To prevent seepage from the reservoir, waste material was used to construct a dam at the mouth of a gulch situated on the south side of the reservoir about 2,000 feet west of the axis of the main dam. The crest elevation of this dam is about 3113. The slopes are about 3 to 1, and the height is about 60 feet. The dam effects some reduction of seepage from the reservoir through sand zones C and D.

28. Drainage Features. Some drainage features were known to be necessary and were included in the specifications. The need for others was discovered during and after construction, and these features were added later. To collect seepage waters under and through the dam, continuous downstream toe drains were provided in the specifications design. Starting high on the left abutment at about station 54+00 and about 75 feet downstream from the axis, 8-inch open-joint sewer pipe was to be placed beneath the zone 2 section in a ditch excavated to a depth of 4 feet below ground surface and surrounded by a graded gravel filter. At elevation 3045, the size of the sewer pipe was to be increased to 12 inches. This drain was to be placed parallel with the toe of the dam to about station 43+50 where it was to be installed parallel to Frenchman Creek and to discharge into the creek at the downstream toe. The right abutment drain was to start at about station 41+25 and about 130 feet downstream from the axis, and was to be installed down the slope of the abutment and to exit at the toe of the dam into the Frenchman Creek channel.

During construction, the toe drain design was modified because it was believed that additional foundation treatment performed during construction made extensive drains unnecessary. Toe drains as installed are shown on figure 11. Significant modifications included: (1) an extension of the left abutment 8-inch drain section beyond the embankment to discharge into fluvial sands below the waste material and organic soil overlying the area; (2) the elimination of that portion of the drain between stations 43+65 and 47+10 and between 190 and 197 feet downstream; (3) joining the 8-inch drains adjacent to the right abutment; and (4) discharging the right abutment drains into the outlet works stilling basin. These changes were based on a restudy of the seepage problem which indicated that the fluvial sands exposed during foundation excavation would be sufficient for drainage when considered in conjunction with additional blankets and cutoffs.

About 2 weeks after closure was made on October 23, 1950, seepage measurements were started. On March 31, 1951, an open drainage ditch was excavated along the left side and toe portion of the downstream waste area to drain waters from the left abutment area. The ditch was originally about 1, 200 feet long and a maximum of about 10 feet deep. Subsequent erosion, particularly in the portion along the left abutment, has altered these dimensions. After construction of the open drainage ditch, sand boils started near station 50+00 about 400 feet downstream from the axis of the dam. To prevent serious piping, 2 feet of gravel was dumped into the sand boils. These special gravel filters successfully controlled the boils.

A restudy of foundation conditions was inaugurated in January 1952 as a result of conical slumps near station 43+00, about 200 downstream; increased seepage as the reservoir was raised; erosion along the left abutment near the toe of the dam; and large quantities of sediment discharged by the 8-inch toe drain. Two test shafts were installed to provide a means of observation and to aid in drainage. (See also section 23.) Test shaft No. 1 was located at the upstream end of the No. 3 drain at station 43+65 and about 190 feet downstream from the axis of the dam. The shaft was placed so the open upstream end of the drain could be plugged. The shaft is 30 feet deep and consists of a 54-inch inside-diameter precast concrete pipe. A 24-inch-diameter gravel-filled drain well extends about 10 feet below the shaft. Details are shown on figure 10. The shaft was designed to function as a pressure relief well, drainage point, and inspection shaft. Test shaft No. 2 was installed at station 41+20, 160 feet downstream from the axis of the dam.

During and after the seepage investigations, four drain wells were installed downstream at the locations shown on figures 8 and 10. Drain wells No. 1 and 2 were located so that permeability of the alluvium could be determined. They were also arranged so that they could be incorporated into the drainage system. Drain wells No. 3 and 4 were placed to reduce hydrostatic pressures and to prevent piping near the toe of the dam. Drain wells No. 2, 3, and 4 penetrate the ancient channel and part of the Ogallala formation. These wells were incorporated in the drainage system. The 24-inch perforated

sheet metal well casing of drain wells No. 3 and 4 is protected against corrosion by magnesium anodes. The elevations of the ground surface and the base of the drain wells are as follows:

Post construction investigation indicated that a method of draining the left abutment area was necessary to prevent erosion of the abutment, to lower the ground-water table downstream from the dam, and to prevent possible piping from developing. Accordingly, 850 feet of 12-inch inside-diameter, interlocking, porous concrete drain pipe was installed in the abutment. Location and details of the left abutment drain are shown on figure 8. The drain was incorporated in the original drainage system. The porous concrete sewer pipe was placed in a trench excavated to about elevation 3052, 5 to 10 feet in depth, on 1 to 1 side slope with a 4-foot base width. The pipe is surrounded by a graded sand-gravel filter with a minimum thickness of 1 foot. One-hundred percent of the filter material passed the 3/4-inch screen and 0.2 to 0.4 percent passed the 100 screen. The gradation of the filter material was selected to prevent the washing of fine base materials into the porous pipe.

B. Embankment

29. Design Considerations. Due consideration was given to economy, earth materials available, and hydrologic requirements in designing Enders Dam, including the dike, to provide a stable water barrier. Prior to and during construction, the design given in specifications No. 1410 was modified as additional studies, exploration, testing, and excavation disclosed facts previously unknown. Enders Dam and dike were designed to use most advantageously those earth materials available in the immediate area. The basic embankment design concepts did not change throughout construction, even though several modifications in design were made. Changes in specifications design resulted primarily from modified hydrologic conditions and resulting changes in the spillway design. Design of Nebraska State Highway No. 61 was virtually a part of the embankment design, since the highway is routed across the crests.

30. Zoning Requirements. The zoning requirements for the embankment for the Enders Dam, including the dike, were based on the zoning reports. In general, the zoning reports as written anticipated the problems that actually occurred during construction, and few variations were necessary as the job progressed. There was plenty of impervious earth material available. The major materials problems were the scarcity of pervious earthfill material and of rockfill material. The zoning report of September 22, 1947, recognized this scarcity and recommended that the pervious zone be reduced in size to the narrowest width practicable, that the rockfill be carried only to elevation 3065, and that some other method of downstream slope protection be adopted above that point.

Both the dam and dike were designed to be virtually homogeneous, with a relatively large impervious zone 1 section enclosed by a thin shell of pervious zone 2, zone 3, and riprap sufficiently large to protect the underlying zone 1 as shown on figures 11 and 12. When the shortage of pervious materials became apparent, various methods of downstream slope protection were considered, but the best plan appeared to be to continue the rockfill as planned and to modify its character. If zone 3 were to be constructed not of rock alone but of a mixture of the rocky Ogallala materials along with some uncemented or deteriorated portions of the formation, it would be possible not only to continue zone 3 to the crest of the dam as originally designed but also to utilize a large amount of the required spillway and outlet works excavation which otherwise would have to be wasted. (Because of the difficulty of segregation, it was impossible to utilize only the rocky portions of the formation in zone 3.) The change in the character of the zone 3 rockfill was made, and the resulting modification to the zoning plan from that given in the original

Surface of highway

340

105

100

3100

zoning reports was covered in an addendum to the zoning report. Subsequent explorations for suitable materials, changes in the location of borrow areas, and variations in the amount of usable materials from required excavation did not affect the overall porportioning of the various zones.

Selections of materials for each zone of the dam were based upon the properties of the materials as determined by standard laboratory test procedures, availability, workability, and relative economy. The most available material was the loess. Materials from required excavation were used in zones 1, 2, and 3 and the toe blankets of the dam and dike. Distribution of materials in the zones was based upon the laboratory properties. Unsuitable material was wasted. If a borrow area did not yield sufficient material with the proper physical properties, new areas were developed.

31. Description of Zones. - Enders Dam (fig. 11) consists of a zone 1 impervious section, which includes the upstream cofferdam; a zone 2 pervious section upstream and downstream; a zone 3 rockfill section downstream; a protective gravel blanket underlying the riprap upstream; and blanket waste disposal areas upstream and downstream. The dike consists of essentially the same zone 1 and zone 2 sections with riprap on part of the upstream slope and selected material on the remainder, and with seeded topsoil on the downstream slope.

The zone 1 section of the dam and dike was specified to be constructed of selected impervious materials consisting of clay, sand, and gravel increasing in permeability toward the outer slopes and rolled to 6-inch compacted layers. Actually, the zone 1 was constructed of silty materials. Variable slopes were specified for the dam, a downstream slope of about 2 to 1 and an upstream slope of about 1.5 to 1 being indicated. As built, the upstream and downstream slopes are about 2.5 to 1. The flattened slopes of the zone 1 compensate for the reduced sections of free-draining material. Slopes extend from a point 6 feet below the crest to the cofferdam upstream and to the enlarged zone 2 section above the toe drain downstream. Slopes of the zone 1 section of the dike were to have been 2 to 1 downstream and 3 to 1 upstream. As constructed, the dike zone 1 slopes are variable on the downstream side and about 3 to 1 to station 142+50 upstream, and variable beyond this station.

The zone 2 section of selected sand, gravel, and rock, increasing in coarseness toward the outer slopes and rolled in 6-inch compacted layers, forms a shell that is parallel with the slopes of and encloses the zone 1. On the crest of the dam, the zone 2 is about 6 feet thick vertically. On the upstream side, zone 2 material is about 10 feet thick horizontally to elevation 3125 and about 16 feet thick horizontally to the downstream slope of the cofferdam. About a 10-foot horizontal thickness of pervious soil comprises the zone 2 on the downstream slope to elevation 3058.2. Below this elevation, a large triangular section of zone 2 is placed over the toe drain. Zone 2 on the dike is about 10 feet thick upstream and downstream. Slopes are variable beyond station 142+50.

Zone 3, rockfill from required excavation, forms the downstream slope protection and is about 10 feet thick horizontally at the crest and about 80 feet thick at the downstream stripping line. The upstream slope is protected by 3 feet of riprap on 12 inches of gravel from elevation 3070 to elevation 3118 and by 2 feet of riprap on 12 inches of gravel from elevation 3118 to crest elevation 3137.5. Originally 3 feet of riprap from the upstream berm to the crest was specified. However, because the reservoir would rise infrequently above elevation 3118, it was believed that the thickness of riprap could be safely reduced above this elevation, especially with the substitution of Guernsey dolomite, which is more dense and durable than the Franklin quartzite originally proposed for riprap. The upstream gravel blanket was designed to protect the underlying zone 2 from wave action. Initially, this material was specified as reasonably well-graded gravel from 1/4-inch to 2-1/2 inches in size. Since gravel meeting these specifications was not available locally, the more durable cemented Ogallala material from the right abutment was substituted. The Ogallala material was crushed and graded from 1/4-inch to 3-inch sizes.

32. Dimensions. Some of the dimensions of Enders Dam and dike were altered to correspond with major spillway design changes. The crest width of 30 feet for the dam and dike was one of the dimensions unaltered from that originally designed. However, the

crest elevation, location of the downstream berms, and outer slopes were revised as a result of these design changes. The crest elevation of the dam was lowered 1.5 feet to elevation 3137.5, and the crest elevation of the dike was lowered 1.5 feet to elevation 3135.5. These revisions reflected the new flood routing scheme which changed the maximum water surface requirement.

The upstream slopes were unchanged except for the addition of the waste area to increase toe stability. Upstream slopes are 2-1/2 to 1 from crest elevation 3137.5 to elevation 3115; and 3 to 1 from elevation 3115 to elevation 3070 with a 23.7-foot berm at the last elevation. A 20-foot berm was originally specified. From the upstream edge of the berm, the slope of the cofferdam is 4 to 1 to the stripping line. Superimposed over the cofferdam, beginning at elevation 3070 is the waste area on about a 10 to 1 slope.

Downstream slopes of the dam were changed to compensate for the decreased crest elevation, and a waste area was added during construction to increase toe support. The elevations of the three berms were raised to accommodate slope and crest elevation alterations. Originally, downstream slopes were 2 to 1 from the crest to a 10-foot-wide berm at elevation 3115, 2-1/4 to 1 to a second 10-foot-wide berm at elevation 3090, 2-1/2 to 1 to a third 10-foot-wide berm at elevation 3065, and 4 to 1 to the downstream stripping line. Subsequent downstream slope changes included: 2 to 1 to a 10-foot-wide berm at elevation 3117±, 2-1/2 to 1 to a second 10-foot-wide berm at elevation 3091.5±, 2-1/2 to 1 to a third 10-foot berm at elevation 3066.5, and 4 to 1 to the stripping line. The downstream disposal area started at about elevation 3065 and continued on about a 20 to 1 slope to the ground surface.

The upstream slope of the dike is 3 to 1 from station 99+39 to station 142+50. Beyond station 142+50 the slopes were flattened to 8 to 1, so that selected material from required excavation forming a beach slope could be substituted for the specified riprap. The downstream slope of the dike was unchanged at 2 to 1. The downstream slope is protected by seeding on 12 inches of topsoil. The seeding was selected for slope protection because it was thought to be most economical, since topsoil was available from borrow area and foundation stripping.

33. Design Details. --Design details were governed by local conditions. The dam was designed originally for 8.3 feet of freeboard at maximum water surface and the dike for 6.3 feet of freeboard. Changes in hydrologic conditions reduced the freeboard by 0.3 foot. Camber was added to the crest of the dam to compensate for loss of freeboard caused by postconstruction compression of the embankment and foundation. About 1.5 feet of camber was added at station 44+00 and lesser amounts along the crest. Since Nebraska State Highway No. 61 crosses the crest of the dam, the crest was surfaced with 4 inches of gravel. Steel guardrails and guardrail posts were placed on the upstream and downstream edges of the crest as a means of traffic protection. Access to the spillway and outlet works is by means of Nebraska State Highway No. 61 and the county road downstream from the dam. An access road was built from the county road to the valve house.