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Because of the nature of this loess material, preconsolidation was deemed necessary in foundation areas which would be subjected to wetting after loading. A geological section at the centerline of the dam is shown on figure 3.
The alluvial materials consist of clays, silts, and fine sand. The material for the most part is poorly graded; the fine and worn fractions exist in separate beds which are often layered and entwined by crossbedding. As studies of the dam foundation progressed, it became obvious that some type of impervious cutoff would have to be incorporated in the dam structure to control seepage.
The Pierre shale, essentially a firm plastic shaley clay, serves as a foundation for the dam and appurtenant structures. It is soft and friable near the surface and becomes harder as the depth increases. Silt, clay, and colloidal material with a small percentage of fine sand are the principal constituents of the shale. It is believed that all joints in the material are watertight. This material is highly impervious and, as it exists in a completely saturated state, high pore pressures develop after application of a load.
5. Cost. - The following tabulation gives the cost of construction of Trenton Dam and Reservoir and the railroad and highway relocations, by major groupings, as of August 28, 1957. A breakdown of these groupings is shown as appendix A, and pay item breakdowns for the two principal specifications, Nos. 2689 and 3047, are shown as appendixes B and C, respectively.
Land rights $ 854, 156
Preliminary surveys and investigations 932,989
6. General. - Exploration work for Trenton Dam (see sec. 3) was started in October 1947 and consisted of hand auger holes, core drill holes, penetration tests, water-pressure jetting holes, and test pits. During the exploratory period, extending to January 1951, 49, 176 linear feet of core drill, water-pressure jet, and hand auger holes were drilled and 316 linear feet of 4- by 6-foot test pits were excavated. Extensive laboratory tests were made for determining physical properties of the undisturbed samples of loess, sands, and shale obtained at various depths during the exploration operations. Logs and locations of some of the exploratory holes are shown on figure 4.
An examination of the geologic profiles indicated a valley fill of alluvium to a depth of about 40 feet overlying the Pierre shale. Loess deposits covered the alluvium on the gentle slopes of the right abutment. A 10- to 20-foot layer of partially cemented sand, covered by a thicker layer of loess, was exposed on the more abrupt river valley slopes of the right abutment.
Permeability of the alluvium in the dam foundation was determined by gravity tests at different depths in several drill holes. Leakage studies indicated that the alluvium was relatively permeable. Studies indicated that considerable excavation would be necessary for construction of a cutoff trench to shale south of station 22+50, with no apparent place for terminating the trench. It was considered advantageous to excavate the main cutoff trench to the Pierre shale and to terminate the south end of the trench in an auxiliary cutoff trench, excavated to shale and positioned at right angles to the main trench so as to accommodate the conduit of the canal outlet works. The auxiliary cutoff trench provides a shale foundation for the conduit, and the impervious backfill provides a cutoff in the right abutment. This cutoff provides a percolation path about 740 feet long for water that may enter the pervious sands in the vicinity of the old river channel.
Tests revealed that considerable consolidation upon saturation could be anticipated in the loess mantle of the right abutment. Provisions were made for preconsolidation of the right abutment by inundating the foundation area south of the canal outlet works for 60 days prior to placement of embankment. This method was feasible because the pervious strata below the loess mantle would provide drainage upon loading. Saturation of the loess was to be accomplished by progressively flooding separate sections of the dam foundation by basins and ditches in the abutment.
7. Construction Materials. - It was estimated that the following quantities would be needed for construction of the dam and appurtenant structures:
(1) Impervious material, cu. yd. 6, 537,000
(5) Gravel or crushed rock blanket
(a) Earth Materials. --The flood plain area was investigated as a possible source of earth materials suitable for construction. The investigations indicated that this area was not suitable for use as a source of construction material. Subsequent investigations were carried on in areas located in close proximity to the north and south ends of the proposed dam. The north borrow area consisted of several separate areas which were designated as N-1, N-2, N-3, and N-4. An area which included a portion of
Figure 4. --Plan of exploratory holes in Trenton Dam foundation area.
(Sheet 1 of 2.) From drawing 328-D-993.