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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 sheaf 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:

No. of Percent

Material tests unacceptable*

Impervious 5,400 17.4

embankment

Pervious 146 6.4

sand and gravel

*Requiring additional compaction.

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.

C. Concrete

50. Materials. - (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) Coorse Aggregote. 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. and dolomitic limestone. About 0. 5 percent of the aggregate consisted of chert which reacted deleteriously with high-alkali cement. The aggregate contained an excess of deeply weathered and otherwise unsound particles, but because of the good service of this aggregate in numerous types of concrete structures in the region, these deficiencies were discounted. About 86, 088 tons of aggregate were obtained from this deposit. The aggregate was received in three sizes--3/16 to 3/4 inch, 3/4 to 1-1/2 inches, and 1-1/2 to 3 inches--and each size was stockpiled separately. When gravel was drawn from the stockpiles, all three sizes were intermingled to prevent them from rolling off the conveyor belt during transportation to the rescreening plant. Also, the rescreening plant could handle more tons of material per hour when supplied with several sizes of intermingled aggregate.

During the latter part of 1953, when the Brannon Sand and Gravel Co. was unablto obtain sufficient railroad cars for aggregate shipments, the contractor purchased 1, 244 tons of crushed gravel from the Guernsey Rock Co. at Guernsey, Wyo. This gravel, composed of limestone, was intermingled with Clear Creek aggregate for use in concrete mixes. Batching weights were altered slightly to compensate for the variance in the specific gravities of the two materials.

i

About 3, 425 tons of aggregate crushed to 3-inch size were obtained from Table Mountain quarry at Golden, Colo. This material was composed mainly of granite gneisses. No physically unsound particles or deleteriously reactive materials were found in the samples tested.

(c) Cement.— Because of the alkali-reactive material in the concrete sand and in the foundation materials, type n low-alkali cement was used for Trenton Dam. This cement, a mixture of pozzolan and portland cement, manufactured at the Louisville, Nebr., plant of the Ash Grove Lime and Portland Cement Co., was low-alkali type and conformed to Federal specifications No. SS-C-192.

As soon as the portland-pozzolan cement was available, the field laboratory conducted tests to determine the most economical mix consistent with workability, durability, and strength requirements. The maximum water-cement ratios indicated in specifications No. 3047 for parts of structures subjected to certain conditions are indicated below:

Watercement ratio

a. Parts of structure subjected to

alternate wetting, drying,

freezing and thawing 0. 47

b. Parts of structure exposed to

weather 0.53

c. Parts of structure covered with

backfill 0.60

However, in order to meet a minimum compressive strength requirement of 3, 000 pounds per square inch at 28 days, the water-cement ratios for structures b and c, listed above, were reduced to 0. 51 and 0. 53, respectively. For concrete mixes using 3-inch maximum aggregate the cement content was increased for structures under conditions b and c from 1. 00 to 1. 23 barrels and 1. 00 to 1.12 barrels, respectively. Concrete mixes using other sizes o.f aggregates were also altered.

51. Reinforcement Steel. - In accordance with specifications No. 3047, all reinforcement steel was to be furnished by the Government. Approximately 45 percent of the total amount required was obtained from a surplus supply at Enders Dam, Nebr. Attempts were made to purchase the remainder through competetive bids on the open market, but because of national defense priorities in effect at the time and the general steel shortage, no bids were received for a sufficient quantity to supply the job requirements. A large quantity was procured from a surplus supply at the Army Engineer's Office at Atlanta, Ga., and a surplus at Shasta Dam at Redding, Calif. ; the remaining

[graphic]

Figure 31. --Panels for forming spillway wall sections. Form was held rigidly in position by five steamboat jacks bolted to the concrete footing. P328-701-3134, September 19, 1951.

[graphic]

Figure 32. --Form for the 5.5-foot circular conduit being assembled. P328-701-3137, September 24, 1951.

quantity was obtained from, several sources. Because numerous sizes of steel were received which did not entirely conform to construction drawings, some substitution of steel sizes to provide equivalent areas was made in parts of the spillway.

Some of the steel was rusted, partly painted or bent. Paint was removed by an acetylene torch or by wire brushing. Cutting and bending or straightening of reinforcement bars was done at the steel storage yard located about one-half mile downstream from the spillway crest.

52. Forms. - Forms for concrete structures were constructed of a variety of materials. These materials included metal, plyboard of various thicknesses, tongueand-groove flooring material, masonite, and heavy timbers. Cables, bolts, and jacks were used for keeping the forms in position during concrete placement. Forms for walls and other exposed surfaces were generally faced with 1- by 4-inch flooring material or plyboard. For reasons of economy, some forms were designed for many reuses.

Spillway wall forms (fig. 31) consisted of panels 32 feet by 19 feet 4-1/2 inches, and were faced with flooring material. The flooring material was placed vertically and was backed by diagonally placed 2- by 6-inch tongue-and-groove material which was solidly attached to 4- by 6-inch timbers. The facing material was held in place by tapered bolts which extended through the forms. Jacks with their bases anchored to a concrete base were used to hold the back form rigidly in position. After these panels were used about 12 times, the floor facing began to curl and required smoothing with a sander. When the curling became excessive, the flooring material was replaced with new lumber. Two replacements were necessary on each form during the entire construction program.

Forms for the upstream end of the outside bridge piers of the spillway were faced with 1/4-inch plyboard and backed with two layers of 3/16-inch masonite and a layer of 1/4-inch plyboard. Frames for these forms consisted of a double layer of 2by 12-inch planks spaced at 15-inch intervals and cut to fit the pier curvature. Support for the forms was provided by a 12- by 12-inch strong back with a number of small studs placed at the leading edge of the pier.

Three 5-foot I-beams were placed in blockouts left in the piers and were used to support the forms and reinforcement steel for the center bridge spans over the spillway structure. Heavy timbers were placed across these beams at 5-foot centers for supporting the bridge forms. Correct elevation for the forms was obtained by wedging.

Forms for the 5. 5-foot circular conduit were assembled in a jig to one side (fig. 32). After the inside and outside forms with reinforcement steel were assembled, the jig was placed in position with a dragline. The interior form was faced with masonite and arranged for easy removal. The outside form was constructed of shiplap. Metal forms were used for forming the 8-foot 2-inch diameter conduit.

53. Batching and Mixing. - Concrete for Trenton Dam structures was batched and mixed at a central plant located about 500 feet east of the spillway outlet channel at station 53+00. This plant consisted of a partitioned 225-ton gravel bin, a 100-barrel cement bin, a 90-cubic-foot batching hopper equipped with a full reading dial scale for cumulative weighing of aggregates, a separate automatic hopper for weighing cement, a semiautomatic water batcher, a 2-cubic-yard tilting mixer equipped with a timing and locking device, a 160-cubic-foot concrete collection hopper, and conveyor systems for the cement and aggregates. Cement was stored in a 1, 750-barrel storage silo located adjacent to the mixing plant. The aggregate bins were provided with vibrating screens. Air-entraining agent was added to the mix by means of a dispenser. Mixing water was obtained from a shallow well located under the plant. A view of the layout of the mixing plant is shown in figure 33.

Three sizes of aggregates were used in the concrete. The percentages of concrete placed with the maximum size of aggregate used are indicated below:

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