May 2015
ACP
DIXIE
CONTRACTOR
Amec Foster Wheeler and Hayward Baker ACP 1028 Shelby Street Indianapolis, Indiana 46203 ELECTRONIC SERVICE REQUESTED
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Collaborate on Successful Lake Manatee Dam Rehabilitation
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Amec Foster Wheeler and Hayw and Add Years to the Lake Mana Debra Wood
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hen erosion issues appeared in Florida’s Lake Manatee Dam, some of the best minds in dam construction and engineering put their heads together and came up a sophisticated solution that would save the dam and the area’s water supply, and protect downstream homes and businesses. “The dam was in danger of a breach in high-release conditions, because of seepage that developed through the years and erosion patterns around the structures of the spillway,” says Mark Simpson, Manatee County Water Division Manager. “In a breach situation, there would be potential loss of downstream lives and property, and loss of the water supply.” Engineers from Amec Foster Wheeler in Tampa, Florida, found evidence of internal erosion and piping during a supplemental annual inspection of the 50-year-old dam facility, which provides potable water for 350,000 people. The county, south of Tampa, deemed the dam in need of emergency repairs and put together a team that could design and complete the repair. “We managed to re-establish the seepage barrier all the way across,” Simpson says.
“We designed the project, came under contract and did the work within 10 months.”
The Problem The existing zoned-earth dam contained sand shell surrounding a 10-foot to 20-foot wide clay core that was designed to extend down to an underlying natural clay layer beneath the soil. Together, the clay core and natural-clay layer were designed as a seepage barrier to limit the amount of water seeping through the embankment and foundation. Soils in the earthen embankment shell, outside of the clay core, provide stability for the dam. The Florida Department of Environmental Protection has classified it as a high-hazard dam, due to the significant risk of property loss and loss of life if a breach occurred. “We were doing due diligence, found it and reacted,” Simpson said. “We fixed it, and our customers continue with their lives, unaffected by it at all.” The engineers, in 2014, noted the internal erosion problem, which likely occurred around and under the clay core. What began as slow seepage carrying away fine soil particles, with time grew as resistance
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to the flow decreased. This greater flow could then carry larger particles, meaning more soil was leaving the dam as seepage rates increased. “There was evidence of water seeping underneath the clay core,” Simpson says. “We knew we had to get someone in as soon as possible.”
Collaboration is Key The County Board of County Commissioners gave approval for an emergency repair. The engineers recommended installing a barrier seepage cutoff wall and bringing on Hayward Baker, due to its full range of capabilities, says Glen Andersen, Project
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yward Baker Improve Safety anatee Dam
“It was a very high-energy situation and fulfilling the way the teams came together to resolve it.” Jim Hussin, a Hayward Baker Director in the Tampa office Manager for Amec Foster Wheeler. “We worked with the county and Hayward Baker to arrive at the final solution,” Andersen explained. “We were able to prevent the dam from failing.” Andersen describes the collaboration as similar to a design-build project, even though the parties held traditional contracts. Everyone met for early morning and late night meetings to brainstorm ideas. “It was a very high-energy situation and fulfilling the way the teams came together to resolve it,” says Jim Hussin, a Hayward Baker Director in the Tampa office. The experts came up with an idea to form
a 105-foot deep seepage cutoff wall along the length of the dam, just to the outside of the existing clay core. The new wall extends 35-feet to 45-feet deeper than the former clay core, so it reached a uniform clay stratum. “The professional teamwork by all members enabled the successful completion of this challenging dam remediation program,” Hussin says. The $17 million Phase 1 repair was significantly less expensive than a new dam would have cost. Amec Foster Wheeler is currently designing repairs to the remaining portion of the dam, but they are not
considered life threatening. Bids for those repairs should be released later this year.
Construction Progress Hayward Baker built a 3,000-foot long seepage cutoff wall, with a permeability of 1x10-6 centimeters per second, through the middle of the existing earthen dam, without draining the lake. Crews used a jet-grouting system at the spillway and a Trench Remixing Deep (TRD) method through the clay core of the earthen dam, 1,000 feet south and 2,000 feet north of the spillway. “Every little bit of each step was making the dam stronger,” Simpson said. The jet grouting injected grout at high velocities into the existing soils. The energy erodes the soil and blends it with the grout slurry, leaving behind soilcrete, a soil and concrete mix. “The jet grout monitor was advanced to the maximum treatment depth, at which time high velocity grout jets were initiated from ports in the side of the monitor,” Hussin reports. “The grout slurry injected from the jets eroded and mixed with the soil as the drill stem and jet-grout monitor were rotated and raised.”
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Project Partners • Owner: Manatee County Water Division, Florida • Consulting Engineer: Amec Foster Wheeler, Tampa, Florida • Contractor: Hayward Baker, Tampa, Florida
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The jet-grouted portion of the wall generally consisted of 4-foot-diameter jet-gout columns constructed on a 3-foot center-to-center spacing. Near the north and south edge of the stilling basin slab, the approach wall foundations restricted the drilling locations so 8-foot-diameter columns were constructed on approximately 6.5-foot centers at these locations, Hussin adds. “The jet grouting technique allows construction of the 4-foot and 8-foot diameter columns through small diameter holes cored through the existing concrete spillway, minimizing damage to the spillway,” Hussin explains. Engineers vertically cored completed columns to assess for column integrity and monitored seepage with multiple nests of piezometers. Where it found evidence of
continued seepage in a couple of locations though the jet-grout wall, Hayward Baker installed additional columns to complete the barrier. The TRD machine cut in a straight line and created mixed-in-place walls that restrict groundwater flow, using a specialized vertical cutter post mounted on a base crawler machine, according to Hussin. “The vertical cutter post, resembling a large chain saw, is inserted vertically in segments by the crawler machine until the design depth of the wall is reached,” Hussin explains. The crawler machine then advances along the wall alignment while the cutter post cuts and mixes the in-situ soil with cementbased binder slurry injected from ports on the post. The vertical mixing action blends the entire soil profile eliminating any stratification and creates a soilcrete wall with a high degree of homogeneity and low permeability, Hussin continues. Hayward Baker configured the TRD equipment to construct a 22-inch-wide wall. In addition, to help assure that no grout slurry entered the reservoir, Hayward Baker excavated a 3-foot-deep trench along the alignment ahead of the TRD rig to contain the excess soilcrete produced during wall construction. A backhoe continuously bailed the trench, placing the excess material in dump trucks for trans-
port to an on-site storage area. The excess material was later used as fill to widen selected sections of the dam. “The TRD method is a cost effective way to construct a deep, safe cutoff wall,” Hussin says. “Since the cutter post stays in the ground as it is mixing, and it leaves a thick mixture of soil and cement behind it, it minimized the risk of stability issues which is very important in an operating dam with stability concerns.” On the north side, Hayward Baker found an unmapped sheet pile wall. Amec Foster Wheeler redesigned a 7-foot offset for the TRD to avoid hitting the approximately 170-foot long wall. “[The project] was a lot of work, and the result means this wall gives another 50 years of life to the dam, maybe more,” Simpson said. “We ended up with a better dam than was originally built.”
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