Palisades Dam and Powerplant, Constructed 1951-1957, Palisades Project, Idaho

north 650 west. This variation is in part due to warping of the beds when they were tilted, in part due to distortions accompanying the andesite intrusion and, in part due undoubtedly to the lens-like nature of some of the beds. The extreme eastward strike is found only on the right abutment upstream from the axis of the dam. In the right abutment exploration tunnel the strike is north 15° west. The average principal strike

of the series in the damsite area is about north 100 to 150 west. Other local variations in the strike and dip of the sediments have been recognized notably near the left abutment where the sediments warp around the upstream end of the intrusive andesite mass, and in the area near trench D on the right abutment. In the latter instance, the strata are believed to have been disturbed by landslide action.

PART II--DESIGN

CHAPTER III--Design--EARTH DAM

A. Foundation Design

9. General Treatment. The design of the embankment foundation for Palisades Dam, including both the valley floor and the abutments, followed a typical pattern of earth dam foundation treatment. Investigations were made to determine the characteristics of the foundation as to stability, permeability and suitability of material for use in the dam embankment. Figure 6 shows the general plan and sections of Palisades Dam.

To form a proper bond between the foundation and the dam embankment, specifications required that all materials that might interfere with the proper compaction of materials in the foundation and embankment be removed from the area to be covered by the dam embankment. Steep slopes in common excavation were to be reduced to provide bonding surfaces which could be compacted with tamping rollers. The excavation required for the foundation of the dam embankment was divided into two types. The first type, stripping, consisted of the removal of material which was considered to be unsuitable for embankment material and was composed mainly of talus from the abutment slopes and topsoil containing organic material. The second type consisted of excavation for the cutoff trenches in the foundation, excavation for the embankment foundation on the right abutment, and possible excavation on the left abutment. The material from stripping operations was to be wasted. Studies of the results of foundation exploration indicated that most of the material of the second type would be usable in the dam embankment, either directly or after separation.

The entire area to be occupied by the dam embankment was required to be stripped. The logs of the foundation exploration (fig. 7) indicated that there was approximately 3 to 5 feet of talus and topsoil that should be removed. On the basis of these logs, the area to be stripped was divided into three portions: the flood plain to be stripped to a depth of 5 feet, the right abutment above elevation 5400 and the left abutment above elevation 5415, both of which were to be stripped to a depth of 3 feet. Any earth underlying the proposed zones 1 and 2 and overlying the andesite which was not removed by the stripping operation was to be removed prior to placing the embankment.

The strata of clay, silt, and sand strike about parallel with the general trend of the valley. In order to provide a longer percolation path through the more pervious sand strata, two cutoff trenches were required to be constructed: the main cutoff trench located at the axis of the dam, and the upstream cutoff trench approximately at the upstream toe of the embankment. The main cutoff trench is 300 feet wide in the valley portion of the foundation and reduces in width on the abutments. It extends from the andesite of the left abutment across the valley floor and up the right abutment to the crest of the dam. The 100-foot-wide upstream cutoff extends from the left abutment across the valley and up the right abutment. Both cutoff trenches were excavated through the river sands and gravels and contact the formation bedrock which consists of sedimentary strata.

Where the andesite on the left abutment is exposed by excavation of the main cutoff trench, two concrete cutoff walls were required to be constructed. Excavation for the footings of the walls was to be performed in a manner to prevent shattering of the rock beyond the limits of excavation to insure a watertight joint between the rock and cutoff wall footings. Water tests in exploration holes on the right abutment indicated some strata to be pervious. By increasing the percolation path, the leakage was held to a minimum. To accomplish this, a blanket of zone 2 material was placed on the upstream right abutment. Also, a blanket of zone 3 material was provided downstream from the axis to serve as a filter to prevent piping. It was necessary to remove some portions of the right abutment in order to provide for uniform slopes of the blanket and to provide a 15-foot minimum width of zone 2 material.

10. Drainage. - Specifications required that toe drains be constructed in trenches excavated in the embankment foundation to collect seepage water and convey it