elevations. While this heavy-arch dam is in no sense a gravity section it has been called an arch-gravity dam for the reason that the loads are carried partly by arch action and partly by gravity action. In all of these studies, including both the arch and gravity sections, uplift pressures have been assumed to act over the whole area of the base, varying from full hydrostatic pressure at the upstream face to one-half hydrostatic pressure at the drainage wells, and diminishing uniformly thence to zero or tail-water pressure at the downstream face. The vertical components of water pressure on the upstream and downstream faces have been included in the cantilever loads. The cantilever studies include the effect of convergence of the sides and the shear and moment deflections of the foundation. The arch studies include the effect of shear, rib shortening, and abutment deflections due to thrust, moment, and shear. The formulæ for calculating the yielding of foundation and abutments are taken from the paper Über die Berechnung der Fundament Deformationen by Dr. Fredrik Vogt. The effects of yearly temperature changes were included in the preliminary computations. The final studies will include consideration of the effect on the stresses of the setting heat remaining in the concrete at the time of grouting the joints. It is also intended to study in the final computations the effect on the stresses of saturation of the concrete. By giving the cantilever elements an initial water load before grouting the vertical contraction joints, which can be accomplished in the normal procedure of filling the reservoir, it may be possible to better distribute the water load between the arch and cantilever elements, thereby reducing the arch stresses, which are the maximum stresses. The system for grouting the vertical contraction joints has been designed with this procedure in mind, and the specifications will make provision for beginning storage in |