Keep Tanks Underground Where They Belong

Important measures you take as a system installer or maintainer can prevent the disaster caused by a buoyant septic tank.
Keep Tanks Underground Where They Belong
When these gas station fuel tanks were emptied, buoyant forces pushed the tanks through soil and asphalt to bring them to the surface. The same situation can occur when a septic tank is not properly installed.

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Three years ago, during an installer class conducted in cooperation with the California Onsite Wastewater Association, we discussed installation procedures for sewage tanks. We highlighted the importance of avoiding or recognizing when a tank may float to the surface. The class broke for lunch and when we reconvened, a participant asked if we had seen the tanks at the gas station on the corner that had come up through the asphalt. We had not  – so we were soon taking a short class field trip. The photo accompanying this story shows what we found.

Gasoline in the tanks had been drained so pumps could be replaced during a remodeling project. The evening before, several inches of rain fell, the water ran into the area excavated for the tanks. Since the tanks were empty and not tied down, they floated to the surface. This can also happen with septic tanks if installers and pumpers don’t use caution during system construction or maintenance later on.

MIND THE EXCAVATION

All tanks have the potential of being floated out of the ground due to forces acting on the tank in saturated soil. At the gas station, the tank hole was excavated into relatively solid or dense soil and then backfilled with a less dense material that will allow water to collect in the excavation. This is a reason to keep the excavation as small as possible; be careful to backfill in layers and to compact the fill material when installing the tank.

Even though the soil around the excavation is not saturated, the volume that was excavated can become saturated. If the upward force (buoyant force) is equal to the weight of the water displaced by the tank and the weight of the tank and soil cover do not exceed the buoyant force, the tank will float. This is what happened in this case. Where the soils around the excavation are seasonally saturated, this risk becomes even greater.

So the obvious question the onsite professional should be asking is: Do I need to calculate the buoyancy potential for tanks I install or service? The answer is no; but you should recognize when you are installing or servicing tanks in conditions or with products where buoyancy is a potential problem. The most obvious condition is the one mentioned above; that is, in areas of high water tables or seasonally saturated soil conditions.

This is where being able to use soil color to help identify sites with seasonal or perched saturated zones becomes helpful. Any soil condition that provides a limiting layer beneath the tank, reducing movement of water, can result in the condition at the service station. This could be dense soil layers or bedrock or other limiting layers.

CHECK WITH MANUFACTURERS

The Consortium of Institutes for Decentralized Wastewater Treatment (CIDWT) Installation of Wastewater Treatment Systems manual is a good reference for calculating tank buoyancy potential. An installer should check with the tank manufacturer and state and local regulations to determine what practices should be employed for the specific products.

Service providers pumping tanks in areas with high water tables, limiting conditions or times of heavy precipitation should recognize it is probably not advisable to pump tanks dry and leave them. In fact, most manufacturers require tanks be refilled to some level based on elevation of the water. This creates an interesting situation if you are servicing tanks to relieve pressure on the system during wet periods. The service provider needs to balance the potential for larger problems versus the customer’s desire to get rid of the water.

We reviewed our state rules regarding tank installation to see what they say about buoyancy. Here is the requirement from 7080.2000 Part I: “Sewage tanks placed below the level of the periodically saturated soil must employ a method to protect against flotation under periodic saturated soil conditions when the tank is empty.” This is a performance-based code item, placing the burden on the designer/installer to determine if there is a potential problem and a method used to solve it. This means consulting with the product manufacturer and following their specifications and requirements is extremely important.

During installation, there are several ways to provide protection against a floating tank. The first is the soil backfill itself. Backfill over the top of the tank provides a force to counter buoyant forces. If the tank is installed under a driveway, the additional weight of the concrete or asphalt would provide additional counter force. We do not want to see tanks covered with concrete or asphalt, but there are times when this happens. Two important points to note: With shallower tank installations, the amount of backfill over the top is usually about a foot, which is usually not adequate by itself to provide protection. And if the tank is too deep, the soil may exceed its structural capacity to withstand that depth of burial. This information is product-specific.

SECURE THE TANK

Two other primary methods may provide this protection. They are the use of concrete deadmen or screw anchors along with the appropriate strapping material to tie down the tank. Most manufacturers will have specific requirements for the straps and the dimensions of the concrete deadmen and the number of straps required, as well as their placement on the product.

Often concrete parking curbs or traffic barriers act as the deadmen. They can be poured in place and the installer can make them out of half pipes filled with concrete. There are also specific requirements for the screw anchors and how they are to be inserted into the soil. These requirements must be followed exactly to maintain any product warranty. If there is a deviation from the requirements, it should be made only under a specification by a professional engineer, and have a conversation with the engineer about how the change was determined.

The final backfill is also extremely important to provide proper sidewall support for the tank. It is important the backfill be installed in lifts and properly compacted, then completed by mounding the backfill over the top of the tank.

Hopefully, this discussion helps you avoid what can be an embarrassing and expensive situation when a tank floats to the surface. Most homeowners never want to see their tank that up-close and personal.



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