Yes it's true, if an aerobic sewer system is not installed properly then it is very possible it will “pop” up out of the ground. Most laugh at the concept of a concrete tank that weighs over 7 tons popping out of the ground, but what it does happen and it is a very difficult problem to fix.
I work for a family owned company that my father started 17 years ago. During this time, we have installed and operated on thousands of sewers. We take pride in our work and do the best job, which is why during march of 2001 we were shocked when an owner of one of our recently installed sewers called us with a surprising bit of information. His sewer had literally floated, or “popped” out of the ground.
In order to describe the situation I must first explain how and why it happens. Any and every object that is sealed water tight is subject to a process called “hydraulics”, defined as the applied science dealing with the mechanical properties of liquids. If that object has greater fluid pressure around or underneath it then it will be pushed to the point of little resistance. Another term for this phenomenon is “buoyancy”. It is easier to understand if you think of an ocean bound ship or a submarine. Even though the ship may weigh thousands of tons, it still floats on the water.
In this particular case the sewer we had installed was made of fiberglass, a much lighter material than concrete. Even though it is lighter we still made sure to fill it with water as we back-filled dirt around the tank. The idea was that the water inside would hold the tank down until the fill dirt could settle; where we went wrong was the back-fill dirt. The dirt we used was what we had dug out of the ground, a thick, clotty, red clay. When the clay was distributed around the tank it did not set into a packed fill. Furthermore, the next few days were typical of March in East Texas- it rained nonstop. Enough rain water had managed to penetrate and saturate the soil that it saw into the hole around the tank. Because the fill dirt was not packed enough, the water soon spread around the tank.
Seepage of water around the tank after a rain was something we were used to. In most circumstances it is a good thing, it packs the dirt even more around the tank causing the ground to settle; depending on the soil, slightly or a lot. Afterwards we would fill in the low spots and be done. Yet so much water managed to get around this tank that the pump chamber (the area with the least amount of water) was pushed out of the ground in what can best be described as a submarine surfacing from under the sea.
The next few days were at best difficult. The tank was pumped out and removed, a new tank was installed with special flaps on both ends that worked as anchors, and better dirt was brought in to be filled around the new tank to keep any problems with seepage from coming again. Needless to say, our company gained much experience from this problem. Now if the on site dirt does not meet our standards, then topsoil or sand is brought in as fill. Also, all tanks are completely filled with water during the install to make sure that any sudden rain showers pose no threat. To this date we have had trouble with our systems floating out of the ground.
If you are a homeowner planning to install an aerobic system, make sure you have a water source available to fill the tank quickly. If you are on a water well that might not be able to handle the load, contact your local fire department or a contractor that has a water truck to bring in enough water. Also, if you are planning to have another in ground system that does not hold water such as a storm shelter installed, check to make sure it is anchored properly.