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.

Figure 28. --A side-elevating loader in operation in South borrow area. Excavated material is being loaded on a 30-cubic-yard truck. P328-701-2801, June 12, 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.

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 Spillway Areas.-- 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-1 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 left is spreading riprap rock. P328-701-3967, July 9, 1952.

to June 1951. (See fig. 5.) Because of this requirement and the reluctance of the railroad to permit the continuance of the haul-road grade crossing on the existing railroad line, the contractor rerouted the existing highway immediately adjacent to the railroad and constructed a haul-road overpass structure to span them both. This structure was completed in February 1951 and was used until removal of the railroad in December 1952.

48. Earth Fill Placement. In general, a standard pattern was followed for placing earth fill at Trenton Dam. A major quantity of impervious material was hauled in bottom-dump trucks and dumped in windrows which were spread to about 8 inches in thickness by bulldozers. Motor-driven scrapers were used for hauling material adjacent to spillway structures. Sprinkler trucks were used when it became necessary to increase the moisture content of the placed material. A heavy-duty disc was used to distribute the moisture in the sprinkled area. During warm and windy weather, it was necessary to sprinkle the previously compacted layers to offset moisture loss by evaporation. Embankment placing operations are shown in figure 29.

Compaction with tamping rollers was employed on the major portion of the embankment. Air-powered tampers and a hydraulic tamper mounted on a half-truck were used on areas inaccessible to tamping rollers.

During the placement of impervious materials, low densities were obtained from numerous tests even though moisture content, thickness of layers, and roller passes met all requirements. After some experimentation, satisfactory densities were obtained by decreasing the weight of the roller from 40,000 pounds to 34, 000 pounds and without increasing the number of roller passes. Better compaction with the lighter roller was due to shear resistance of the compacted material being less than the unit weight of the heavy roller.

The gravel and sand pervious zone was placed with tractors and scrapers. A minor portion was placed with trucks. The material was placed in layers about 8 inches thick, watered heavily and compacted with four passes per layer with a crawler-type tractor. Satisfactory compaction resulted when the tractor rolling operation followed closely after watering.

The material in zone 3 was placed in approximate 1-foot layers and compacted with the placing equipment. Moisture was added to the material when necessary with sprinkler trucks. Because the moisture content was satisfactory for most of the material, only a small amount of water sprinkling was required.

Topsoil was placed on the downstream slope of the embankment with bottomdump trucks, motor-scrapers or bulldozers. On steep slopes the material was dumped along the crest or bottom berm and spread on the slope with bulldozers. The material was placed in two layers, each about 7 inches thick, and compacted with a 7-foot-diameter, 10-foot-long, water-ballasted, smooth-surfaced roller.

(a) Control.-- Needle-moisture tests, needle-density tests, and field-density tests were used for control during embankment placement. Because of unreliability of readings and for other reasons, the needle-density and the needle-moisture tests were discontinued early in the testing program and the field-density test was adopted as a reliable test for both moisture and compaction.

The field-density test was conducted in accordance with the Bureau's "Earth Manual", except the density holes were excavated only to a depth slightly in excess of 6 inches. By limiting the depth of holes, the test was confined to the most recently placed and compacted layer and resulted in positive control. By obtaining the minimum of compaction permitted within the required moisture range, it was reasoned that satisfactory density would result after placing and compacting successive layers. Also corrective measures could be applied, if necessary, to the easily accessible layer. This method of testing was readily adaptable to predominantly silt and homogeneous loess materials, which comprised the impervious zone of the embankment.

In the pervious zone, samples for density tests were obtained from depths extending from 1 to 2 feet below the surface. At these depths, work in the saturated soil was avoided and more accurate samples were obtained.

Numerous tests and observations indicated that the density increase, because of consolidation, was greatest when embankment was placed from 1 to 2 percent dry of optimum. During 1950, several density tests were taken at various depths of the embankment to study the effect of compaction after successive layers were placed. The results of these tests, although not conclusive, indicated that if loess was placed between optimum and 3 percent dry of optimum, the tamping rollers were effective in compacting a depth of 3 feet or more of embankment. The number of density tests taken, together with the percent of acceptable tests, are indicated below for the entire construction period:

49. Crushed Rock and Riprap Placement. - Crushed rock was shipped in hopperbottom gondola cars from a quarry at Golden, Colo., to the contractor's railroad switchyard and hauled in trucks to the embankment. The trucks with rock were lowered down the slope with a winch tractor and their loads dumped on the embankment slope for placement (fig. 30). A bulldozer was used to spread the rock to required thickness. In general, the rock was placed after the embankment construction had progressed to a height of about 10 feet and the slope had been trimmed to grade. The riprap was hauled, dumped, and spread in a similar manner on the embankment except a dragline was used for spreading the riprap. The crushed rock blanket was placed 18 inches in thickness on the 3 to 1 slope, diminishing in thickness at the 2-1/2 to 1 slope to 12 inches at the crest. The riprap rock was placed 3 feet in thickness on the 3 to 1 slope, diminishing in thickness at the 2-1/2 to 1 slope to 2 feet at the crest.

A car tipper was used for unloading riprap from railroad cars at the switchyard into a pit, and a power shovel was used to load the material into trucks. Crushed rock was unloaded from the gondola cars into a hopper under the tracks and loaded on trucks by means of a rubber conveyor belt. The delivery schedule of the rock supplier and the construction schedule of the dam contractor were coordinated so that a minimum of rehandling and stockpiling of rock would be necessary. However, because of delays in construction of the embankment, the contractor was unable to place all the rock after delivery; it was necessary to stockpile and rehandle approximately 176,000 tons.

50. Materials.

C. Concrete

(a) Sand.-- Sand for concrete construction was obtained from a deposit near the dam site and processed in a plant located on the north edge of the spillway outlet channel at station 62+00.

The plant was of a washing and screening type and was capable of producing about 35 tons of sand meeting specification requirements per hour. Sloping stationary screens with 1-inch square openings were used for removing large gravel, lumps and trash. The larger fractions of sand were controlled by altering the slope of a 1/4-inch screen or by substituting appropriate screen panels. Fine fractions were controlled by regulating the outflow of wash water from the collection hopper.

(b) Coarse Aggregate.-- Three sizes of coarse aggregate used in the concrete were purchased by the contractor from Brannon Sand and Gravel Co., Denver, Colo., Cass Co., Golden, Colo., and Guernsey Rock Co., Guernsey, Wyo. These aggregates were obtained from Clear Creek deposit near Denver, and from quarries near Golden, Colo., and Guernsey, Wyo.

Aggregates obtained from the Clear Creek deposit consisted of material that was rounded to regular in shape, being composed chiefly of granites and gneisses with smaller quantities of schists, basalts, andesite porphyries, cherts, quartz, quartizites,