HE Guernsey Dam is a part of the North Platte

Federer irrigation project of Nebraska and Wyoming and is built across the canyon of the North Platte River about 3 miles upstream from the town of Guernsey, Wyo. The Guernsey Dam provides a regulating and storage reservoir of 72,000 acrefeet capacity and also hydrostatic head for the development of 6,000 kv. a. of electrical power.

The main storage reservoir for the North Platte project is the Pathfinder Reservoir, located in central Wyoming about 160 miles upstream from the point of diversion to the irrigated lands. The Guernsey Reservoir is located only 9 miles from the main point of diversion to the canals and serves as a regulating reservoir to control the water released from Pathfinder and also to store the inflow into the river between the two reservoirs. The electrical energy developed at Guernsey Dam is interconnected with the other Government power plant at Lingle, Wyo., and serves the project and the towns in the North Platte valley between Casper, Wyo., and Scottsbluff, Nebr.

The dam site is in a comparatively precipitous canyon through which the river flows between steep hillsides. Solid rock consisting of sandstones of varying degrees of hardness, together with limestone and some shale and with occasional streaks of iron ore, is generally near the surface at the dam site and is exposed in irregular cliffs over portions of the abutments for the dam. Solid rock in the stream bed lies beneath a deep layer of sand, gravel, and bowlders so great as to render closure between the dam structure and the underlying bedrock impracticable. Test holes were carried to a depth of 100 feet below the stream bed surface, but failed to reach solid rock. This condition was a dominant factor in reaching decision as to the general design of the structure. The territory surrounding the dam is covered with a scattering growth of pine and cedar trees and withal presents a very picturesque setting for the

structure.

DESCRIPTION OF DAM

The Guernsey Dam is a sluiced clay and sand and gravel structure with its downstream portion of heavy rock fill. It is 105 feet in height above the original stream bed, 560 feet long on the crest, and has a base width measured up and down the stream bed of 1,000

feet. The 3:1 upstream slope is protected from wave action by a 3-foot layer of dumped rock riprap. The theoretical top width of the embankment is 20 feet. The downstream surface has a slope of 2:1 for the top 30 feet then breaks to a slope of 8:1 for the next 50 feet in elevation from the bottom of which slope it is carried level for a maximum distance of 160 feet and terminates in a 3:1 slope to the river bed. The design. first contemplated and provided for a percolation slope in the seepage water through or beneath the structure. of 8:1 which was changed during construction by the addition of the level portion of the rockfill to 9:1.

The central portion of the dam is a clay puddled core founded in an open trench 30 feet below the river bed and extended above continuously through the main embankment to the crest. The trench in which this puddled core is founded has a bottom width of 10 feet and side slopes of 12:1. On each side of the clay core is a sluiced sand and gravel fill and on the downstream side of the embankment is a heavy body or blanket of rock fill.

The accompanying drawing shows the general plans, elevations, and sections of the Guernsey Dam and also the general features and dimensions of its related. structures.

RIVER DIVERSION

The North Platte River at the site of the Guernsey Dam has a large annual run-off, the maximum of record being about 2,636,000 acre-feet measured at the Whalen Diversion Dam 9 miles downstream. The maximum rate of discharge of record at this same point is about 20,800 second-feet. The river above the Guernsey Dam drains a large area in the States of Wyoming and Colorado and it is thought that the maximum possible discharge is much greater than the recorded amounts. It was also necessary during construction to pass the required water for the irrigation of the North Platte project and to meet other vested water rights. Due to these conditions and the fact that the construction of the dam would require more than one full calendar year it was necessary to make ample provision for caring for the river discharges during construction.

River diversion during construction was accomplished by a large tunnel excavated through the solid rock of the right abutment about on the level with the original stream bed. This tunnel was the first work of construction. This tunnel is lined throughout with

concrete and provides a net horseshoe section 25 feet in diameter for 273 feet at the intake end and 30 feet in diameter for 797 feet at the outlet end, making a total length of 1,070 feet. The designed thickness of the concrete lining was 18 inches throughout, but due to the varying, laminated, and seamy nature of the sandstone and limestone in which this tunnel was excavated the overbreakage was large and necessitated increasing the concrete placed in the lining by 47 per cent over the designed thickness.

The rock excavated from the tunnel was placed directly into the downstream portion of the dam by building out from the abutments across the river channel and restricting it until after the tunnel was com

dam could then be unwatered and its construction proceeded with. The foundation for the dam was first stripped of all loam, brush, trees, and vegetable matter. The cut-off trench for the base of the puddled core was dug into the sand, gravel, and bowlders forming the river bed to a depth of 30 feet with 10 feet bottom width and 12:1 side slopes. This trench was not completely unwatered, as this would have been impracticable, and the clayey material for filling it was dumped on the two sides and sluiced into place.

In order to tie the embankment into the rock abutments and prevent seepage along the planes of contact three concrete cut-off walls were provided on each side of the river channel. These walls were keyed not

pleted and closure could be made. The tunnel muck, therefore, provided largely for the permanent dam and was used for the downstream cofferdam during the construction of the earth and gravel-fill portion of the embankment. The excavation from the intake end and from the intake portal served likewise for the extreme upstream toe of the dam and for the upstream cofferdam.

Suitable concrete portal structures are provided at each end. The upstream structure provided for the permanent closure of the tunnel after its use for diversion and the downstream portal provided a suitable outlet for the permanent use of the outlet end of the tunnel for spillway and sluiceway discharges.

Upon the completion of the diversion tunnel for the passage of the river flow the upstream and downstream cofferdams were completed and the river diverted through the tunnel. The foundation for the main

less than 2 feet deep into the solid rock abutments and built not less than 5 feet high above the rock surface and extended from the original river bed to the top of the dam on each side. The trenches in which these walls were placed were back filled with puddled clayey material, and this material was carried into the dam embankment over and around the tops of the cutoff walls to a minimum depth of 5 feet.

CONSTRUCTION OF THE DAM

The construction methods used in building the main sand, gravel, and clay portion of the dam determined to a large extent its detailed composition. The borrow pits from which the main embankment material was obtained. contained clay, sand, and gravel in about the right proportions for the embankment except that especially clayey material was added during construction to furnish sufficient extra clay for the puddled clay core.

The material was hauled to the dam by trains and dumped from trestles on each side of the embankment. A pool of water was maintained between these two piles of dumped material and the finer materials from them were sluiced into the pool which thus formed the puddled core and left the coarser and more gravelly material in the upstream and downstream portions of the clay, sand, and gravel embankment. The trestles. from which the material was dumped were brought up in several stages until the work was completed. The result is a structure conforming substantially to the original design but with considerable irregularities in the thickness and position of the puddled clay portion. The attached progress chart of the dam fill indicates the methods used and the results obtained. Besides the material excavated from the diversion and power tunnels, the solid rock excavated from other structures, especially the north spillway, was used in completing the rock fill on the downstream portion and top of the dam. The best and most durable of this material was selected and used for the 3 feet of riprap on the upstream face.

The dam and power plant are reached from the outside from the higher land above the right abutment. A good roadway was therefore excavated down to the top of the dam and the crest of the dam was made into a roadway to reach the north spillway structure. To give access to the power plant, a branch road was built onto and across the downstream side of the dam as shown on the plan. The roads are surfaced with tailings from the Sunrise Iron Mine which is located within a few miles of the dam. A concrete bridge is provided across the north spillway channel and it is possible that at some time in the future the top of the dam will be used as a main public highway to the country to the north and west of the dam.

The crest of the dam is reinforced with a concrete parapet wall extending 3 feet above the top of the embankment on the upstream side and with a low concrete curb on the downstream side. Between these a roadway 26 feet wide is provided.

The designed plans called for placing an earth blanket on the reservoir floor immediately above the upstream toe of the dam. This earth blanket was partially placed as contemplated, but was largely secured in a very suitable manner by the natural settling of the silt from the water due to its impounding while it was being diverted into the diversion tunnel and through the north spillway.

THE SPILLWAYS

The north spillway structure is built in the solid rock of the north or left abutment of the dam. This provides the main facility for passing large flood discharges and also for the regulation of the flow of irrigation water past the dam. It has an estimated discharge capacity, with the gate wide open and the

reservoir water surface at full level which is 13 feet below the top of the parapet, of 52,000 second-feet. The structure has a waterway area at the control gate of 50 by 50 feet and a trapezoidal concrete lined discharge channel 585 feet in length with a bottom width of 25 feet, side slopes of 12: 1 and depth of 40 feet. Downstream from the gate the channel is in solid rock. Upstream from the gate the dam side of the channel is formed by a vertical reinforced concrete counterforted retaining wall 66 feet high at the maximum point. The foundation rock along the toe of the retaining wall, across the channel under the control gate and behind the wing wall on the north side, was pressure grouted to avoid leakage past the reservoir. The concrete lining of the spillway channel is anchored to the solid rock by 14-inch anchor rods at 10-foot centers both ways grouted 5 feet into rock and firmly attached to the steel reinforcement in the lining consisting of 3/4-inch steel bars at 15-inch centers both ways. Four-inch diameter tile drains are provided beneath the lining on the sides and bottom at 10-foot intervals and discharge into a 30-inch-square manway drain excavated beneath the lining along the center line of the channel and running from the control gate to the outlet end.

The control gate is a structural steel vertical Stoney roller gate 50 feet 9 inches high by 54 feet 71⁄2 inches wide. The gate weighs 434,000 pounds and is counterweighted by concrete blocks. The gate leaf is built up of 12 plate steel girders 6 feet deep laid horizontal and suitably braced and covered on the upstream side with a 1-inch steel skin plate. The gate operates on 6 sets of caterpillar rollers on each side which run on an H-beam track. The web of the H-beam track provides flexibility to take care of any deflection in the gate or other movement that might throw the bearing surface of the rollers out of plane. Each of the three lower caterpillar roller sets contain 24 rollers of chilled cast iron 8 inches in diameter by 12 inches long, and each of the three upper sets contain 15 rollers of the same size. The gate is rendered watertight by the use of brass pipe staunching rods that move with the gate and which the water pressure forces against the 6 by 6 inch seal angles which are embedded. in the concrete on each side of the structure.'

The gate is suspended by a large metal link chain on each side that passes through the operating machinery which is placed directly above and returns to the counterweights that are placed in concrete chambers on each side of the gate. The chain is made up of high-tensile bronze pins 5 inches in diameter and 17 inches long spaced at 10-inch centers and connected by 4 lines of 1 by 9 inch high carbon steel plate links with a combined cross section area of 36 square inches.

These chains are made with extreme accuracy, are each 66 feet 2 inches in length and weigh 283 pounds per linear foot.