the gate level while in motion failed to operate fast enough, permitting the gate to get out of plumb and bending the guide rail on the downstream side of the gate structure, between two adjacent supports. The gate was then leveled and the lowering operation resumed, largely by manual control of the hoisting apparatus. As the gate was being lowered into position, the fillets in the lower corners of the gate chamber were removed as provided for in the design, and the gate was finally lowered to its seat without further trouble. The river was at a low stage at the time of closure, as shown in figure 42. After closure, the rear face of the gate and the head of the diversion tunnel were inspected. The only noticeable leaks were at splices in the cross girders, where openings existed that could not be entirely closed; and at several points along the gate seals, as the result of rust spots. The rubber seals were found to be in good condition, having been coated with shellac to prevent weathering. Verdigris on the bronze bar seals interfered to some extent with their action, but this was easily corrected by jarring at the locations of the springs. With 10 feet of water above the top of the gate, the total leakage from all sources in and around the gate was about 25 gallons per minute. There was also some leakage through the concrete lining of the tunnel, principally at construction joints, for a distance of about 400 feet from the portal. FIGURE 42-CLOSURE OF GATE AT OUTER ARIZONA TUNNEL 60. Closure of Nevada Gate. The bulkhead gate at the outer Nevada diversion tunnel was closed May 6, 1936, when the reservoir surface was at elevation 938.0. The water was 298 feet above the gate sill, so that the hoisting apparatus, as well as the gate, was deeply submerged. The gate was closed by remote control, the control station having been located at a point at elevation 1026 on the Nevada canyon wall. The reservoir surface was rising at the time of closure, due to relatively high stages in the river, and at the end of May 1936 had reached elevation 982.4, producing a head on the gate in excess of that for which it was designed. Because of this, the original proposal to unwater the tunnel behind the gate for inspection purposes was abandoned, and the temporary outlet conduits in the tunnel plug were closed with concrete as quickly as possible. While the bulkhead gate was being lowered, one of the temporary outlet gates in the tunnel plug was partly open and the others closed. Closure was therefore made with practically balanced water pressures on the two sides of the bulkhead gates. As the gate approached its closed position, the downstream pressure dropped slightly, forcing the gate lightly against the top and side seats. The wedges were then seated. Due to the use of the temporary outlet gates, the rollers of the bulkhead gate were subjected to little if any load while the gate was being closed. On the other hand, the gate and wedges, when the gate was closed, were loaded to about 125 per cent of their designed capacity, which they could safely withstand. UPSTREAM TUNNEL PLUGS 61. General Description.-After the four diversion tunnels had served their purpose in bypassing the river around the dam site during construction of the dam and powerhouse, they were incorporated into permanent features of the project, as contemplated by the general plan shown in figure 1. The outer tunnels, from points approximately under the downstream ends of the spillways, were converted into spillway outlet tunnels. This was done by placing concrete plugs in the tunnels, upstream from the junctions with the inclined tunnels leading to the spillway channels above. Similar plugs were placed in the inner diversion tunnels upstream from their junctions with the inclined tunnels leading to the upstream intake towers, so that the downstream. portions of the tunnels could be used as locations for the header pipes of the two lower units of the penstock and outlet system. Thus four upstream tunnel plugs were required, one in each of the four diversion tunnels. The downstream tunnel plugs required in the inner diversion tunnels were incorporated in the tunnel-plug outlet works and are described in chapter VIII. The four upstream tunnel plugs were planned to seal off all flow from the reservoir below the level of the intake towers. The plugs were designed to withstand full reservoir pressure. In addition to the forces acting on the plugs and the coefficients of friction, the design of the plugs considered the necessity for providing adequate rock coverage between the upstream faces of the plugs and the inclined tunnels. As explained in chapter II, keyways for the upstream tunnel plugs were provided during the excavation and lining of the diversion tunnels. Total costs of the four upstream tunnel plugs are given in section 8. More detailed cost data for the upstream plugs installed in the two inner diversion tunnels are given in section 101. Similar data for the upstream plugs and the bulkhead gates at the inlet portals of the two outer diversion tunnels are given at the end of this chapter. 62. Design of Plugs. The plug in each outer diversion tunnel, immediately upstream from its junction with the inclined tunnel, has an over-all length of 393 feet. The downstream part of each plug forms the lower part of the transition from the inclined spillway tunnel to the horizontal tunnel. Each plug, as shown in figures 43 and 44, consists of three tapered sections, designed to provide adequate anchorage and to prevent seepage along contact planes between the plug and the surrounding rock. Each of the upstream two sections of each plug is 142 feet long, with diameters of 66 feet and 52 feet at upstream and downstream ends, respectively. The plug in the outer Nevada tunnel, shown in figure 43, was provided with four 6 by 7.5-foot conduits, equipped with control gates, for temporary regulation of stream-flow after closure of the diversion tunnels and until water could be released through the intake towers and the permanent outlet system. Flow through each conduit was controlled by means of two high-pressure slide gates placed in tandem, the upstream gate in each instance being provided for emergency operation should the lower gate fail to function, see figure 45. The gate openings were of the same size as the conduits. The gates were designed to operate under a head of 300 feet, and to withstand a hydrostatic head of 570 feet. Operation of the gates was by means of hydraulic cylinders, using oil as a medium, with a working pressure of 1,000 pounds per square inch. The conduits, from the upstream face of the plug to a point 16 feet downstream from the lower gates, were provided with cast-steel linings which were made in 6-foot sections with flanged couplings, as shown in figure 45. |