By Phillip Cutler, P.E. and Kayla Hanson, P.E.
Out of sight, out of mind.
It’s a popular expression used in situations where a troublesome issue or cause for concern can be ignored because it can’t be seen. But abiding by such logic in the construction industry is a dangerous game.
When a structure is buried underground, adverse conditions can create difficulties for designers, contractors and precast manufacturers. For example, the buoyancy or uplift force on any structure boils down to the weight of fluid displaced by the structure. Many below-grade precast concrete structures installed near a high water table face the hidden challenges of buoyancy. If these challenges are not taken into account, flotation can occur, meaning bad news for the project.
Thankfully, these potential issues can be avoided with a little planning and the help of the National Precast Concrete Association’s buoyancy calculator.
Big tank, bigger ballast
A building expansion project for the Upham Woods Outdoor Learning Center in Wisconsin Dells, Wis., called for a large aerobic treatment system to handle the property’s wastewater. Project plans specified a multi-tank precast concrete solution. Wieser Concrete Products of Portage, Wis., secured the work.
“The project presented many challenges, including the high seasonal groundwater in some of the tank locations,” said Vice President Mark Wieser. “One of the most severe locations involved a tank that was 20 feet long, 12 feet wide and just over 9 feet deep. We had only 2 feet of soil cover on the top of the tank and the seasonal distance to groundwater was only 1 foot.”
When facing difficult site challenges due to high seasonal water tables, designers commonly consider buoyancy or flotation calculations as part of the project details. These calculations are not difficult to make as long as the structure and site parameters are known.
“Flotation calculations for the tank at Upham Woods required more than 52,000 pounds be added to the tank,” Wieser said. “The challenge with adding this much weight is where to put it and how to attach it to the tank.”
If a high water table exists, flotation can occur when the tank in question is pumped. To prevent this, ballast can be added to the product. Typically, the amount is equal to or slightly greater than the weight required to equal the buoyancy force when the structure is empty, plus a marginal factor of safety. Although the factor of safety varies slightly, a good general rule of thumb is 1.1 times the calculated uplift force. For severe or continuous cases, a factor as high as 1.25 may be considered.
There are as many ways to add ballast to a structure as there are design possibilities for that structure. Sometimes weight is added by attaching a concrete anchor system with cabling. In most cases, this method requires additional site excavation. Another approach is to design the structure with additional depth and fill the void with concrete to achieve the desired weight. Adding ballast this way increases impact to the site by requiring a much deeper excavation and would only be considered if minimal weight is needed.
A more common solution is to add an extension at the base slab, much like a footer. Doing so takes advantage of the additional concrete that forms the extension as well as the weight of the saturated soil bearing directly above it. Wieser Concrete Products adopted this method for their Upham Woods project.
“The outside of our tank had exterior ribs on it, so we placed 13 cubic yards of concrete along the sides, on top of the ribs,” Wieser said. “This created a shoulder all the way around the tank, providing additional protection against flotation.”
Tools and resources
Scouring the Internet for reference material about buoyancy can be frustrating. NPCA has developed a series of resources to help simplify the process.
- The Buoyancy White Paper provides you with an all-inclusive, step-by-step resource on buoyancy as it relates to precast concrete structures. The White Paper includes multiple examples detailing calculations and solution methods. Download the Buoyancy White Paper at
- NPCA’s newly revised Buoyancy Calculator offers designers, engineers and precast manufacturers a tool to determine a number of parameters related to buoyancy and flotation. The calculator allows input of structure parameters and site conditions for both round and rectangular structures. These values are used to calculate uplift forces and the amount of ballast required to counteract buoyancy issues. Download the Buoyancy Calculator and the Buoyancy Soil Values References Sheet at precast.org/buoyancy.
If you have any questions, please contact Phillip Cutler, P.E., at [email protected] or (800) 366-7731.
Phillip Cutler, P.E., is NPCA’s director of quality assurance programs.
Kayla Hanson, P.E. is a technical services engineer with NPCA.