Twenty-six test pits were excavated during the 1934 investigations. An additional exploration program in 1941 and 1942 consisted of prospecting nearly 1,000 acres by means of 86 test pits, 40 auger holes, and 4 trenches. At that time it was estimated that about 22,000,000 cubic yards of embankment materials were available. The following table summarizes the investigations of 1941-1942: The primary purpose of the preliminary investigation was to locate materials rather than to select the most desirable areas. Consequently, laboratory testing, including determination of rock content and amount of oversize, was not undertaken at that time. Some 400 samples were taken, however, and stored in the project laboratory for future testing. Detailed borrow-area explorations were resumed in 1945. This program consisted of 123 test pits, 283 auger holes, and 7 trenches in areas C, D, and the Wheat Area. The initial investigations which had been confined to the comparatively shallow topsoil material indicated the most probable areas in which impervious materials existed. The detailed explorations were resumed not only for purposes of gaining additional information on the characteristics and extent of the impervious soils, but also to explore the underlying gravelly and rocky materials and, to some extent, to determine the probable field-moisture conditions. Samples of the various strata were secured as the 4- by 6-foot test pits progressed. Samples containing rock were screened on the 5-, 3-, and 1-1/2-inch screens, and the respective weights determined for complete mechanical analyses. Composite samples, conforming to possible shovel cuts averaging approximately 20 feet in depth, were prepared in the project laboratory for additional testing. (a) Borrow Area A.-- Area A (L) is located on the right side of the river immediately adjacent to and partially upstream from the right abutment of the dam. Initially, the area was divided into two parts. The lower portion consisted of a terrace deposit of river gravels, cobbles, and boulders underlying a shallow layer of silt. The upper portion consisted of slope-wash debris composed of clayey silt, small angular rock fragments 1/4 to 5 inches in size, and occasional rock blocks 6 inches to 5 feet in size. The detailed investigation program did not include additional work in this area for the following reasons: (1) Preliminary design studies indicated that the lower terrace deposit of pervious material would be inundated during construction. (2) The upper slope wash (impervious material) was considered a questionable source of borrow because of the indicated high cost of clearing and stripping, and because of contemplated difficulty in excavating the relatively shallow deposit of suitable material from the steep hillside. (b) Borrow Area B.--Area B (not used in specifications design) is located on the left side of the river, extending upstream from the damsite a distance of about 3,000 feet. The material consisted of a mixture of clay, silt, small angular rocks, and large rock blocks. The area was thickly covered with aspen and pine trees, 1 to 18 inches in diameter. The topsoil contained a dense mat of roots requiring stripping to depths of 18 inches. This area was also considered to be a questionable source of embankment material because of: Relative inaccessibility Unsatisfactory moisture conditions Excessive amount of rock fragments and large The detailed investigations of 1945-1947 did not include additional test holes in this area. (c) Borrow Area C.--Area C (K) is located on the left side of the river, extending from about 3,000 feet to 8,000 feet upstream from the damsite. The outwash material consisted of a variable depth of clayey silt overlying a mixture of silt, sand, gravel, and boulders varying in size to 1-1/2 cubic yards in volume. A portion of the area was densely covered with pine trees, but since the area lies within the reservoir, the clearing cost was not considered to preclude use of the area as borrow. From 6 to 12 inches of stripping would be required. The deposit of fine-grained, rock-free soil ranged in thickness from 10 to 35 feet. Unsatisfactory moisture conditions were expected in portions of the area, particularly along the southeastern boundary. It was believed that it might be necessary to restrict use of the area to periods of the construction season when moisture conditions are favorable, and to exploit the area to the fullest extent during such periods. (d) Borrow Area D.--This area (M) is located 1/2 to 1 mile downstream from the dam on the left side of the river. The area consisted of two parts. An out wash deposit along the side of the canyon was composed generally of a variable depth of clayey silt topsoil overlying a mixture of clay, silt, sand, gravel, and boulders with no particular stratification. For investigation purposes, this portion of the area was designated D-1 and considered separately. The area was densely covered with aspen and pine trees. Unsatisfactory moisture conditions were indicated over much of the area. The A second portion of the D area, designated area D-2, included a terrace deposit of clean sand and gravel overlain by outwash deposits from two tributary streams. upper outwash material consisted primarily of a layer of silty topsoil of variable depth. A layer of silty gravel intervened between the topsoil and the underlying river sands and gravels. About 43 percent of the area was covered with buck brush and small aspen trees; the remainder was under cultivation. Stripping of 6 to 12 inches would be required. (e) Wheat Area. The Wheat Area (N) is an outwash deposit located on the right side of the river about 1-1/2 to 2-1/3 miles downstream from the dam axis. The material consisted essentially of three types of soil. A layer of clayey silt, ranging in depth from 0 to 35 feet, overlay a coarser material composed of a mixture of sand, gravel, and clayey silt in portions of the area. In other locations the top layer of fine-grained soil covered a stratum of sand and gravel which in turn was underlain by clayey silt. It was considered that moisture conditions in this area would be more favorable for embankment construction than in any other borrow area. A part of the area was covered with a dense growth of small aspen trees, and the remainder with scattered sage brush and buck brush. From 6 to 10 inches of stripping would be required. (f) Riprap and Rockfill.--Samples of andesite were obtained from the left abutment and tested in the laboratory. The rock was considered suitable for use as riprap. More than an adequate amount was available. A large quantity of rock from required excavation, as well as oversize rock from borrow areas, would be available for use in the embankment, and the specifications design did not contemplate borrowing rock for either riprap or rockfill purposes. 16. Selection of Borrow Areas. To construct the dam as designed for the specifications, it was planned to obtain 3, 540,000 cubic yards of fine-grained zone 1 (impervious) material from area K. A requirement was placed in the specifications for early clearing in an attempt to improve the moisture conditions. Use of the area was restricted to portions of the construction season when the moisture condition was favorable. The specifications item of 3, 540,000 cubic yards was considered a minimum quantity which would be required from this area, and, for this reason, full use was to be made of the area when conditions permitted. A minimum quantity of 1, 286,000 cubic yards of material was contemplated from area L; investigations were not complete enough to permit a firm estimate of the quantity available. Upon advice from the field forces, the area was extended to insure inclusion of at least 1, 500,000 cubic yards of material. It was impossible on the basis of investigational data available to delineate the portions of the area requiring separation from those containing rock-free material, although it was definitely indicated that considerable quantities of rock-free material did occur. Hence, the total specifications item for excavation in this area included separation. It was anticipated, however, that about 600,000 cubic yards may go directly to zone 1 without being processed through the separation plant. Oversize rock from the separation plant (estimated to be about 5 percent of the material requiring separation) was to be used in the cobble and boulder blanket on the downstream slope of the embankment. Area D-2 was extended to include additional sand and gravel at the northeast corner of the area. A portion of this area had been explored as a source of concrete aggregate and had been called area E. The entire area was designated in the specifications as area M. The area would provide a variety of materials. The upper portion was to be excavated to provide zone 2 (semipervious) material, and the depth of cut controlled so that the remaining material would be free-draining zone 3. In order to uncover the clean sand and gravel, more than one cut was required in zone 2 material in portions of the area. In other portions, the shallow silty topsoil would be combined with some underlying gravel to produce comparable zone 2. In some portions of the area, the overburden was less than 2 feet in thickness. The contractor was required to excavate the topsoil in these areas and deposit it in similar locations, thus providing minimum cuts of zone 2 material of 5 feet which would meet the requirement of at least 40 percent topsoil. The contractor was to be paid stripping prices for excavation of the shallow (less than 2 feet) overburden and receive payment again under the item for "Excavation, common, in borrow area M, separation and transportation to dam embankment." Cobbles and boulders separated from the zone 2 material were to be used in the downstream cobble and boulder blanket, in the cobble and boulder blanket beneath dumped riprap, or, if in excess to these requirements, in zone 4 rockfill. In addition to embankment material, this area would supply concrete aggregate, access road surfacing, and bedding material for drains. It was contemplated that 2,000,000 cubic yards of zone 1 material would be obtained from area N. Underlying the zone 1 material in area N were large quantities of gravelly soil which could be exploited for use in the embankment if necessary. D. Zoning 17. General. The detailed investigations of 1945-1947 are summarized in a zoning report dated May 1, 1947. The investigations were conducted on the basis of a zoned structure utilizing three types of material: Three possible zoning schemes, based on an embankment requirement of approximately 11,000,000 cubic yards, and on various combinations of material from required structure excavation and borrow areas, were presented. Estimated quantities of available borrow material were as follows: In addition to rolled embankment zones 1, 2, and 3, a zone of sluiced sand and gravel and a large quantity of rockfill (to be obtained from required structure excavation) were incorporated into the sections. The final zoning was based on considerations of the following elements: (1) Stability of the section. (2) Provision of an upstream impervious blanket. (3) Maximum utilization of required structure excavation. (4) Economical exploitation of borrow areas. Stability studies, discussed in sections 12 and 13, which were based on laboratory tests of the fine-grained topsoil containing no rock, indicated the desirability of confining this material in a thin core. The depth of deposit of this fine-grained material in two of the principal borrow areas, K and N, precluded mixing gravelly material with the first cut. At the same time, quantitative considerations required that this material be used. In the M area, the shallow overburden did not provide satisfactory shovel cuts by itself, and the economics of this situation dictated a cut combining some gravel with the overburden. In so doing, ample quantities of clean sands and gravels would be made available. It was believed that the heterogeneous deposit of slope wash in area L would provide both fine-grained material containing no rock and gravelly impervious material. It was determined that the gravelly material from both areas L and M would be impervious and would require processing through a separation plant to remove the plus 5-inch oversize. It was further determined that the required common excavation for structures fell into two classes; namely, clean river sands and gravels, and impervious material containing various amounts of rock including oversize. In view of these conditions, and from a stability standpoint, the embankment zoning shown in the specifications was developed. In this specifications design, zone 1 impervious material composed the central core of the embankment and provided an upstream impervious blanket. The material was to be obtained from borrow areas K, L, and N. Cuts in these areas were to be regulated to prohibit inclusion of oversize rock so that separation would not be necessary. Transition sections of zone 2 material flanked the core on either side. This material, although containing as much as 40 percent rock, was relatively impervious. Both zones 1 and 2 were to be compacted to 6-inch layers by tamping rollers. Next to the transition zones, both upstream and downstream, was zone 3 material-clean river sands and gravels. This material was to be compacted to 12-inch layers by crawler type tractors. The sands and gravels were to be obtained from required common excavation for structures and from borrow area M. The pri The downstream outer zone below elevation 5450 was zone 4 rockfill. mary source of material for zone 4 was specified as rock from structures and tunnel excavations. Cobbles and boulders from separation plants were also permitted in zone 4 if the yield was in excess of the requirements for downstream slope protection. Above elevation 5450 on the downstream slope, a cobble and boulder blanket of variable thickness was planned to utilize most of the oversize from the separation plant. The specifications required that the upstream slope of the embankment be protected by a layer of riprap obtained from rock excavation required for structures. It was believed that suitable rock for riprap could be obtained from open-cut rock excavation because of the deep cuts. The andesite rock was considered of good quality and would be suitable for riprap provided specifications sizes could be obtained. The rock was to be reasonably well graded from the maximum size of 1 cubic yard down to 1/2 cubic foot. Figure 38, the materials utilization chart, shows the sources of the materials which were to make up the various zones of the embankment, based on the design used in the specifications. E. Miscellaneous Details 18. Location of Dam Axis. - The approximate location of the dam embankment axis was selected as a result of the preliminary investigations and studies made in 1946 and 1947. The final location of the axis was greatly dependent on the location of the spillway, outlet works, and power system, all of which were located on the left abutment to obtain rock foundations and to provide the shortest possible tunnels. The locations of the power intake structure and powerplant were definite limiting factors in determining the location of the axis intersection with the left abutment so that the slopes of the embankment would not interfere with the flow of water to the intake structures or the discharge from the powerplant. When study of the right abutment disclosed the possibility of its being permeable, it was decided to place an impervious blanket (protected with a sand, gravel, and cobble blanket and riprap) over the right abutment upstream from the dam embankment to increase the distance of percolation, and to place a zone 3 blanket (protected with a cobble and boulder blanket) on the right abutment downstream from the dam embankment to stabilize the abutment. The location of these blankets on the right abutment to most effectively perform their functions was a limiting factor in determining the exact location of the axis intersection with the right abutment. The dam axis is a straight line between the points of intersection with the abutments. 19. Freeboard. - Bureau of Reclamation experience and study indicate that a minimum freeboard of 10 feet above the normal water surface is desirable for a dam of this height and importance. Routing of the design flood resulted in a maximum water surface at elevation 5621 which encroaches 1 foot on the 10-foot normal freeboard for short periods of time. A study of probable wave height plus wave rideup substantiated the choice of a 10-foot normal freeboard and a 9-foot freeboard above the maximum water surface, thus placing the crest at elevation 5630. 20. Crest Treatment. - The crest width of 40 feet was determined largely by previous study and experience. This is considered a minimum desirable width for dams of this height and is adequate in this case so far as stability is concerned. This width is also utilized to provide a roadway across the crest to connect the Forest Service road with U. S. Highway No. 26. A 6-inch layer of gravel surfacing is provided on the crest for a wearing surface. Concrete guard posts at 25-foot centers delineate each edge of the crest. The roadway over the crest is crowned 3 inches in the center to provide drainage. Zones 1 and 2 of the embankment are terminated at elevations 5620 and 5625, respectively, to permit sufficient working width at the top of the zones 1 and 2. The embankment above elevation 5625 is constructed of pervious sand and gravel (zone 3). This protects the crest against damage from frost action and forms the subgrade of the roadway. Camber was provided on the crest of the dam so that consolidation of the embankment and settlement of the foundation will not reduce the amount of freeboard. The maximum camber of 2 feet is provided at axis station 21+00 by parabolic curves. |