CHAPTER VI--CONSTRUCTION

A. Earthwork

1. Preliminary and Excavation

88. Clearing and Stripping. The areas to be occupied by Enders Dam, dike, highway, borrow and rock deposits, and appurtenant works were first cleared of all trees, stumps, roots, brush and other objectionable material that might interfere with the progress of the work. Such debris was burned or otherwise disposed of as directed by the contracting officer.

The areas to be occupied by the dam and dike were then stripped of all soil, rubbish, vegetable matter, and other perishable and objectionable materials that might interfere with proper compaction of the embankment materials. Stripping throughout the valley bottom involved the removal of sandy and silty alluvial materials. Stripping outside the cutoff trench consisted mostly of removing a few feet of sod and organic topsoil. Directly under the topsoil were deposits of fine-graded to medium-graded sand, the upper portion of which contained enough organic material to stain the sands a light-grayish color. It was considered advisable to remove this organic sand in the downstream curve of the foundation area until clean sand was encountered. Portions of this area proved to be an old swamp covered with 18 inches to 2 feet of blow sand. Between stations 43+00 and 46+50, it was necessary to excavate to a depth of 9 feet in some places to remove the peaty material. These organic soils were removed to eliminate the possibility of excessive differential settlements and to achieve proper bonding of the embankment and foundation. Additional stripping was also performed south of the right spillway wall. The dike foundation was stripped to an average depth of 3 feet to remove sod and topsoil containing a high percentage of roots.

89. Unwatering. · Because the water table was within a few feet of the ground surface throughout the river bottom, unwatering activities were necessary almost from the start to stabilize excavation slopes and to eliminate free water prior to embankment placement. Unwatering in the stripped areas on each side of the cutoff trench was accomplished by means of open sumps and the use of 2- to 6-inch centrifugal pumps. No unusual difficulties were presented here because of the low heads and comparatively small flows involved.

The problem of unwatering the cutoff trench was given much study by the contractor. Several methods were considered, and the method ultimately selected was use of a well point system. The plans were satisfactory to the Government, and preparations for unwatering the cutoff trench were begun July 9, 1947. At this time, work was started on the upper stage of the well point system. Six-inch header pipes with 1-1/2-inch well points on 6-foot centers were used. The well points were 3 feet long and of the type having a jet nozzle incorporated in the tip. Water or a mixture of compressed air and water was used in the jetting work. In some areas it was possible to jet the well points into position by means of the point nozzles, but in hard ground it was necessary to use a 6-inch jet casing. In a few instances, even the jet casing was insufficient to carry the points down to the desired depth. The irregular character of the materials encountered in excavation was not ideal for the use of a well point system, and it was necessary at times to resort to the use of auxiliary sumps, particularly where underlying impervious strata caused perched water tables. In areas where the perched water tables offered the greatest trouble, 6-inch holes were jetted in back of the line of points. The holes were filled with sand, thereby creating a drawdown channel for the water. By August 7, the upper stage of well points, consisting of 250 points, was completed. This stage was pumped by four 8-inch centrifugal pumps, the discharge ports being reduced to 6 inches to fit the header lines.

By August 9, the excavation was sufficiently deep to permit installation of the second stage of well points. This second stage completed August 26, contained 164 points and was pumped by three 8-inch centrifugal pumps. After the second stage of points was in operation, two pumps were sufficient in the upper stage. By the time the excavation reached the required depth, there was considerable leakage past the well points on the

contact between the river sands and the Ogallala formation; and it was necessary to install four sumps, two on each side of the cutoff trench. Seepage was led into these sumps by a system of graveled tile drains along each toe of the excavation. Discharge for the entire system, well points and sumps, draining the cutoff trench area between stations 43+20 and 50+20, amounted to approximately 5.5 second-feet.

Unwatering of the dam foundation through the area under the temporary diversion channel adjacent to the right abutment was not as difficult a problem as it had been farther out in the valley bottom. Unwatering of the foundation on each side of the cutoff trench was accomplished by means of shallow sumps and 2- to 6-inch centrifugal pumps. Heads were low and quantities were not large. Sumps and centrifugal pumps were also used for unwatering the cutoff trench. After excavation had reached the required depth, ditches were dug in the Ogallala formation along each toe of the excavation, and in these ditches were laid tile drains embedded in gravel. Flow from the drains was collected in two sumps, one on each side of the trench at station 43+90. Through a manifold suction line hookup, the sumps were pumped by either one or two 8-inch centrifugal pumps discharging through a single 6-inch discharge line. Grout return pipes were connected to the tile drains at appropriate points; and as backfill of the cutoff trench progressed, these returns as well as the suction lines for the sumps were carried up with the embankment. After the fill had reached sufficient height, the sump and drain systems were grouted full with a silt-clay slurry introduced through the suction lines into the sumps.

Initial unwatering of the spillway stilling basin excavation was accomplished by well points. By May 7, 1948, the header for the first stage of points was placed across the downstream edge of the basin at station 14+94 and elevation 3032. This stage included approximately 103 of the 1-1/4-inch well points. It was necessary late in May to install a second stage of about 47 points with the header at station 14+86 and elevation 3026. Because of the character of the material and the quantity of the water, the well points alone were not sufficient to keep the excavation dry; and it was necessary to use sumps to finish the excavation to grade. After grade was reached, six sumps were excavated below grade, three within the basin area and three just outside. Five of these sumps and the lower stage of well points were pumped during the construction of the stilling basin. Maximum discharge in the area was approximately 5 second-feet.

The unwatering system used during the construction of the stilling basin did not exert influence far enough downstream to make possible the excavation of the spillway outlet channel immediately downstream from the stilling basin. This area was unwatered by means of the downstream stilling basin sumps plus four additional sumps located across the outlet channel as far downstream as possible but within the stilling basin excavation. Figure 34 shows the unwatering units used for the spillway stilling basin excavation.

Excavation for the outlet works stilling basin was unwatered by means of sumps. After excavation was completed and the permanent subdrains were in place, seepage was collected in these drains and led to a sump just outside the left basin wall. A second sump was installed at the end of the left wing wall to collect drainage from the cutoff wall excavation. Pumping was continued in these sumps until the structure was completed.

90. Excavation and Transportation Equipment. The equipment used for excavation and transportation determines in a great measure the contractor's costs of excavation. The principal units used at Enders Dam were as follows:

(1) Shovel, diesel motor, 3-1/2-cubic-yard capacity, equipped with a 65-foot dragline boom, a 4-1/2-cubic-yard drag bucket, and a 2-1/2-cubic-yard extraheavy clamshell. The shovel was acquired new in March 1947 and was used throughout the job except for 6 months, January to June 1947, when it was used on another project by the same contractor.

Figure 34.--Spillway stilling basin--Foundation conditions and unwatering units. From drawing 328-716-1.

-Sta 14+40.5

(2) Shovel, diesel motor, 1-3/4-cubic-yard capacity, equipped with a 60-foot dragline boom, a 2-1/2-cubic-yard drag bucket, and a 2-cubic-yard clamshell. shovel was used sometimes with a 90-foot boom and a 3/4-cubic-yard drag bucket. The shovel was acquired used in April 1947 and was used throughout the job.

(3) Dragline, gasoline motor, equipped with a 45-foot boom, a 1-1/2-cubic-yard drag bucket, and a 1-cubic-yard clamshell. The dragline was acquired used in July 1949 and was used throughout the remainder of the job.

(4) Dragline diesel motor, equipped with a 65-foot boom, a 2-1/2-cubic-yard drag bucket, and a 2-cubic-yard clamshell. The dragline was acquired used in August 1948 and was used throughout the remainder of the job.

(5) Gasoline-powered wheel crane, equipped with a 45-foot boom, a 3/4-cubicyard drag bucket, and a 3/4-cubic-yard clamshell. The crane was acquired new in June 1947 and was used throughout the job.

(6) Wheel-type tractor scraper units, diesel motor, 12.1-cubic-yard rated capacity. Three were acquired new in March 1947 and were used throughout the job. One was acquired used in June 1949 and was used throughout the remainder of the job.

(7) Scraper drawn by crawler-type tractor, 15-cubic-yard rated capacity. One was acquired new in March 1947 and was used throughout the job.

(8) Motor patrols, diesel motor. Two were acquired new, one in February 1947 and the other in April 1947, and were used throughout the job.

(9) Ripper. The ripper was acquired in June 1947 and was used throughout the job.

(10) Bottom-dump trucks, diesel motor, 13-cubic-yard rated capacity. Six were acquired new in February 1947; one was acquired new in October 1947. All seven were equipped with new diesel motors during the winter of 1947-48. seven were disposed of in the spring of 1949.

All

(11) End-dump trucks, diesel, 9-cubic-yard capacity. Two were acquired in 1947. One was disposed of in December 1947, and the other was converted to a water truck in July 1948.

(12) U. S. Army trucks, 6-ton capacity, 6-wheel chain drive, hydraulic enddump body, 8-cubic-yard capacity. Six were acquired on loan basis from the Army in July 1948 for test purposes and were returned in early winter of 1949.

(13) End-dump trucks, gasoline-powered, 8-cubic-yard capacity. Four were acquired used in May 1949 and were used throughout the remainder of the job.

(14) End-dump trucks, gasoline powered, 7-cubic-yard capacity. Four were acquired used in September 1945 and were used to December 1949.

91. Required Excavation. Excavation of the outlet works was started May 3, 1947, and was completed in May 1948 except for final trimming of the slopes and removal of the access road crossing the channels, which work was deferred until completion of all other operations in the outlet works and spillway stilling basin areas. Construction of the outlet works involved a cut-and-cover operation throughout the conduit section, with a maximum depth of cut equal to 70 feet. Excavation was accomplished principally by use of the 3-1/2-cubic-yard shovel, and the excavated material was transported by 13-cubic-yard bottom-dump trucks and 9-cubic-yard end-dump trucks. Toward the bottom of the cut, the trench became so narrow that it was no longer possible to operate the shovel and trucks in the restricted area. Excavation below this point was therefore accomplished by the use of a ripper, three wheel-type tractor scraper units, and a scraper drawn by crawler-type tractor. The stilling basin and outlet channel were excavated by the use of the drag bucket and clamshell on either the 3-1/2-cubic-yard shovel or the 1-3/4-cubic-yard shovel.

As was indicated in the "Investigations and Zoning Progress Report, dated February 11, 1947, it had been anticipated that most of the outlet works excavation material would be used for the embankment either as earthfill or rockfill. It had not been anticipated that excavation by shovel would be difficult; only in the area between the gate chamber and the valve house was there any rock that appeared to be hard enough to require drilling and shooting. However, after the cut was opened, it was found that much of even the more poorly cemented formation was too hard to excavate without blasting. Even those parts of the formation which did not require shooting were stratified, some of the layers being soft and others quite hard. The hard strata varied from a few inches to a few feet in thickness. Softer parts of the formation were excellent for use in the impervious fill; and the harder parts, while not ideal, were acceptable rockfill--best to be found locally.

As the excavation of the outlet works approached grade, considerable apprehension was felt about the possibility of differential settlement under the conduit. The Ogallala formation at grade varied typically in gradation and hardness. When the excavation had reached a point about 18 inches above grade, work was suspended temporarily, and an extensive drilling program was initiated. Dennison samples taken throughout the length of the conduit section subgrade were sent to the Denver laboratories; and