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CHAPTER II--INVESTIGATIONS

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. Leaka ge 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:

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(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

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Figure 4.--Plan of exploratory holes in Trenton Dam foundation area.

(Sheet 1 of 2.) From drawing 328-D-993.

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Figure 4. --Plan of exploratory holes in Trenton Dam foundation area.

(Sheet 2 of 2.) From drawing 328-D-993.

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Figure 5.--Plan of borrow areas and logs of several test holes.

the spillway excavation was also investigated. The south borrow area consisted of two ridges divided by a dry wash and comprised an area about 3,500 feet square. The locations of borrow areas are indicated in figure 5.

The north borrow areas were explored with hand auger holes, power drill holes, and test pits. Samples taken from these areas indicated a fairly homogeneous soil which was dominantly silt and some fine sand. Laboratory tests indicated maximum density ranging from 101.7 to 109.8 pounds per cubic foot at optimum moisture contents ranging from 15.5 to 18.7 percent. Permeability tests indicated an average coefficient of 0.1 foot per year. The average for the in-place density tests indicated a shrinkage factor of 23 percent.

The south borrow area was investigated with hand auger holes which in some instances extended to a depth of 70 feet. The soil was classified into three general types: silt with very fine sand; medium to fine sand with excess silt and small amount of clay; and medium to fine, silty to fairly clean sand. The soil was somewhat heterogenous with irregular recurring layers of sand, silt, and sand containing excess silt. A relatively homogeneous material could be obtained by controlling the depth of cuts in the borrow area. Laboratory tests of the silty sand indicated maximum densities ranging from 108 to 122.7 pounds per cubic foot at optimum moisture contents ranging from 8.0 to 13.6 percent. The shrinkage factor was about the same as that of materials in the north borrow areas.

(b) Riprap and Rock-Blanket Materials. -- Several sources for riprap and rock-blanket materials were considered. The three most feasible sources were: Franklin, Nebr., for quartzitic sandstone; Golden, Colo., for basalt or granite; and Guernsey, Wyo., for dolomite. A study of the freight rates, quality, quantity, and production costs indicated that the hard, sound, and durable metamorphosed granite with a specific gravity averaging about 2.7 obtained from a quarry at Golden, Colo. would be the most suitable for the riprap and rock-blanket of the dam. Rock obtained from the Golden quarry required blasting and processing at the quarry, shipment by railroad to Trenton, Nebr., and hauling by truck for a distance of 2-1/2 miles to the dam.

8. Railroad, Highway, and County Road Relocation Studies. - (a) Railroad. -Relocation studies for the Chicago, Burlington and Quincy Railroad were started in 1947. These investigations included several routes having a combined length of 62 miles. The railroad company provided data and assisted in the selection of the route. Some of the studies for the relocation work included:

(1) Cost estimates for raising the railroad grade to the crest elevation of the dam.

(2) Widening cut sections for additional borrow materials.

(3) Stability of embankment.

(4) Increased earthwork because of flattening fill slopes.

(5) Costs of road crossings, cattle passes, etc., versus combined structures for passes and drainage.

(6) Costs of road crossings and cattle passes versus severance costs for rightof-way.

(7) Switching details for the Trenton switchyard.

Preliminary data, which included alinement and profile, structure topography, section corner ties, right-of-way requirements and quantity estimates were completed during April 1949. The railroad company approved the relocated line in October 1949. Final plans and specifications were submitted to the railroad company in April 1950 and approved during June 1950. The selected route is shown on figure 6.

(b) Highway.-- Studies for a relocation of U.S. Highway No. 34 were begun in April 1948. Several routes were studied and cost estimates were made of each to

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