Offshore Shoring
Standing outside Orlando’s Stanton Energy Center, one would never know that last October this was the site of one of the most complicated and unique excavation projects on record in Orange County, FL. It was here that Central Florida Underground (CFU), an Orlando-area excavation company employed by general contractors McCartney & Sons and The Saxon Group, was tasked with installing a concrete intake structure that would supply cooling water to the natural gas-fueled power plant. One catch: The water would be taken from a nearby lake, and the intake equipment was to be installed inside of it—underwater and 30 feet offshore.
The Stanton Energy Center is jointly owned and operated by Southern Power, a subsidiary of Atlanta-based energy giant Southern Co. and the Orlando Utilities Commission (OUC). Its 656-MW generation powers much of the Central Florida region, and this cooling station was the first step in allowing for any future plant expansion, so for Central Florida Underground and its partners this was an opportunity to demonstrate its capabilities in a big way. The project had a 12-week timeframe, in which the excavation was to be dug and shored, the equipment was to be installed, and the system to be activated. Failure to meet this deadline meant budget overages and possible delays of future plant growth plans.
Installing the intake system required a dry excavation measuring 51 feet wide, 33 feet long, and 21.5 feet deep, so CFU turned to the Orlando offices of Baltimore’s Mabey Bridge & Shore Inc. to provide the engineering, dewatering, and excavation shoring technologies needed to keep water out and allow the project to proceed. Owner Roy Ward worked with Mabey in-house engineers and product specialist Dave Howard on the design of a cofferdam system that would utilize Mabey’s 40-foot-long PZ40 steel interlocking sheets, two levels of Powerbrace framing systems, and corner braces.
 |
Photo: Powerbrace
Constructing a watertight barrier in 9 feet of water is no minor task. |
Once installed, the sheets would interlock, forming a watertight barrier. The Powerbrace frames consisted of hydraulic walers pinned at the corners to side rails of varying length. Once engaged, the walers expand and place outward pressure on the sheets. Corner braces would ensure that the pressure exerted on the excavation by the 9 feet of water would not collapse the excavation at its corners. “This was a unique project for Mabey,” explains Howard. “Here in Florida, we’re used to wet soils and high water tables, so the interlocking sheets really come in handy. But this was a lot of water.”
To install the equipment, Central Florida Underground used a crane and a conventional diesel hammer to drive a few “pilot” sheets into the ground at the site location. The hydraulic Powerbrace frames were then hung from the exterior of these sheets, allowing the primary sheeting to be brought in and installed outside of them. The sheets were driven through the water and mucky lakebed and, once stable, would form a sort of lake within a lake. After the Powerbrace’s hydraulics were engaged and corner bracing installed, over 35,000 cubic feet of water were pumped out of the excavation, utilizing a well-point system onshore. “The excavation was watertight initially, but the tremendous pressure from the water on the sheets created some problems,” says Howard.
These problems included small leaks that, while not an issue at first, would lead to standing water over days and weeks if left unattended. Again working with Mabey engineers and product specialists, project manager Brad Ward’s team installed plastic sheeting on the outside of the sheets to impede the flow of water. Additionally, a mixture of well-point sand and Oil Dry was applied to the outside edges of the sheets to plug any small leaks. Once placed along sheeting joints, water pressure forced the material through any tiny gaps that existed and closed them
for good.
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With a relatively dry excavation completed, Central Florida Underground was able to install the concrete intake structure and connections to the plant. Removal, however, was another obstacle to be tackled. “We couldn’t just pull the sheets out,” Howard says. “If we did, 9 feet of water would rush back in at once and that would be too much pressure at one time. It could have damaged the equipment.” The solution was to shut off the well-point pumps and allow water to slowly rise inside the excavation. Once the water inside was high enough, and the pressure inside and outside of the pit had equalized, the bracing and sheets could then be pulled out safely.
In all, the Stanton Energy Plant project took just over 11 weeks to complete. The intake structure began operation soon after and now supplies cooling water to the power plant. For Central Florida Underground and Mabey Bridge & Shore, the project remains an example of how quality products, ingenuity, and persistence can make an excavation possible in almost any environment.
Author's Bio: Greg Richards is a journalist based in Jacksonville, FL.
November-December 2009
Offshore Shoring
Photo: Powerbrace
Standing outside Orlando’s Stanton Energy Center, one would never know that last October this was the site of one of the most complicated and unique excavation projects on record in Orange County, FL. It was here that Central Florida Underground (CFU), an Orlando-area excavation company employed by general contractors McCartney & Sons and The Saxon Group, was tasked with installing a concrete intake structure that would supply cooling water to the natural gas-fueled power plant. One catch: The water would be taken from a nearby lake, and the intake equipment was to be installed inside of it—underwater and 30 feet offshore.
The Stanton Energy Center is jointly owned and operated by Southern Power, a subsidiary of Atlanta-based energy giant Southern Co. and the Orlando Utilities Commission (OUC). Its 656-MW generation powers much of the Central Florida region, and this cooling station was the first step in allowing for any future plant expansion, so for Central Florida Underground and its partners this was an opportunity to demonstrate its capabilities in a big way. The project had a 12-week timeframe, in which the excavation was to be dug and shored, the equipment was to be installed, and the system to be activated. Failure to meet this deadline meant budget overages and possible delays of future plant growth plans.
Installing the intake system required a dry excavation measuring 51 feet wide, 33 feet long, and 21.5 feet deep, so CFU turned to the Orlando offices of Baltimore’s Mabey Bridge & Shore Inc. to provide the engineering, dewatering, and excavation shoring technologies needed to keep water out and allow the project to proceed. Owner Roy Ward worked with Mabey in-house engineers and product specialist Dave Howard on the design of a cofferdam system that would utilize Mabey’s 40-foot-long PZ40 steel interlocking sheets, two levels of Powerbrace framing systems, and corner braces.
|
Photo: Powerbrace
Constructing a watertight barrier in 9 feet of water is no minor task. |
Once installed, the sheets would interlock, forming a watertight barrier. The Powerbrace frames consisted of hydraulic walers pinned at the corners to side rails of varying length. Once engaged, the walers expand and place outward pressure on the sheets. Corner braces would ensure that the pressure exerted on the excavation by the 9 feet of water would not collapse the excavation at its corners. “This was a unique project for Mabey,” explains Howard. “Here in Florida, we’re used to wet soils and high water tables, so the interlocking sheets really come in handy. But this was a lot of water.”
To install the equipment, Central Florida Underground used a crane and a conventional diesel hammer to drive a few “pilot” sheets into the ground at the site location. The hydraulic Powerbrace frames were then hung from the exterior of these sheets, allowing the primary sheeting to be brought in and installed outside of them. The sheets were driven through the water and mucky lakebed and, once stable, would form a sort of lake within a lake. After the Powerbrace’s hydraulics were engaged and corner bracing installed, over 35,000 cubic feet of water were pumped out of the excavation, utilizing a well-point system onshore. “The excavation was watertight initially, but the tremendous pressure from the water on the sheets created some problems,” says Howard.
These problems included small leaks that, while not an issue at first, would lead to standing water over days and weeks if left unattended. Again working with Mabey engineers and product specialists, project manager Brad Ward’s team installed plastic sheeting on the outside of the sheets to impede the flow of water. Additionally, a mixture of well-point sand and Oil Dry was applied to the outside edges of the sheets to plug any small leaks. Once placed along sheeting joints, water pressure forced the material through any tiny gaps that existed and closed them
for good.
With a relatively dry excavation completed, Central Florida Underground was able to install the concrete intake structure and connections to the plant. Removal, however, was another obstacle to be tackled. “We couldn’t just pull the sheets out,” Howard says. “If we did, 9 feet of water would rush back in at once and that would be too much pressure at one time. It could have damaged the equipment.” The solution was to shut off the well-point pumps and allow water to slowly rise inside the excavation. Once the water inside was high enough, and the pressure inside and outside of the pit had equalized, the bracing and sheets could then be pulled out safely.
In all, the Stanton Energy Plant project took just over 11 weeks to complete. The intake structure began operation soon after and now supplies cooling water to the power plant. For Central Florida Underground and Mabey Bridge & Shore, the project remains an example of how quality products, ingenuity, and persistence can make an excavation possible in almost any environment.