foundation fractures and behind the lining, the foundation rock for a depth of 20 feet was consolidated by grouting and by tieing the concrete to the foundation by means of anchor bars extended 15 feet into rock. A system of open-jointed sewer pipe drains was provided behind the floor and sidewall lining downstream from the spillway gates, to prevent a large buildup of hydrostatic pressure under the lining. For computing stresses in the gate structure, pier, sidewalls, and gate supports, the following alternative loadings were investigated: (1) Reservoir empty; dry backfill loads along the tops of the walls and a surcharge of 400 pounds per square foot on top of the backfill. (2) Reservoir water surface to top of gate, elevation 5620; gates closed; backfill loadings with and without surcharge. (3) Reservoir water surface to elevation 5620; one gate wide open and the other closed; backfill loadings as above. (This loading condition was also investigated with the stoplogs installed in lieu of the closed gate.) To check the stability of the inlet and gate structure sidewalls against lateral overturning, it was assumed that the 15-foot-thick zone of the anchored rock mass would act integrally with the concrete as a stabilizing force to resist the horizontal overturning force due to reservoir pressure. For this assumption, the resultant fell safely back of the toes of the sidewall. The pier and the crest slab were designed to resist the internal stresses induced by moments caused by the most critical of the above loadings, combined with the bridge load and the highway traffic loads. To assist in resisting the overturning moments induced by conditions of single gate operation, the crest slab and abutment linings were anchored into the foundation rock. No longitudinal joints were provided between the floor and walls of either the inlet or the gate structure. Also, no lateral joint was provided at the junction of the open channel and tunnel lining at the tunnel portal. The gate structure thus was analysed as a W-shaped monolithic structure on an elastic foundation. The radial gate arm bearings are supported on brackets attached to the tunnel portal concrete headwall, which was anchored into the portal rock facing. (c) Operating Bridge.--The operating bridge which supports the gate hoist mechanism is a composite reinforced concrete beam and slab deck 48 feet long and 20 feet wide. It is divided into two equal spans, each simply supported on its outer edge by the channel sidewalls of the gate structure and on its inner edge by the center pier. The deck was designed to carry the hoisting machinery loads, loads resulting from the gate hoisting operation, and a live load which might be caused by snow or by the temporary placement of stoplogs or other equipment on the deck. For normal design stresses, in addition to the dead load weight of the beam and slab members, the following load assumptions were used: Live load--200 pounds per square foot of area. Weight of hoist motor--4, 000 pounds. Weight of hoist drum housing --9, 500 pounds placed Pull on hoist cable--31, 250 pounds, applied 7.5 feet to each side of the hoist motor. The above assumed loadings were distributed equally between the two downstream deck beams. The upstream beams were made the same size and were reinforced similarly to the downstream beams. The design of the bridge beams and deck slab was checked for the assumption that a gate could bind, thus placing an increased pull on the hoist cables equal to 300 percent of the normal gate pull. For this condition, an increase in working stresses was allowed. (d) Highway Bridge.--The highway bridge crossing the spillway just upstream from the spillway tunnel portal was shown in the specifications as a two-lane beam and slab deck span, designed in accordance with the AASHO loading criteria for trailer truck loadings of H 20-S 16-44. After the excavation for the spillway inlet structure was completed, the bridge was redesigned, principally to cover the open gap between the bridge and headwall and thereby prevent rock which might ravel from the portal cut from falling into the inclined shaft and tunnel. The 3-foot depth of sand cushion placed over the bridge slab was selected on the basis of tests 1/, assuming that adequate protection was needed against a rockfall of 2-cubic-yard volume dropping freely a distance of 75 feet. Impact forces from the falling rock were computed on the basis that a cube of rock would penetrate the sand cover about half the depth of a free falling bomb of the same weight and falling from the same height. It was also assumed that the force of the fall would be distributed on the bridge slab over a square area whose side length is equal to the width of the rock plus twice the depth of the sand cover. For this loading, an increase of 33 percent in the normal allowable working stresses was permitted. The design of the bridge slab on the above basis proved more than adequate to accommodate an H 20-S 16 truck-trailer loading. To maintain the necessary clearance between the bridge and the radial gate arms, the redesigned bridge span was changed to a 3-foot-thick flat slab, in lieu of the deeper slab and girder design. The bridge consists of two equal spans, each supported on its outer edge by the channel sidewall and on its inner edge by the gate structure center pier. (e) Inclined Shaft.--The transition in the inclined tunnel shaft and vertical bend, from the inlet portal to the 28-foot-diameter horizontal tunnel leg, varies first from a cross section having vertical sides and arched top and bottom, to a 34-foot circular section; and then tapers uniformly to the 28-foot-diameter section of the main tunnel. At the upper portion of the transitions, the center pier of the crest structure extends into the tunnel to provide a strut between the top and bottom arches. For computing design stresses, the top and bottom arches along the strutted section were considered fixed at the spring line and at the center pier ends. The design analyses were based on an assumed uniform vertical loading of 3, 800 pounds per square foot acting over the projected horizontal width of the top arch combined with external hydrostatic pressures equal to the reservoir head above the considered section. Rib shortening due to stress, and to shrinkage caused by temperature drop, were considered in the stress analyses. For computing design stresses in the tunnel transition downstream from the pier, the bottom and top arches were considered symmetrically fixed at the spring lines. The vertical sides of the transition were considered as columns which were loaded with the moments, thrusts, and shears from the top and bottom arches and with a hydrostatic load acting along the side. Analyses of stresses were made by the method of virtual work, and alternate check computations were made by the column analogy method and elastic center analyses method. General expressions for the moment and abutment reactions for the two arch conditions noted above were computed 2/ for unit loading, to aid in the analyses of the various transition sections. Designs were based on the most critical moments, thrusts, and shears resulting from any one of the following assumptions of loading: 1/Smith, Sherwood B., "Air Raid and Protective Construction," Military Engineer, July-August 1941. 2/ Moody, W. T., "Analyses of Fixed-End Arch Sections--Palisades Spillway, August 31, 1953, and October 5, 1953 (included as appendix F). Case I II III Top Arch Assumed loadings Dead load weight of arch plus uniform vertical super- Dead load weight of arch, plus uniform superimposed Dead load weight of arch, plus uniform superimposed IV V VI Lower Arch Vertical external hydrostatic pressure, plus stress due Stress due to shrinkage resulting from a 15° F. drop Radial external hydrostatic pressure, plus stress due For computing design stresses in the circular sectioned portion of the transition, the lining was considered as two 180° arches, each fixed at the horizontal centerline of the tunnel. Moments, thrusts, and shears were computed in accordance with the expressions shown in the reference memorandum 2/. Loadings were predicated on an average external pressure equal to the hydrostatic pressure on the tunnel centerline, applied radially, and on a uniformly distributed vertical load caused by the weight of a 120° rock wedge whose base width equalled the excavated width of the tunnel. Designs were based on the most critical moments, shears, and thrusts resulting from any one of the following assumptions of loading: The stresses were assumed equal for the top and bottom arches, making the reinforcement requirements symmetrical above and below the tunnel centerline. (f) Tunnel.--Beyond the inclined shaft, the spillway tunnel grade levels off, extending from station 7+72 to station 24+00 on a slope of 0.01911. The 28-foot-diameter circular tunnel, which has an average concrete lining thickness of 29 inches, is unreinforced between stations 9+29 and 23+20. From station 7+72 to station 9+00 the tunnel lining is reinforced with the same pattern of reinforcement employed upstream, the design being based on the same analyses considered for the transition section upstream. Also, from station 23+60 to station 24+00 the same pattern of reinforcement as used between stations 7+72 and 9+00 was provided. Adjacent to the unreinforced lining, from 2/ Moody, W. T., "Analyses of Fixed-End Arch Sections--Palisades Spillway," August 31, 1953, and October 5, 1953. station 9+00 to station 9+29 and from station 23+20 to station 23+60, a band utilizing lesser reinforcement serves as a transition to the fully reinforced linings. (g) Conduit.-- Beyond the spillway outlet portal, a closed cut-and-cover conduit serves as a transition to the open-cut outlet channel. Its top also serves as a bridge for a service access road. Below its horizontal centerline the conduit is cast against the excavated slopes of the channel, and above the centerline it is formed as a 2-foot thick 180° arch. Backfill over the conduit fills the trench to the level of the outside of the arch at its crown. The conduit arch was designed to support the backfill load as well as the wheel loads of a 60-ton truck-trailer, on the basis that the powerhouse transformers might be transported across this access road. The conduit floor was designed for the most critical stresses induced into the structure either on the assumption that the foundation reaction was uniformly distributed over the width of the base or on the assumption of a triangular distribution varying from a maximum at each side to zero at the center of the base. (h) Outlet Channel.-- The lining of the 1/2 to 1 sloped outlet channel and the channel floor was cast directly against the foundation andesite and is anchored by reinforcement bars extending 8 feet minimum into the rock. Where the channel sides extended higher than the bedrock level, concrete gravity retaining walls were placed above the top of the lining on a bench excavated to firm rock. The floor end sill and flip extension was designed as a cantilever, being tied to the upstream floor slab and to the 6-foot-deep transverse cutoff. The flip bucket was designed to withstand the centrifugal force of the flow produced by the upward deflection of the jet. This force was computed from the equation, 3/ P = force normal to surface of floor, pounds per square foot, W = velocity of flow, feet per second, r = radius of curvature of floor, feet, d = depth of flow, feet, and g = acceleration of gravity, feet per second per second. C. Outlet Works and Power Penstock System Design 33. Hydraulic Capacities. Reservoir releases are required for downstream prior right flows, for irrigation demands, and for flood control regulation. Such releases are made first through the powerplant turbines to the extent of power demands, and additional releases if required are made through the outlet valves and gates of the outlet works system. Irrigation demands vary from no flow in the winter to a flow approximating 15,000 second-feet at the peak of the irrigation season. As discussed previously, a flood control release capacity was established at 30,000 second-feet with the reservoir at minimum power head level, elevation 5497. 5. Discharge capacities of the outlet works control gates and valves and of the power penstock system bypass gates, for the arrangement shown on figure 53, are plotted on figure 52. Releases through the powerplant turbines, with four units operating under full gate at minimum power head, approximate 7,000 second-feet. These flows, together with the approximately 23,000 second-feet which can be released through the outlet tunnel, will satisfy the flood control release requirements of 30,000 second-feet. With the powerplant units shut down, the power penstock bypass gates can discharge approximately 10,000 second-feet at the minimum reservoir head. At normal reservoir elevation 5620, the capacity of the controls served by the outlet tunnel approximates 33,000 second-feet. With the power units shut down, the power penstock tunnel bypass gates can release approximately 14,000 second-feet. The total 3/Gumensky, D. B., "Design of Side Walls in Chutes and Spillways," Transactions A.S.C.E., vol. 119, 1954, p. 359, formula 6a. |