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Figure 8. --Trenton Dam--General plan and sections. (Sheet 4 of 4.)
From drawing 328-D-945. A. Arrangement of Structures
13. General. - Trenton Dam is a rolled earth-fill structure 144 feet in height above the lowest elevation of shale bedrock in the cutoff trench, 100 feet in height above the riverbed, and 8,600 feet in length (fig. 8). It has a maximum base width of 780 feet, a crest elevation of 2793.0, and a crest roadway 30 feet wide.
A 1,100-foot-long spillway structure having a maximum discharge capacity of 126,500 second-feet (determined by model studies) is located on the left abutment. The gate structure for the spillway is a combination gravity overflow crest and slab and buttress abutments. It is 90 feet in height and 438 feet in length. The overflow gravity section contains three 42- by 30-foot steel radial gates for flood control and two 6- by 7.5foot gated sluiceways for river control. A 266-foot-wide channel extends from the end of the spillway stilling basin a distance of about 1 mile downstream to the river channel.
An irrigation canal outlet structure is located under the embankment in the lower right abutment. This consists of a vertical intake shaft, about 50 feet high, and a circular conduit leading to an emergency gate chamber located near the axis of the dam. A horseshoe conduit, about 244 feet long and containing a 56-inch steel outlet pipe, extends from the emergency gate chamber to a control house and stilling well structure. An outlet is provided in the stilling well for the Meeker Irrigation Canal.
14. Embankment. - The scarcity of free-draining materials in the vicinity of the dam site limited the zoning possibilities of the structure. Maximum use of material from structure excavations was incorporated as a requirement in the embankment design. The sand and gravel obtained from foundation excavation and the Ogallala sands obtained from the spillway excavation were considered satisfactory zone 2 material. This material intercepts seepage through the zone 1 embankment material and lowers the saturation line at the downstream portion of the embankment. At the lower end of the downstream toe of the embankment, specifications provided for placement of zone 3 material which consisted of selected material from dam foundation and structure excavations. The locations of the various zones are shown on figure 8.
The embankment design was initially based on the use of approximately 750,000 cubic yards of zone 2 material. During later stages of exploratory work and early phases of construction, it became apparent that a much larger quantity (about 1,100,000 cubic yards) of zone 2 material would be available. In order to utilize this larger quantity of material in the embankment, the top surface of zone 2 material was extended from elevation 2710 to about elevation 2725. The downstream slope was set in 12 feet from the finished 4 to 1 slope and impervious material placed in that area for slope protection.
The slopes of the embankment were determined largely by the properties of the material, including density, shear strength, and load consolidation characteristics. A variable slope was selected in order to obtain approximately the same stability for embankment at all elevations.
(a) Upstream Slope. -- The upstream slope of the embankment was provided with a 20-foot berm at elevation 2710 (fig. 8). Below the berm, a slope of 6 to 1 was used for the upstream toe. Immediately above the berm a slope of 3 to 1 was established to elevation 2760 and a slope of 2-1/2 from that elevation to the crest of the dam (elevation 2793).
The upstream slope above the berm was protected with 3 feet of rock riprap placed on 18-inch gravel or crushed rock bedding to an elevation 8 feet above the normal water surface and diminishing to 2 feet of riprap on a 12-inch bedding at the dam crest. The superimposed weight of the rock riprap and crushed rock bedding on the embankment of low permeability provides sufficient resistance to movement of the fine materials during reservoir drawdown. The bedding serves as a filter and the rock riprap protects the slopes against damage from wave action. In order that the riprap rock would have sufficient weight to withstand anticipated wave action, a minimum specific gravity of 2.65 for the rock was specified. A provision was also made that not more than 25 percent of the riprap materials should consist of pieces smaller than one-half cubic foot in size and at least 30 percent should consist of pieces larger than 3 cubic feet in size. Toe trenches, filled with rock riprap to a minimum depth of 5 feet, were provided at the base of the riprap to prevent undermining of the riprap base by wave action.
(b) Downstream Slope. -- Two 10-foot berms, one at elevation 2735 and the other at elevation 2760, were provided on the downstream slope of the embankment. These berms reduce erosion, collect rainfall runoff, and provide access for irrigation of seeded areas. The downstream slopes of the embankment were flattened progressively with distance downward from the crest. These slopes range from 2 to 1 to 10 to 1. (See fig. 8.) A 12-inch layer of top soil was placed in the downstream slope for a seedbed for grass seeds. Various drought and frost resisting grasses in conjunction with a legume were planted to provide protection against erosion.
A 12-inch toe drain was constructed near the outer toe of the downstream section of the right abutment south of the canal outlet works. This drain provides an outlet for seepage water which otherwise would impair the stability of the downstream portion of the embankment. The outer limit of the downstream embankment was selected for the toe drain construction so as to reduce the velocity of the water entering the drain and thereby minimize the movement of fine particles into the drain. To insure effective interception of seepage flow, the trench for the toe drain was designed with a 4-foot minimum depth below the ground surface. The drain was constructed with open joints and embedded in graded gravel and selected sand. Coarse material was placed adjacent to the drain to form a filter and prevent clogging of the drain by movement of fine material. Because of the pervious materials and alluvial foundation, toe drains were not provided in the other sections of embankment.
(c) Crest and Camber Details. -- A freeboard of 8 feet was selected above the maximum water surface, to prevent the dam from being overtopped by waves. Factors influencing wave height and wave ride-up, including maximum wind velocity, direction and duration, offshore reservoir depth, slope of dam and shore, and nature of the surface protection, were considered in selection of freeboard.
To allow for settlement of the foundation and consolidation of the embankment without a reduction in freeboard, a camber was provided over the crest length. From a maximum camber of 1.5 feet, anticipated settlement and consolidation at different locations was determined by vertical parabolic curves. The crest of the dam was covered with 12 inches of gravel and topped with 3 inches of gravel surfacing for a roadway. Beamtype guardrails, conforming to State highway requirements, were erected along the crest of the dam.
C. Spillway and River Outlets
15. General. - The spillway is located in the left abutment and consists of a concrete gravity gated overflow section, with slab and buttress wing walls extending into the embankment on each side. The crest structure is ogee shaped with a 1 to 1 upstream slope. Discharge of the spillway is controlled by three 42-foot-wide by 30-foot-high radial gates, which are arranged for automatic or manual operation depending upon the water surface elevation of the reservoir. Four 8-foot-wide piers separate the gates and provide support for a spillway bridge and gate control equipment. The spillway was designed for a discharge of 133,000 second-feet at reservoir water surface elevation 2785.0, but model studies indicated a maximum discharge of 126,500 second-feet with the reservoir water surface at elevation 2785.6.
Two river outlets located in the base of the intermediate piers pass through the overflow crest and discharge into the spillway chute. Each outlet has an invert elevation of 2710.00 and is provided with two 6- by 7.5-foot high-pressure gates installed in tandem. The upstream gate is for emergency use only, and the downstream gate is for regulating the flow through the outlet. The river outlets are an integral part of the spillway and are designed to discharge 1,000 second-feet with reservoir water surface at elevation 2720.0. At the top of the spillway gates, elevation 2773.0, the river outlets and the spillway have discharge capacities of 4,300 and 76,000 second-feet, respectively. A plan and sectional views of the spillway are shown on figure 9. Views of the completed structure are shown in figures 10 and 11.
Design criteria used in the design of the spillway structures are indicated in appendix F.