Innovation in Global Power - Parsons Brinckerhoff

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Hydropower – New Technologies, New Considerations http://www.pbworld.com/news_events/publications/network/ the slurry coming directly out of the slot cut. In addition, the contractor installed troughs on both sides of the piers under the deck to collect water that seeped through joints to the underside of the deck. Figure 2: Cross Section View of Pier Slot Cuts. non-overflow section to relieve as much of the built-up stresses as possible, then six sets of two slots were cut in piers along the main spillway. Joints were spaced along the spillway deck about four bays apart or at approximately 120-foot (37-m) intervals. The slot cutting activities occurred during the summer months, so a thermal increase of about 50°F (10°C) was used to evaluate the required width of the slot. This thermal change required a minimum slot width of about 0.4 inch (1.0 cm). The wire slots in the piers on the main spillway extend through existing construction joints in the concrete deck to a minimum depth of approximately 0.51 m (20 inches) across the width of the deck (Figure 2). The concrete deck is reinforced, however, the reinforcement does not extended into the unreinforced pier through the construction joints. In addition, the steel beams also supporting the concrete deck did not extend through the existing construction joints. Therefore, no steel beams or reinforcement were cut. The primary objective of the slots was to provide open joints to allow for thermal expansion of the concrete deck. Re-cutting was required at only three slots in two different piers due to closure of initial cuts. The exposed slot cut areas (top and upstream/downstream faces) were sealed with a foam backer rod and sealant. Initial closure after all slots were cut was approximately 3.0 cm (1.2 inches), and the total accumulated available slot closure was 200 cm (8 inches). The final slot gap widths ranged between 1.5 cm (0.59 inch) and 2.0 cm (0.78 inch). Protecting the Water A unique challenge we faced during the fix was in meeting the client’s request that we prevent the cuttings and the resulting cutting slurry from entering the project waters. We required that a slurry catch basin (collection bin) be installed on the upstream and downstream sides of the piers to collect Water was pumped from the upstream dam reservoir into the collection bin until it was nearly full. This water was then pumped and used as lubrication for the wire for the remainder of cutting activities at that pier, being collected and re-circulated until the slot cutting was completed. After that, the re-circulating water was pumped to a settling tank where solids were allowed to settle out. The effluent from the settling tank, which was clean water, was discharged to the reservoir. This method for collecting and processing the slurry and fluids from the cutting activities was reviewed and approved by the State of North Carolina Department of Water Quality. No unanticipated environmentally related incidences occurred during the slot cutting activities. Instrumentation Narrows Dam has an instrumentation program that includes manual and automated instruments to monitor the overall safety aspects of the site. Prior to the deck remediation program, instrumentation included high order survey pins, piezometers and seepage measurements. We recommended the addition of inclinometers, tiltmeter plates and deck slot monitoring pins prior to the start of slot cutting activities because it was critical that the dam’s behavior be monitored while work was in progress, and equally important that the long-term effects of the new expansion slots be monitored. Tainter Gate Remediation The Tainter gate remediation included: • Removing the existing side rollers, which prevented the gate skin plate from cutting into and binding on the concrete piers • Trimming the edges of the Tainter gate steel • Installing grooves in the concrete piers • Replacing the side seals. Tainter gates are illustrated in Figures 3. The side rollers had been added to the ends of the top and bottom horizontal girders on each side of the Tainter gates in the early 1990s to PB Network #68 / August 2008 42
Hydropower – New Technologies, New Considerations protect the edges of the gates and to facilitate smooth operation during opening. The continued tilting of the piers during the 1990s led to excessive force buildup, however, and local abrasion (due to excessive frictional forces) of the concrete at the upper right side roller. The visual scarring of the concrete was evidence of the local concrete abrasion (Figure 4). Although the side rollers did provide protection for the edges of the gate, they did not stop the gates from binding. Figure 3: Closed Tainter gate viewed looking downstream (top) and upstream (bottom). Figure 4: Image of local concrete abrasion due to excessive frictional forces from gate binding. The gate clearances and the extent of pier tilting were not the same in each spillway bay. To ensure that appropriate gate trimming and concrete grooving were performed on each pier, we determined for the construction specification the tolerances and criteria for determining which Figure 5: Image of gate repaired with groove. http://www.pbworld.com/news_events/publications/network/ gates needed to be trimmed and which piers needed to be grooved. Twenty gates required trimming on at least one side, and twelve required trimming on both sides. Twelve gates required that the concrete pier be grooved (Figure 5) on the right side, and one required pier grooves on both sides. Summary At the time of writing, the slot cutting had been completed for nearly six years and the Tainter gate remediation for more than four years. The deck monitoring pins have shown that the concrete decks between the expansion slots expand and contract seasonally with an amplitude of about 0.65 cm (0.25 inch). All gates continue to operate successfully and open fully. We expect that this work will maintain reliable hydropower generation for the Yadkin Project long into the future by maintaining the associated dam in a safe condition. In conclusion, the installation of spillway deck slots, trimming of the gate skin plates, and the grooving of the concrete piers have significantly extended the useful life of the 86-year old Tainter gates at Narrows Dam. Related Web Sites: • http://www.alcoa.com/yadkin/en/info_page/relicensing_overview.asp Marc Buratto has nearly three decades of experience in the engineering and design of power stations and other types of facilities. Marc’s expertise includes finite element stress analysis and stability analysis of existing structures, preparation of FERC safety inspection reports and Potential Failure Mode Analysis (PFMA) Reports as part of the new Dam Safety Performance Monitoring Program (DSPMP), concrete and steel design and analysis, and Tainter gate assessments and modifications. Tony Plizga has 36 years’ civil-structural engineering and design experience. Since joining PB in May 2000, he has worked on various hydroelectric projects within the Boston Office Hydro Group. Major tasks include site inspections and reports, gate upgrades, instrumentation data evaluation, stability analyses, rock anchors, GIS oversight and FERC relicensing support. Paul Shiers is a PB vice president in hydroelectric power and water resource engineering. Having 38 years of engineering experience, Paul is qualified as an Independent Consultant and PFMA facilitator for FERC Part 12 safety inspections under the new FERC DSPMP requirements. 43 PB Network #68 / August 2008
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