below the bottom of the third lift. The horizontal bars were distributed along both faces of the buttresses, and the area of the steel used amounted to approximately 0.3 of 1 percent of the vertical cross-sectional area of the concrete. This percentage of longitudinal steel appears to have stopped all vertical cracking. Reinforced concrete struts, 18 by 24 inches, are placed between all buttresses, spaced 24 feet on centers both ways. The contractor was given the option of placing precast or monolithic struts. He first chose the precast and afterward changed to monolithic struts, due to limited space for a precasting yard. Precast struts are grouted into pockets left in the sides of the buttresses and likewise the monolithic struts are poured into similar pockets. The pockets for the monolithic struts are formed with tapered sides, which are painted with one coat of water-gas tar to prevent bond with the buttress concrete. The tapered sides prevent spalling of the edges of the buttress around the strut. The contraction joint between the face slab and the buttress, consisting of the corbel seat and the keyed side of the buttress tongue, was coated with about -inch thickness of plastic asphalt putty before the face slab was poured. This putty acts as a waterproofing seal for the joint, prevents bond between the concrete of the face slab and that of the buttress, and insures against spalling of the edges of the buttress corbel. The corbel seat is also tapered slightly to prevent spalling. The plastic asphalt putty was chosen for this purpose upon the recommendation of the Ambursen Construction Company, of New York City, based on previous satisfactory experience with the material. SPILLWAY Provision was made for a spillway discharge of 30,000 cubic feet per second, through three bays of overflow section in the central portion of the dam. Each bay is controlled by a 30- by 30-foot structural steel caterpillar gate of the overflow type. The gates are operated by large screw stem hoists driven by electric motors, the power being supplied by a 15-kilowatt generator, driven by a turbine supplied with reservoir pressure from the needle valve outlet pipes. The gates are mounted on a 45° slope and slide down the upstream face of the dam in opening. The overflow type of gate was adopted on account of the large amount of drift, with an occasional live-oak tree, carried by the river during the flood stage; and also on account of the smaller head required to pass corresponding floods than with the underflow type of gate. This latter criterion was considered important due to the flashy floods which are characteristic of Stony Creek, and the fact that the spillway gates were not designed for automatic operation. An unusual feature of the spillway for a dam of this type is embodied in the gatehouse which encloses the gatehoisting machinery. In addition to housing the machinery, the reinforced concrete walls of the gatehouse form the supporting track for a traveling crane used in handling the heavy machinery parts. OUTLET WORKS The major irrigation outlets, with a maximum capacity of 1,050 cubic feet per second, consist of two 50-inch, riveted-steel, outlet pipes extending through the dam, with 31⁄2- by 31⁄2-foot emergency gates at the upstream end and 42-inch balanced needle valves at the downstream end. Each pipe occupies a bay of the dam. The two bays are housed at the upstream end for the protection of the emergency gate hoists, oil-pumping equipment, etc.; and at the downstream end for the protection of the needle valve operating equipment, battery, and turbine-generator set. Both bays are protected by structural-steel trashracks, supported on reinforced concrete beams, the trashracks lying in the same plane as the upstream face of the dam. The usual bypass valve and air vent pipes are provided in connection with the emergency gates. A 10-inch needle valve outlet, with required capacity of 10 cubic feet per second, is provided for diversion to the Angle Troxel ditch. This outlet includes a 12-inch gate valve and a 12- by 7-inch Venturi meter, installed upstream from the needle valve. The whole installation lies within the emergency gatehouse for the main outlets, and in the same bay of the dam with the south main outlet. The trashrack for the 10-inch outlet lies within the main trashrack, and consists of structural-steel rack with 1inch spacings. The needle valve discharges into a concrete box, provided with an Ensign baffle plate for energy distribution, from which box the water is carried in steel pipe through the dam to the south side of the creek, and in a concrete conduit about 367 feet downstream from the dam to the head of the Angle Troxel ditch. Diversion to this private ditch was occasioned by the fact that the original diversion dam for the ditch was destroyed by the construction of the new dam. CONSTRUCTION Bids were received for the construction of Stony Gorge Dam on August 18, 1926, and the contract awarded to the low bidder, the Ambursen Construction Co. This company sublet the entire camp operation to H. S. Anderson, of San Francisco; and the hauling, excavation, backfill, and the production of sand and gravel for concrete, to A. Haidlen Co., of San Francisco. A good gravel road was already in existence from Fruto to the dam site. Preparatory work was started promptly after the contract was awarded, and foundation excavation on December 6, 1926. The excavation was confined to the two abutment sections until after the winter flood season, and cofferdam construction was not started until April 20, 1927. The creek was diverted to the south channel on May 9, 1927, and concrete work started on June 3, 1927. The creek was diverted from the south channel to the closure bays on October 9, 1927, and excavation for the dam completed in November 1927. Excavation for the dam disclosed good rock for foundations, but generally at greater depth than estimated; so that the excavation quantities were considerably increased. The depth of excavation for buttresses varied from 4 to 27 feet below original surface, with the greater portion running from 15 to 25 feet. The upstream cut-off trench with a bottom width of 2 to 6 feet was from 7 to 29 feet below original surface, averaging 20 feet in depth. The downstream cut-off trench below the spillway apron was excavated to elevation 720 feet, or 10 feet below the top of the concrete apron. The main fault line crosses the dam site near buttress no. 34, about the middle of the dam. So far as exposed by the excavation, the fault seam appeared to be well sealed. and tightly filled with clay gouge of about 1-inch thickness. Two creek channels, separated by a small rocky island at the exact point needed for convenient handling of the stream, made diversion of the stream for construction purposes relatively simple. The north channel was first enclosed by building a cofferdam along the island and across the north channel at the upper and lower ends, thus diverting all the flow to the south channel. This unwatered a considerable area of the stream bed and permitted excavation for the foundations of buttresses no. 30 to no. 34, inclusive. These buttresses were then built to an elevation well above probable high water, and the upstream cut-off trench was completed and filled with concrete to an elevation above the stream bed, the two bays from buttresses no. 32 to no. 34 being left 5 feet lower than the rest of the bays for the passage of the ordinary flow of the creek. At this point a special closure structure was built of concrete connected with the dam, which provided two 8- by 10-foot 3-inch rectangular openings to be closed with stop logs in the final closure of the dam. When the closure structure had been built, the stream flow was again diverted to the north channel, passing through the closure openings, and the south channel unwatered by additional cofferdams. This permitted the excavation for foundations for buttresses nos. 26 to 29, inclusive. These buttresses and the adjoining upstream cut-off walls were thereafter built up to an elevation well above any expected flood. This work was completed in November 1927, well in advance of the December 25, 1927, flood, which peaked at 1,650 cubic feet per second. Earth excavation was done by two different methods, including handwork on the steep slopes, and teamwork using fresno scrapers on the more gentle slopes. Rock excavation in the flatter reaches between buttresses nos. 25 and 47 was done by gasoline shovels, using first an old shovel with 4-yard bucket, and later a new shovel with a 1-yard bucket. The old shovel was converted to a tractor crane and used for loading skips of rock onto trucks for hauling to the waste banks. A considerable quantity of rock was also handled by the steam derrick in the vicinity of buttress No. 24. No excavation was handled by the cableways. Careful blasting was necessary in order to excavate the narrow trenches for the buttress foundations. This was found impracticable in certain portions of the foundation where the rock was close jointed and thin bedded. Instead of excavating buttress trenches by slow hand methods, the contractor preferred to take off part of the ridges between trenches without payment; so as to permit the use of the gasoline shovel to greater depths in excavating the trenches. The buttress trenches as excavated were generally wider than necessary for the concrete footings, and in nearly all cases vertical forms were built in the trenches for the footing courses. On many of the footings the forms were omitted for 1- or 2-foot depths at the bottom, permitting the concrete to fill the entire trench up to the bottom of the form. Concrete aggregates were obtained from a gravel bar in Stony Creek, one-half mile upstream from the dam site. The bar covered about 10 acres, and had a depth of 8 to 10 feet of gravel and sand, lying over ledge rock. Materials were only fair in quality, the sand being rather poor structurally and generally rather coarse. There was a small excess of sand and the natural gradation was satisfactory. The small proportion of cobbles, larger than 6-inch diameter, was run through the crusher and combined with the gravel. Cobbles up to 6-inch diameter were used in the massive portions of the concrete. After the completion of the massive work all cobbles were crushed to 3 inches and combined with the gravel. The crushing, washing, and screening plant was set up at one side of the gravel bar, and the material was excavated and conveyed to the plant by drag-line scrapers. A belt conveyor elevated the material to the cobble screen, from which the oversize went to the crusher. A bucket conveyor lifted the gravel, sand, and crushed rock to the smaller screens over the gravel bins. There were three screens of the inclined conical type, and an automatic sand separator. At the capacity of the plant, materials were furnished for about 24 cubic yards of concrete per hour. Operation of the washing and screening plant was generally satisfactory, but close supervision was necessary to prevent overfeeding and to secure thorough washing. The material was separated into sand, three sizes of gravel, and cobbles. Sand, gravel, and cobbles were hauled in trucks from the screening plant to the concrete-mixing plant, erected on the south hillside immediately downstream from the dam. The trucks delivered the three sizes of gravel, sand, and cobbles into separate bins constructed over the mixer. The gravel for each batch was measured in three measuring batches. An inundator was used for measuring and saturating the sand. The mixer discharged concrete into bottom-dump buckets of 11⁄2-cubic-yard capacity, which were pulled by hoist about 75 to 100 feet to points under the main cableways. Bottomdump buckets were landed by the cableway onto small trucks running on a light track supported on the buttress forms. The buckets were then moved along the track delivering concrete to any desired point in the forms. Chutes were used to convey the concrete from the buckets to the face-slab forms, into the corbels, struts, etc. Batches of different mixtures varied from 1.20 to 1.40 cubic yards of concrete. The mixing time was 11⁄2 minutes per batch, and the maximum capacity of the mixing plant was 24 batches per hour, which was seldom attained. Cableways were used for handling reinforcement steel and forms as well as concrete; and with two of the three cableways carrying concrete the ordinary capacity was 20 to 22 batches per hour. With only one cableway handling concrete the capacity of the plant dropped to 10 or 12 batches per hour. A day's run of concrete was from 100 to 200 cubic yards, averaging 150 cubic yards in an 8-hour shift. The greatest amount placed in 1 month was 4,881 cubic yards in January 1928. The rate of placing concrete was generally governed by the time required to get the forms ready, and this was especially true toward the end of the job when walls were thinner and volumes smaller in proportion to the surface area of the forms. A night shift was usually employed on the forms and reinforcement steel, but practically all concrete was placed during the day shift. About 1,000 cubic yards of concrete were placed by a small tower and chuting system, including the stream bed protection below the spillway and the Troxel conduit. The screening plant separated the material into the following sizes: Sand passing a 16-inch ring; gravel passing a %-inch ring; gravel passing a 13⁄41⁄4-inch ring; gravel passing a 3-inch ring; and cobbles passing a 6-inch square opening. The sand, gravel, and cobbles were washed, and the maximum permissible amount of clay left in the sand was fixed at 4 percent by weight. The sand was sampled each day. and tested for clay, fineness modulus, and organic matter. After being washed at the screening plant the sand generally contained from 1 to 3 percent of clay, averaging 1.79 per cent. The fineness modulus seldom exceeded 3.25 and did not in any test fall below 2.69. A number of briquet tests of the sand were made each week and compared with results using standard Ottawa sand. Mortar in proportions of 1:3 almost invariably showed greater strength for the local sand than for Ottawa sand. The tensile strength of the briquets tested, averaged 254 pounds per square inch at 7 days, and 355 pounds at 28 days. Specific gravity of the sand was 2.66 to 2.70 and that of the gravel 2.80. The concrete mixtures were designed to have the desired strength with a large safe margin, and with the lowest workable water-cement ratio. No admixture of any kind was used in the concrete. The work involved four different mixtures of concrete as follows: Mixture no. 1: Massive concrete in cut-off walls and buttress footings, trashrack structure floors, stream bed protection, mass concrete around gates, and buttress below elevation 774 feet. Mixture no. 2: Heavy sections, such as buttresses above elevation 774 feet, spillway training walls, piers for 50-inch outlet pipes. Also used in place of mixture no. 1 when no cobbles were available. Mixture no. 3: Light sections such as spillway gatehouse floor, parapet on top of the dam, upstream face slab, downstream face slab, walls on top of and through dam, and struts between buttresses. Mixture no. 4: Very thin sections such as the roofs and walls of spillway and outlet gate-operating houses. One hundred and sixty holes were drilled in a single line in the bottom of the upstream cut-off trench for pressure grouting. South of the creek the holes were spaced about 6 or 7 feet apart. From buttress no. 32 to the north end the spacing was about 4 or 5 feet. The depth of the holes below the bottom of the trench varied from 18 to 40 feet, the deepest holes being in the vicinity of the main fault between buttresses no. 32 and no. 35. Grout pipes, 21⁄2-inch diameter, were set over the drilled holes, and these were embedded in the concrete of the cut-off trench and the grouting done subsequently to the pouring of the concrete. Pressure used in grouting was from 90 to 100 pounds per square inch. Grout was mixed in the proportions of 71⁄2 gallons of water per sack of cement for the greater part of the grouting, a small quantity being mixed with 10 gallons per sack. No sand was used in the grout. Only five holes took more than three sacks of cement each. The other 155 holes averaged less than one sack each, taking very little more than enough to fill the hole and the pipe. Buttress-footing courses were made of a height equal to twice their width, stepping up or down to conform with the larger irregularities of the rock foundation. The upstream cut-off wall on the side-hill slopes was built in level steps, so that the face slabs started from a horizontal joint, the vertical steps being at 2 feet from the center line of the buttress on the downhill side. The joint at the top of these cut-off blocks, and all face slab joints above, were made normal to the plane of the face. From buttress No. 27 to no. 38, extending across the creek channel at nearly the same level, the construction joints between cut-off blocks were in most cases made on the center lines of the buttresses. Forms for the cut-off blocks, buttress footings, and the first part lift above the footings, being special, were built in place. Above the first full horizontal joint the buttresses were built in 12-foot lifts, and panel forms were used. The panels were 14 feet in height and from 8 to 24 feet in length, built with 14-inch matched lumber for lagging, and lined with galvanized steel sheets. Panel forms were handled by the cableway. The top forms for the face slabs were panels, 15 by 18 feet, the under forms being built in place. Forms were secured in place by steel rods passing through the wall or face slab, and the rods were removed the next day after placing the concrete. The rods were slightly tapered to facilitate removal and to provide a wedge effect in the grouted holes. Rods passing through the corvels were run through pipe spreaders, which were left embedded in the concrete. The large panel forms resulted in smooth surfaces and a very small amount of patching. No cement-grout washing was required, except on the lower portion of spillway apron, where a small amount of washing was done for the sake of a smoother surface. The sides of the parapet wall were rubbed with carborundum to give a pleasing appearance. The top of parapet and all horizontal surfaces requiring a trowel finish were troweled at the time concrete was placed, and no plastering was permitted. The United States furnished the reinforcement steel for the dam, this item amounting to 2,232,840 pounds. At the request of the contractor, most of the cutting and bending of the reinforcement steel was done at the mills, the contractor paying this cost, amounting to $0.2836 per hundredweight. All bars requiring bending were of structural steel grade, and straight bars were of hard rerolled rail steel. The upstream cut-off trench above the concrete was backfilled to a minimum depth of 4 feet, after the concrete was placed and the pressure grouting was done. Selected clay from the hillside near the south end of the dam was used for this purpose. The spaces between the buttresses were backfilled with any convenient class of material to a depth of at least 3 feet above the bottom of the buttress footings to prevent weathering of the rock and to give a finished appearance to the structure. COST ANALYSIS The accompanying table no. 1 shows the detail cost of Stony Gorge Dam, not including the cost of preliminary investigations, right-of-way, etc. |