Preventing Tank Flotation
Posted / Last update: 01-09-1997
Other Major Considerations
Fire codes require that horizontal USTs and ASTs be anchored if more than 70 percent of the tank’s storage capacity would be submerged at the established flood stage. Tanks may be anchored to a foundation of concrete or steel and concrete of sufficient weight. This will prevent movement when the tanks are filled with flammable or combustible liquid and submerged by flood waters or when the tanks are adequately secured from floating by other means. Absent any other requirements, local authorities define the established flood stage.
Bathtub effect is not uncommon in areas with impervious soils such as clay. In such a situation, infiltration of surface water into the tank excavation may cause the UST to float. That is, the tank excavation holds water the way a bathtub holds water. This is called the bathtub effect. Localized flooding can occur while the surround-ing area remains unflooded.
Larger diameter tanks are more susceptible to flotation. Some manufacturers of 12 foot diameter tanks have, in the past, required that they be anchored. Most manufacturers continue to recommend it.
Design plans for the UST installation should include deepening and enlarging the excavation to permit proper installation of restraining devices. Deadmen, for instance, must be located outside of the profile of the tanks to achieve maximum benefit. A wider excavation may be required. Similarly, if a full bottom tank holddown pad is required, the depth of the excavation will need to be increased.
The openings of tank vents and other non-liquid-tight tank fittings should terminate and be made liquid-tight to above the maximum flood stage water level.
When additional anchorage is deemed necessary, an approved method must be selected. The most common alternatives are:
• Increasing burial depth
• Providing deadmen anchors
• Incorporating a thicker paving slab
• Using a full bottom holddown pad
Increased burial depth and a thicker paving provide protec-tion only after the installation is completed. While construction is underway, tanks must be secured against movement without the benefit of the weight of backfill and paving. The best way to do this is to keep the water level in the excavation below the bottom of the tanks.
In severe conditions, this might require continuous pumping or well-pointing. Well-pointing involves dewatering the tank area through the use of large pumps connected to a series of perimeter well-points. Disposal of pumped water will take some advance planning and, if the water is contaminated, disposal permits.
Tank Burial Depth
Increasing tank burial depth increases the weight of the backfill overburden. This method is less costly than the other methods, and safer because workers might not be required to enter the excavation.
In general, normal backfill and paving over the tank will provide adequate restraint if the burial depth is at least 60 percent of the tank diameter.
For example, 60 percent of an 8-foot diameter tank is 4 feet by 9 inches. Therefore, the top of the tank should be at least 4 feet 9 inches below finished grade level.
Adding one foot of additional burial depth to a 10,000 gallon 8-foot diameter tank provides an additional 13,392 pounds to counter flotation.
A concrete paving slab provides the tank with mechanical protection by spreading any live load from vehicular traffic and impeding the flow of water into the excavation. The slab also adds to the weight countering flotation; concrete is 46 percent heavier than the backfill it replaces (87.6 pft3 versus 60 pft3). Each inch of reinforced concrete above the tank can be con-sidered equal to 2 inches of compacted backfill.
As a general rule, design the paving slab to be as wide as the diameter of the tank, plus 4 feet, and as long as the tank, plus 4 feet. The slab should extend 2 feet beyond the tank in each direction (see Figure 2). It is recommended that the corners of pads be mitered at 45 degree angles to prevent cracking and breaking off during use.
When circumstances are not conducive to increasing burial depth, and if supplemental restraint is needed while construction is in progress, deadmen anchors are preferred (see Figure 3). They are made of reinforced concrete (generally between 8 and 12 inches thick and 18 to 36 inches wide), either precast on-site or furnished by the tank manufacturer and delivered to the jobsite with the tank.
Deadmen provide protection during and after construction. This is a popular method because deadmen are effective and reduce the amount of work required in the excavation. The combination of deadmen anchors and the temporary lowering of the water level in the excavation by pumping and deadmen provide sufficient protection, even in severe cases.
Bottom holddown pad
A bottom holddown pad is usually 8 inches of reinforced concrete extending at least 18 inches beyond the tank sides and 1 foot beyond each end. The pad provides a firm foundation for the tank and helps to offset buoyancy as well. (See Figure 4) Calculate the thickness of the pad, as well as the number and size of anchors and reinforcement for each installation to determine the offsetting value.
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