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44. Foundation Area. - The entire area to be occupied by the dam embankment, except that included in the cutoff trench and canal outlet excavation, was stripped to an average depth of 1 foot to remove all topsoil and all objectionable materials. Tractordrawn scrapers and motor patrol graders were used in the stripping operations (fig. 25). The removed materials were used in embankment construction or stockpiled for later use as topsoil on the downstream slope. All objectionable materials were wasted.
After completion of the stripping operations, water was applied to several areas in order to consolidate the foundation material with the embankment material. Approximately 26, 000, 000 gallons of water was ponded and allowed to seep into the foundation area between stations 7+00 and 21+00. After 2 weeks of ponding, auger holes were drilled and samples taken to determine the amount and depth of water penetration. Steel rods were driven at several places to determine preloading consolidation. Foundation settlement crossarms were installed at stations 15+00 and 21+00 (see chap. VI). After completion of this work and prior to embankment placement, the entire foundation area was smoothed and rolled.
45. Main Cutoff Trench. - The main cutoff trench extends from about station 90+00 to station 22 ;-50, where it connects with an auxiliary cutoff trench which forms the foundation of the canal outlet works. The centerline of the trench is 100 feet upstream of the dam axis from station 22+50 to station 76+10, where it is deflected at an angle and contacts the dam axis at about station 83+70. The trench, with a base width of 50 feet and side slopes of 2-1/2 to 1, was excavated in firm shale. Originally the trench was designed to extend 50 feet beyond the spillway structure, or to station 88+60. During excavation, it was considered advantageous to extend the trench to station 90+10 in order to lengthen the path of percolating water through the underlying sand strata and away from the spillway structure. Excavation operations are shown in figure 26.
Excavation work was started on August 1, 1949. Draglines and scrapers were used for the sand excavation. To prevent fracture of the shale surface, rubber-tired equipment was used when shale was encountered. Because of the river diversion requirement and anticipated winter shutdown by about October 15, excavation work was started in the area between approximate stations 44 and 54 and it was planned to complete the excavation and backfill work for river diversion before the winter shutdown. Fortunately, mild weather continued until December 10, and the contractor was able to complete considerable work in addition to the planned work. The excavation work was resumed in February 1950.
Because of the high water table in some sections of the trench, it was necessary to install well points and pumps for dewatering. This method was very successful. Because the well points were not embedded in the shale, shallow ditches and sumps were used to dewater the bottom 2 feet of the trench.
A major portion of the excavated material above the shale bedrock was satisfactory for use as backfill. The shale in the valley portion required only a minor amount of leveling and smoothing prior to placement of backfill.
46. Auxiliary Cutoff Trench and Other Excavation Work. - The auxiliary cutoff trench is located with its centerline at dam axis station 22+50 and at right angles to the dam. This trench extends beyond the toe line of the dam and accommodates the canal outlet and control structures; it has a length of about 570 feet, a base width of 50 feet and side slopes of 1-1/2 to 1. The outlet works structure, averaging about 12 feet in width, is located along the centerline of the trench in from 4 to 9 feet of shale (fig. 27).
The excavation work was performed under two contracts. Under the first contract the trench was excavated to the ground-water table elevation, and under the completion contract the trench excavation was completed to the required grade. All excavation work was performed with tractors and scrapers and all suitable excavated material was utilized in the dam embankment. Only a small portion of the material was wasted.
A well point system and pumps were installed around the perimeter of the auxiliary cutoff trench excavation for dewatering during excavation operations below the
Figure 27.--Auxiliary cutoff trench excavation completed to bedrock. One stage of well points was used for dewatering. In the background forms are being set for the canal outlet works structure. P328-701-3138, September 24, 1951.
ground-water table. The pumps handled an average of 200 gallons per minute during the dewatering operations. The dewatering system was.also utilized during construction of the outlet works structures and backfilling operations.
An approach channel was excavated upstream from the intake structure of the outlet works. This channel has a base width of 20 feet, 1-1/2 to 1 side slopes, and a length of about 1, 500 feet. When the excavation was in a fine to medium sand below a silty sand strata, it was over excavated on the sides and base for placement of an impervious blanket. Because the excavation was above the ground-water table, no unwatering was necessary.
B. Embankment Operations
47. Borrow Area Operation. - (a) South Borrow Area. — The south borrow area is located on the uplands of the south abutment. It is about one-half mile southwest of the dam and comprises an area about 3, 500 feet square. This area supplied about 4, 425, 000 cubic yards of material for the impervious zone of the embankment.
Irrigation of the borrow area was started in September 1949 and excavation was started soon afterward because of the need for backfill in the cutoff trench. Initially the moisture penetration was not deep enough to permit satisfactory cuts. Later, a more extensive irrigation system was set up. This consisted of sprinkler heads placed at 500-foot centers, piping, and two electric pumps, each having a capacity of 1, 100 gallons per minute.
The excavation work was done by 3- to 5-cubic-yard shovels and draglines, and motor scrapers. The excavated material was hauled to the embankment in bottom-dump trucks, having capacities ranging from 13 to 17 cubic yards. Fine sand and silt were the principal constituents of the excavated material. The dragline and shovel method of excavating yielded a satisfactory mixture. During the completion contract (specifications No. 3047) the excavation work was performed by a side-elevating loader drawn by two tractors (fig. 28). Trucks having capacities from 17 to 32 cubic yards were used for hauling the excavated material. Because of the sand layer and mixing limitations of the loader, it was necessary to adjust the depth of cut and to subdivide the borrow area for excavation operations. This involved making several short turns and some deadheading of the equipment.
(b) North Borrow and Spittwoy Areos.— Approximately 4, 050,000 cubic yards of material was excavated from the north borrow area and spillway area and placed in the impervious zone of the embankment. About 3, 500, 000 cubic yards was Peoria loess and the remainder the slightly coarser, high-clayey, Loveland loess. Water sprinklers were used for adding moisture to these areas prior to excavation.
Under specifications No. 2689, about 1, 150, 000 cubic yards were excavated and placed in the cutoff trench between stations 54 and 76 and in the dam foundation between stations 52 and 75. The major part of the excavation work was performed by the same equipment as was used in the south borrow area. All of this material had to be hauled across the existing Chicago, Burlington and Quincy Railroad and U. S. Highway No. 34. About 70, 000 truckloads were transported without any accidents at the crossings.
At the north abutment, the area to be excavated was changed from that indicated in specifications No. 2689 to include an area on which the caretaker's residence was to be constructed and an area which would later be north of the relocated railroad. This was done to facilitate the early construction of the caretaker's residence for use as a field office and laboratory, and to remove as much as possible of eventually required material from a location which would require crossing the relocated railroad at a later date. The sprinkler method of irrigation was also employed for this area. Because of the high vertical percolation rate in these soils, the moisture penetration was uniform to a considerable depth. Therefore, it was not necessary to restrict the depth of cut during excavation.
Under the contract for specifications No. 3047, the completion contract, approximately 330, 000 cubic yards of material located in north borrow area N-l north of the southern boundary of the relocated railroad right-of-way was to be excavated prior Figure 29.--Embankment placing operations. The dragline in the upper