reaction with the sealing compound. It is probable that the reaction of the sealing compound with the concrete was caused by a specific ingredient in the compound rather than the overall composition. Figure 54 shows concrete curing compound on the spillway floor. Figure 55 is another view of the spillway floor showing the placing of sand for use in water curing. The placing of the wet sand is also the first step for cold weather protection.

109. Cold Weather Protection. As cold weather approached in the fall of 1948, the contractor was faced with the necessity of providing protection for fresh concrete. In a letter to the construction engineer, the contractor requested that the curing period be reduced from 14 to 7 days on the basis that steam curing under tarpaulins would be used. This request was not granted, because the plan did not outline how the steam would be confined nor what temperatures would be maintained. However, steam curing was used successfully on the valve house superstructure and numerous floor panels and wall sections of the spillway stilling basin. Steam curing as used on this work gave excellent results, and was considered one of the best curing procedures. For curing and protecting the concrete in floor slabs immediately below the spillway crest section, during cold weather, the fresh concrete was covered with a 6-inch layer of wet sand as soon as sufficient set had been attained, and the sand was covered by about 12 inches of straw overlain by a tarpaulin.

These methods of cold weather curing and protection were very satisfactory and were used until concrete work was discontinued on December 22, 1948. The placing of concrete was resumed on April 12, 1949, after which time the weather was sufficiently mild so as not to require any special protection for the concrete.

C. Miscellaneous Work

110. Driving Steel Sheet Piling. - The plans for Enders Dam included the driving of an interlocking steel sheet piling cutoff wall around the downstream end and along the sides of the stilling basins for the spillway and the outlet works. However, after completing the excavation for the outlet works stilling basin and upon drilling several exploratory holes in and near the basin foundation, it was found that the subgrade was a particularly hard, well-cemented, Ogallala formation through which it would be extremely difficult and impracticable to drive piles. Since this foundation was found to be hard and competent, the requirement for piles around the outlet works stilling basin was eliminated by the Denver office.

In contrast with the outlet works stilling basin, the foundation under the spillway stilling basin was, in many places, of a less compact nature and varied appreciably in quality, texture, and hardness. After considerable on-the-job study by both the construction personnel and design engineers from the Denver office, it was decided to drive piling around the spillway basin so far as practicable. Accordingly, steps were taken by the contractor during the summer of 1948 to start driving the 30-foot-long piles. These piles weighed 36 pounds per linear foot and had an overall width of 19-5/8 inches. A 150-horsepower boiler was obtained, together with a single-acting steam pile-hammer. The weight of the striking head of the hammer was 5,000 pounds, and the normal drop was 3 feet, producing a striking energy of 15,000 foot-pounds per blow. With a steam pressure of 80 pounds per square inch or more, the hammer would deliver at least 60 blows per minute. The head used was fabricated to drive two piles at a time. This equipment was adequate and proved to have sufficient driving force. Some of the piles curled and split at the bottom, and some became so battered that it was necessary to cut them off on top to straighten the driving edge.

Driving was started at the left side of the stilling basin on August 21, 1948. The left wing-wall piling was driven first, and this operation was followed by driving of the left channel wall piling. Each pile was numbered consecutively, and the length of each pile was painted on the metal in 1-foot increments starting from the bottom end (fig. 56). This numbering facilitated the inspectors' duties of maintaining accurate records and keeping up-to-the-minute information on the progress of the work.

With the exception of 2 or 3 feet at the beginning of driving on some of the piles, the driving was very hard all the way down, and almost the maximum resistance to penetration was encountered by all piles. The piles along the downstream face of the basin seemed to be stopped in a hard, partially cemented, brown silty fine sand material. This material blended into a hard, gray, Ogallala rock upstream from station 13+25. 5, and it was not practicable to drive all of the piles upstream from this station on the 2 to 1 sloping portion of the wall footing. Nineteen piles on the left channel wall slope and 13 piles on the right channel wall slope, were omitted entirely on the upstream ends of the se two sections.

The formation under the left channel wall downstream from station 13+25.5 appeared to contain numerous large, isolated boulders. Persistent, hard driving generally forced the piles through these boulders and into the sand below and made it possible to drive a few to full depth. Likewise, in a few instances, the cemented layer along station 14+41 was penetrated, and the piles were driven to full depth into the sand stratum lying below the cemented silty fine sand to the Ogallala formation.

Shortly after starting the work, it was found that the piles could not be driven to refusal under the heavy impact of the hammer, but continued indefinitely to move a very small fraction of an inch under each blow. Under this situation, it was realized that if the contractor drove the piles to bedrock or to full length, as specified, he would be engaged in the work an unduly and unreasonably long time with very little benefit to the structure. It was therefore agreed, at the contractor's request, to establish other limits than those prescribed in specifications No. 1410. Accordingly, it was established that the piles were to be driven to refusal, which was defined as a penetration of 0.5 inches or less under 10 blows maintained for a depth of at least 6 inches. Likewise, it was established that driving should continue until the pile had been forced down a distance of not less than 5 feet. This requirement proved to be reasonable and to give satisfactory results, with the exception of a few cases where it was necessary to abandon driving before 5 feet of penetration had been reached. Most of the piles were accepted with a penetration of from 0.25 to 0.30 inch under 10 blows continued over the last 6 inches of depth. Figure 57 shows the variation in the depths of penetration. A graphical record of the depths of penetration is presented on figure 58.

Figure 58.--Steel Sheet piling in spillway stilling basin. From drawing

328-716-13.