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Figure 3. --Sketch showing the axis positions of three sites considered during
the O'Sullivan (Potholes) dam site investigations.

Exploratory investigations were begun by the Bureau early in 1943. Many possible dam site locations were investigated and considered. 17-12/ Seventy-two test holes were drilled for studying the rock structure, ground-water conditions, and the character and extent of overburden materials. Three test pits and one exploratory trench were dug for visual observations of subsurface conditions. In the left third of the dam site area are disturbed basalts which have been severely crushed and sheared by the folding and faulting of the Lind Coulee flexure. Because of the underlying basalt beds in this area only minor reservoir seepage was expected.27

Of the many sites investigated by the Bureau, three were worthy of detailed consideration, as indicated previously. Dam site No. 1 (fig. 3) is located across the Drumheller channels, and would not require an excessive amount of embankment yardage. Because of the roughness of the area, however, the estimated foundation treatment costs were considered excessive, and it was recommended that consideration of this site be abandoned. The remainder of the surface and subsurface explorations and testing were therefore confined largely to defining the conditions relative to sites Nos. 2 and 3, located just above the Drumheller channels and along the upper edge of the Drumheller channels, respectively (fig. 3). These investigations demonstrated that the several lava flows which constituted bedrock in this vicinity were essentially impervious, but that the interflow zones or horizons of porous, broken basalt at the contacts between successive lava flows were relatively pervious. Thus, the movement of ground water in a horizontal direction was aided by the permeable interflow zones, whereas its movement in a vertical direction was impeded by the lava flows themselves.

The drilling operations revealed that, although the basalt flows were severely faulted and sharply folded along the Lind Coulee flexure, they were elsewhere essentially flat-lying. Along the northern edge of the Lind Coulee flexure zone, the basalt flows were so severely crushed as to create a vertical (underground) barrier which effectually blocked the regional southward movement of ground water through the interflow zones. As a result artesian ground-water conditions existed north of and up to the very northern edge of the flexure, whereas normal ground-water levels existed in the central and southern portions of the flexure and in the undisturbed basalt flows to the south.

Although site No. 2 was considered superior to site No. 3 with respect to geologic conditions, it was recognized that it possessed certain engineering disadvantages. In order to utilize the Lind Coulee drainage in the reservoir it would have been necessary to build a small dam across Lind Coulee and connect the two sections of the reservoir by a canal. The embankment volume for a dam at this site, including the small dam across Lind Coulee, was estimated at 9 million cubic yards, which was considered excessive.

Dam site No. 3, the site selected for the dam, is located along the upper edge of the Drumheller channels (fig. 3). The entire site lies downstream from the tightly sheared section of the Lind Coulee flexure. The drill hole cores showed that a large percentage of the rock is good quality and relatively tight. A dam at this site opens the main section of the reservoir to the Lind Coulee drainage portion without the necessity of a small separate dam. A dam at this site was estimated to require about two million cubic yards of embankment less than one at dam site No. 2.

The foundation conditions, revealed as excavation and construction of the dam proceeded, were not materially different from those visualized during the investigations. The western half of the dam foundation consisted of essentially horizontal basalt flows, whereas the eastern half lay largely within the zone of folding and faulting of the Lind Coulee flexure.

10. Materials Investigations. - The search for 10 to 20 million cubic yards of suitable embankment materials for the construction of O'Sullivan Dam was started January 15, 1945. Initial investigation showed that more than one type of suitable material was available for construction of the dam. Field investigations were therefore conducted on the premise that a zoned structure would be built. All investigations prior to construction, other than foundation exploration, were centered in three borrow areas designated as areas 1, 2, and 3 (fig. 4).

Borrow area 1 is located immediately upstream from the dam near the left abutment. The area is covered by sedimentary silts and fine sands deposited by Crab, Lind . Coulee, and Spring Creeks, together with more recent deposits of windblown sand. The silt and fine sands were roughly stratified except for localized deposits of the windblown

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sands. In general, composite cuts of about 15 feet resulted in a relatively impervious material which could be classified as a poorly graded sand-silt mixture (SM). Selective cutting and thorough mixing, however, were required to maintain acceptable permeability coefficients. The area was investigated by 15 hand-dug test pits and 12 hand-auger holes. Twenty-one composite samples were obtained and tested in the project laboratory.

Borrow area 2 is located on the slope of the Frenchman Hills just above the west, or right abutment of the dam. The area is covered by a relatively thin deposit of windblown silts and fine sands. No particular stratification of this material occurs in the area. The silts and mixtures of silts and fine sands were deemed sufficiently impervious for use in the core of the dam, but the sands by themselves have undesirable high percolation rates. In portions of this area where silts could not be conveniently mixed with them, the sand was wasted or left in place in the pits. The area was investigated by 30 hand-dug test pits, and 27 composite samples were tested in the project laboratory.

Borrow area 3 is located on the sand and gravel terrace between Lind Coulee and Crab Creeks, approximately one mile north of the left abutment of the dam. The terrace is a deep outwash deposit of poorly graded coarse sands with varying amounts of gravel, overlain by a thin layer of silty topsoil. A cut consisting of a mixture of 2 feet of the silty topsoil and approximately 10 feet of the underlying sand and gravel provided a suitable material for the semipervious zones of the embankment. A second cut of clean sands and gravels was used for zone 3 material. The area was explored by 34 hand-dug test pits and three hand-auger holes. Representative samples of both the proposed first cut and the clean sands and gravels from a second cut were tested in the project laboratories.

Localized areas of pervious sands were found to be more numerous and extensive in borrow area 1 than was indicated by exploratory holes. As a result, the amount of suitable material was less than anticipated. In an effort to obtain additional zone 1 material, several other areas were explored. Area 1-A, located adjacent to area 1, was prospected by four hand-auger holes. It was estimated that this source contained 130,000 cubic yards of suitable zone 1 material. Because of a rising water table and an excess of fine permeable sand, the quantity of suitable material fell short of the estimate. Area 1-B, located downstream from the dam axis and adjacent to Crab Creek, was propected by 11 hand-auger holes. An extension of borrow area 1 was prospected by 44 hand-auger holes. Tests showed that the material from these areas was suitable for zone 1 material.

Borrow area 7 was explored after area 1 was depleted and certain portions of the fill were too far away for an economical hauling distance from area 2. Tests showed that the material obtained from area 7, located one-half mile below the dam, contained suitable zone 1 material. This area was divided into three portions based on the permeability of the material. These portions were designated as 7a, 7b, and 7c, with 7a material having the lowest percolation rate and 7c the highest. The material designated as 7c was not used in the embankment.

When the specifications for the spillway were changed (sec. 20), it became necessary to locate an additional suitable rock source for surfacing the dam. Three sites of suitable rock were located (fig. 5) and designated as borrow areas 4, 5, and 6. These are discussed in section 33(f). All of these borrow areas contained an overburden of soil and loose rock of varying depth, up to a maximum of 4 feet. An examination revealed that the rock obtained from these areas was suitable for use on the dam.

Quantities of material obtained from each borrow area are tabulated below:

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