By Claude Goguen, P.E., LEED AP
Ever heard of a precast concrete canoe?
You probably won’t find one at your local sporting goods store, and it’s certainly not what Lewis and Clark used on their expedition in 1804. However, reinforced concrete boats were built just a mere 44 years later by Joseph Louis Lambot in Maraval, France, in 1848. Concrete was also used to build barges during World War II to replace scarce steel supplies. And now, each year, the American Society of Civil Engineers holds a competition where teams of university students from around North America build and race their concrete canoes.
Although precast concrete never became the material of choice for boat builders, the fact remains that concrete floats due to the scientific concept known as buoyancy.
Buoyancy is defined as the tendency of a fluid to exert a supporting upward force on a body placed in the fluid. The fluid can be a liquid, as in the case of a boat floating on a lake, or it can be a gas, as in a helium-filled balloon floating in the atmosphere. An elementary application of buoyancy happens whenever you try to push an empty bowl downward into a sink full of water. When applying a downward force to the bowl from your hand, the bowl will stay suspended in place. But as soon as you remove your hand from the bowl, it will float to the surface. The buoyant force on the object determines whether or not the object will sink or float.
Using those same principles, a concrete structure will float if there is not enough downward force to counter the buoyant upward force. An underground structure will eventually lift up out of the ground, causing many problems. And we’re not just talking about small 2×2 hand holes – large vaults can be affected as well if conditions are right.
So how do we determine whether an underground concrete structure will resist buoyant forces? Using basic principles, a concrete structure will not float if the sum of the vertical downward forces is greater than the vertical upward force. When applying this principle to a structure below grade, if a buoyant force is greater than the mass of the structure and the combined mass of soil surcharges and objects contained within the structure, the structure will float. This is why buoyancy is an important factor in the design of an underground structure.
Determining water table levels
Typically, contractors who need precast structures will present precasters with details on what they need and give design requirements and information on the underground conditions.
Not always, however, do they inform precasters about every detail, especially job site conditions and problems in the construction area. Site and subsurface conditions are vital pieces of information needed for the design calculations to optimize the performance of the structure in the installed condition and to prevent flotation.
So how does the design engineer determine when there could be a potential problem with the job site conditions and with flotation? First, the design engineer should review and investigate the plans, specifications and soil reports to gain more insight about the project and the underground conditions. One of the first factors that must be determined when analyzing an area in which the concrete structure will be placed below grade is the water table, or groundwater level. Obtaining this information will help the designers identify sites where flotation may or may not be a factor in the design. If no soil reports or previous water table data are available for fluctuations (seasonal and regional), most engineers will design the structure on the conservative side. This will ensure that the structure will be able to withstand seasonal and regional fluctuations.
Computing downward (gravity) forces
After the water table level has been identified, the design engineer needs to look at computing all the downward forces that will be acting on the structure. All vertical downward forces are caused by gravitational effects, which need to be calculated in the design of an underground structure. At times, these structures will contain fluid, as with septic tanks or grease interceptors. Some will contain equipment, as with utility vaults. The weight of the fluids or equipment in the tank should not be included in the downward force calculation, as the tank may be pumped empty or the equipment removed for maintenance. The following should be considered in calculating the downward force:
- Weight of all walls and slabs
- Weight of soil on slabs
- Weight of soil on shelf or shelves
- Weight of inverts inside structure
- Friction of soil to soil
- Additional concrete added inside structure
- Weight of reinforcing steel
Not all underground structures are the same, and therefore some of the above-listed vertical downward forces may not be included in the summation of total vertical downward force.
Computing upward buoyant force
As stated in Archimedes’ Principle, an object is buoyed up by a force equal to the weight of the fluid displaced. For example, if you had a 6-ft-diameter manhole with 7-in. walls, it was 8 ft tall from the bottom of the base to the top of the lid, and the entire structure was designed to be below the water table, you would calculate the total displaced volume of that manhole. You would come up with a volume of around 323 cu ft. Water weighs around 62.4 lbs/cu ft, so the total weight displaced would be a little over 20,000 lbs. That would be your upward buoyant force. If you’re using a concrete that weights 145 lbs/cu ft, that manhole would weight around 19,500 lbs. That means the buoyant force upward is greater than the weight of the manhole, and you’re relying on soil friction to keep the structure in place. In this case, the designer may consider using additional measures.
Several methods can be used to overcome a buoyancy problem:
- Base extension. Use the additional weight of soil by adding shelves to counteract buoyancy.
- Antiflotation slab. Anchor the structure to a large concrete mass (shelf) poured on site or use precast concrete manufactured off site.
- Increase member thickness. Increase the concrete mass (m).
- Lower structure elevation and fill with additional concrete. Set the precast structure deeper than required for its functional purposes. This will add additional soil weight on top of the structure to oppose buoyant forces. Also, with the structure being deeper in the soil, some contractors opt to pour additional concrete into the base of the installed precast concrete structure.
It is a fairly simple concept: Downward gravitational forces need to exceed upward buoyant forces. In the case of the students in the concrete canoes, they rely on the opposite to stay afloat and bring home the trophy.
NPCA developed a Buoyancy Calculator to help determine if the structure is in danger of floating just by entering dimensions and field conditions. It can be found at precast.org/bcalc. For much more detail, consult the Buoyancy White Paper at precast.org/bpaper.
Claude Goguen, P.E., LEED AP, is NPCA’s director of Technical Services and Sustainability.
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