BOY, OH, BUOY, ARCHIMEDES PREVAILS. I. Introduction

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1 BOY, OH, BUOY, ARCHIMEDES PREVAILS M. Leslie Boyd, P.E., Freese and Nichols, Inc. Doug Witkowski, P.E., Lower Colorado River Authority Frederick Lux III, P.E., Schnabel Engineering, Inc. I. Introduction Buchanan Dam is located on the Colorado River in central Texas near the city of Burnet and approximately 100 river miles upstream of Austin, Texas. The Lower Colorado River Authority (LCRA) has owned and operated Buchanan Dam and reservoir since Buchanan is the uppermost dam in the LCRA chain of six lakes known locally as the Highland Lakes. Lake Buchanan has a surface area of 35 square miles and retains 876,000 acre-feet of water at normal pool. The dam is often referred to as a multiple arch dam and is one of the largest in the United States with a length of 2 miles and maximum height of 145 feet. The dam has three gated spillways which contain thirty-seven tainter gates. The original design did not include any provision for dewatering any of its thirty- seven spillway gates. An aerial view of the dam is shown in Figure 1. Figure 1. Buchanan Lake Flooding. (three spillways engaged)

2 II. Background A Comprehensive Facility Review was performed for Buchanan Dam in 2007 by URS 1 and one of the recommendations was that the tainter gates at the dam be thoroughly evaluated for structural adequacy. All gates were noted as having a lightweight look and were notably different from typical tainter gates now commonly in use. HDR performed a gate evaluation study in The results indicated that there was minor overstressing in some of the gate members. The HDR evaluation recommended that strengthening of some gate members be performed on all gates. LCRA recognized that the required physical evaluations, which would include member replacements and trunnion pin removal, could only be performed if a dewatering system was installed. In 2008, LCRA contracted with Freese and Nichols, Inc. (FNI) to prepare plans for a dewatering system for one of the 25-feet high by 40-feet wide tainter gates. The work to be performed once the dewatering system was in place, would include member removal, physical testing, trunnion pin removal and inspection, gate member replacements, painting and seal replacement. Freese and Nichols enlisted Schnabel Engineering and HDR to provide subconsultant services for dewatering and gate design respectively. III. Project Description The three gated spillways are comprised of a seven gate spillway, a fourteen gate spillway and a sixteen gate spillway. The tainter gates in the seven gate spillway are each 25-feet high by 40-feet wide. Each gate in the other two spillways is 15-feet high by 33-feet wide. This presentation only applies to the seven gate spillway and no further reference hereafter will apply to the other two spillways. LCRA selected one particular gate, which had a damaged rub plate in the pier wall to be dewatered. This gate is referred to as Gate Number 2 and it is the second gate from the south end of the seven gate spillway. The spillway crest is at elevation 995 feet and normal water level is normally at elevation 1020 feet except during hurricane season when it is reduced to elevation 1018 feet. There is an operating bridge at elevation 1036 feet and a pier shelf extending upstream of the bridge at elevation 1023 feet. There is no horizontal sealing surface on the crest to rest/seal a dewatering system panel. The crest and pier relationship is shown in Figure 2. Gate 2 is the first gate to be refurbished under the present LCRA modernization program to upgrade all 37 gates. The program will include splashboards and new hoists along with the gate strengthening. The splashboards and hoists will improve LCRA s ability to respond quickly to severe local rainfall events and avoid any gate overtopping.

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4 IV. Dewatering System Design A. Background When LCRA approached the design team about developing a dewatering system for Gate 2, their primary question was What is the quickest dewatering system you can develop? The LCRA had a need to quickly access a typical gate to remove members for chemical and strength testing. This information was needed to provide design parameters and to keep the project moving on its prescribed schedule. The design team was very familiar with all of LCRA s other mobile dewatering systems since they had been involved in their design and deployment. These consisted of floating bulkheads for use at LCRA s Starcke Dam and at the Tom Miller Dam s small gates along with a reverse needle beam dewatering system for the large gates at Tom Miller Dam. The floating bulkheads could not be utilized at Buchanan because they were too long to fit between the piers and the undercut concrete crest at Buchanan did not afford a sealing surface for the bottom bulkhead at the pier face plane. The reverse needle beam system at Tom Miller Dam was designed for a water depth of 18 feet and could not accommodate the 25-foot depth at Buchanan. B. Design Concept The design team knew that to develop a dewatering system quickly that the following concepts were necessary: Utilize existing LCRA components wherever possible Self perform all possible fabrication and modifications with LCRA personnel Have LCRA craftsmen talk directly with engineer to resolve questions and issues Install the system with LCRA personnel supported by contract divers Although the needles for the Tom Miller system were too short for use at Buchanan, it was determined that the carrier beam was adaptable. The carrier beam was 62 feet long and spanned 59 feet at the Tom Miller piers. At Buchanan it would only need to span 49 feet and it could be supported on existing pier shelves rather than suspended from pier brackets as it had been at Tom Miller. Calculations indicated that the 30,000 pound twin HSS carrier beam could be turned on its side to handle the loads at Buchanan. By rotating the beam 90 degrees, new attachments for Buchanan needles could be added to the carrier beam without affecting any of the attachments or connections necessary for its continued use at Tom Miller Dam. The panels utilized in the Tom Miller Dam dewatering were actually panels for dewatering the head gates at LCRA s Wirtz Dam. Using the Wirtz panels at Buchanan could achieve an enormous time and cost savings. Further, the Wirtz panels were designed for much greater hydraulic head than the hydraulic head at Buchanan. Thus, the Buchanan system was designed around using the existing Wirtz panels. With the carrier beam and panel selections completed, the design team turned their attention to the needles. The team had the benefit of lessons learned from previous applications to make improvements on several details related to the needles. At Tom Miller, there were two primary leakage areas. The first area was where the needles rested on the continuous seat. It was very difficult to seal where the steel needles rested on the steel seat. The second leakage area was between the side seals and the pier surface. The seals kept getting pushed through gaps at the vertical pier face, particularly at the bottom corners. To overcome the needle seat issue, the design team eliminated the continuous seating shelf and

5 made the shelf part of each needle assembly. The needles were seal welded to the sealing shelf. Each needle assembly resembles an upside down T as shown in Figure 3.

6 By doing away with the continuous shelf, each needle assembly now mates to the adjoining member near mid panel width and can accommodate a seal gasket (Figure 4). To address the side seal push through, the end needle assemblies were fabricated with adjustable slide plates to minimize the gap between concrete and steel. The plates were cut every four feet to allow better adjustments in abrupt change areas. In addition, conveyor belting was utilized for side seals to afford both flexibility and strength and to easily mate to the pier sealing surface. Figure 5 shows details for a typical end needle beam assembly (ENBA). Figure 4. Needle Beam Fabrication and Matchup.

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8 V. Dewatering System Fabrication LCRA fabricated all components for the dewatering system at their Smithville Railcar Facility. LCRA is fortunate to have a very talented team of employees at this facility who not only maintain 2036 coal railcars at a rate of 100 per month, but were also able to build: ten 60 feet by 10 feet flood gates for Starcke Dam three 12 feet by 16 feet headgates for Buchanan Dam twenty five stop log panels for Wirtz, Starcke & Buchanan Dam a floating bulkhead system for Tom Miller Dam two sets of 50 feet stop logs for Wirtz Dam a suspension walk bridge for Starcke Dam They are equipped with all of the welders, track cutters, cranes and tools that a typical fabrication shop would have. In addition, they have some unique handling devices such as a device called a FlipRite (Figure 6) that can lift and rotate rail cars as well as all the parts listed above. LCRA, Freese & Nichols, Schnabel Engineering and the shop personnel have formed an efficient and innovative team that can handle most fabrication needs. Figure 6. FlipRite with Carrier Beam.

9 VI. Dewatering System Installation The installation of the dewatering system was, perhaps, the most difficult part of this project. The seven gate spillway structure on Buchanan Dam is approximately one mile from the staging area. Access is very limited due to the terrain below the spillway area. Extensive earthwork would have been required to allow truck access to the spillway apron. Once on the apron, however, the reach would still be too great for even a 400-ton mobile crane that was available. A different method was needed to transport and place the components. LCRA owned a small 30 feet by 30 feet barge made up of four Flexifloat H-50 units. Its floatation capacity was 42 tons which could handle the first piece to be installed, the 37,000 pound carrier beam, but the load would hang out 16 feet on either side. No additional barges were available at the time as they were all being used in the Gulf of Mexico for repair work after recent hurricanes. Barge shortages and the inability to use spuds in the deep water eliminated the possibility of using a floating crane. The project team thus had to devise a new approach to install the system without a large crane. Buoyancy became the primary solution to the project using the existing barge along with the use of floatation bags (ship salvage bags). The carrier beam would have to be floated directly into place and somehow left there. It had to be cribbed up nearly 6 feet above the barge deck (Figure 7) to reach the tops of the piers. It also had to be loaded very near the leading edge of the barge to reach its position on the pier tops before the barge would hit the flood gate. And to make matters worse, the lake level was dropping, sometimes ½ inch per day, making the calculations for this arrangement a daily activity. The engineers at Robishaw Engineering Inc, the manufacturer of the Flexifloat barges, were contacted for their opinion on the stability of the proposed arrangement. They were very helpful in performing stability calculations and making recommendations on the use of ballast. Figure 7. Carrier beam on Flexifloat barges.

10 The beam would be set on a stack of 14-inch by 14-inch oak cribbing that was bolted together. It was lashed to the deck with cables and come-a-longs. Ballast to level the barge was provided by a 2500 gallon tank, set on the opposite side from the beam (Figure 8). A series of hoses and valves, along with an onboard gasoline driven pump, allowed filling the tank before placing the beam on the cribbing. This would keep the barge within the recommended 2 degrees of trim. Additional tanks were located under the beam, to provide ballast to help sink the barge from under the beam after it was delivered to the pier tops. The main water tank was also drained, moved next to the beam and refilled, to provide additional weight to sink (lower) the barge. Figure 8. Ballast tank on barge. LCRA Hydro Operations agreed to halt water releases for two weeks to help stabilize the lake elevations while the various components were being delivered. As the time for the operation neared, water releases were necessary, causing the lake levels to drop more quickly than before. This made the placement of the beam even more questionable on the day we had chosen to perform the operation. The barge trip out to the spillway gate went very smoothly. Lake rangers patrolled to keep curious boaters away and the two tug boat captains did a great job maneuvering the barge from opposite sides where they were lashed. The boats maneuvered the barge in close and lines strung to the adjacent piers were used to pull the barge into the opening and to adjust the final right/left positioning. There was uncertainty as to whether we would make the pier elevation so we took along five 2000 pound recovery air bags, strung together with rope and air hoses, for additional buoyancy. It was quickly apparent that the carrier beam was sitting too low, so the air bags

11 were deployed under the leading edge of the barge. One side of the beam slid onto the pier (Figure 9) but the other side lacked a quarter of an inch. We didn t get our beam perfectly centered on the barge. One deck hand realized that we were the problem. We were all up front surveying our misfortune. As soon as we moved to the back of the barge, the end of the carrier beam slid into place. Figure 9. Carrier beam installation on pier The next step was to install the needle beams. These W36 beams are 39 feet long and weigh 7500 pounds. Once again, a crane would not be able to place these beams, partially due to reach but also because a walkway overhangs above where the beams are to be installed. The needles needed to be installed from under the carrier beam and fastened to the down stream side of the carrier beam. Once again, buoyancy came to our rescue. We decide to use a gaggle of air bags (Figure 10) to float the needles out to the floodgate. Two bags were attached to either end of the beam for transportation to the site. An additional group of four bags, that were capable of supporting the entire weight, were attached just slightly above the center of gravity of the beam. This allowed us to float it up against the spillway gate, deflate the bags at the foot end, and slowly rotate the beam to the vertical position downstream of the carrier beam. We used an old section of floodgate chain, wrapped around the beam and attached with bolts, to provide points of attachment for the air bags. This allowed us to place the lift forces directly along the centerline of the beam so it would float plumb. By controlling the amount of air in the bags, we could position the needle beam so that the bolts could easily be installed to the carrier beam. Divers then went down to drill holes for and install anchor bolts to the dam. We eventually changed out the four 2000 pound bags for two 4000 pound bags to reduce the tangle of rigging. This gave a much smoother rotation of

12 the needle beam. The lake level continued to drop, to the point that we could no longer float the needle up to its installed elevation. Chain falls were attached on either side of the needle to pull it up the rest of the way. These were difficult to control and a small gin pole was built to pull the needle beams into final position. The divers then completed installation by installing cross bracing, rubber belt seals and grouting areas where the concrete was too uneven. Figure 10. Floating needle beam into position The final task was to install the panels to complete the dewatering assembly. The heaviest panel weighed only 3500 pounds so we thought a small crane could be found that could be put on our barge that would have the reach and capacity needed. We found just such a crane at LCRA s Sim Gideon gas fired steam power plant. They use this crane on the turbine deck during overhauls. It had just enough reach and capacity. We still had the problem of the overhead walkway but the crane could boom out under the walkway if the rigging was short enough. Instead of using conventional rigging, a square tube strong back was designed that was bolted to the top of each panel. The barge could carry eight panels, stacked two high in the corners around the centrally located crane. As the panels came in several different widths and heights, they had to be loaded so they could be picked in a crisscross pattern in the right order needed so as not to unbalance the barge.

13 VII. Conclusion The entire dewatering system has worked very well (Figures 11 & 12). It has allowed LCRA to inspect the trunnions, replace various marginally sized steel members, and blast and paint the flood spillway gate. The dewatering system is now considered a semi-permanent part of the dam..that is..until the rains return and we can remove it in the manner it was installed. Figure 11. Upstream side of dewatered gate Figure 12. Upstream view of assembled needle beam cofferdam

14 References 1. HDR, Buchanan Dam Radial Gate Inspection and Analysis, Final Report June URS, Buchanan Dam Comprehensive Facility Review, Final Report June 2007.