Concrete septic tanks can and will float out of the ground under certain conditions. Here is a photo to act as proof.
This is a picture of a septic tank that was buried in the ground and had some soil with foliage growing over it. The ground had become saturated and the tank had pumped down so far that it actually became buoyant. It then slowly rose out of the ground like a zombie!
An example of the math behind it:
This is a rough calculation of the buoyancy of a concrete 500 gallon (inside volume) septic tank which is roughly 5 feet in diameter and 5 feet tall with 3″ thick walls.
The volume that the tank displaces is = π * radius * radius * height = 3.1416 * 2.5* 2.5 * 5 = 98.175 cubic feet
The amount in gallons of water that is displaced is = 7.48 g/cu.ft * 98.175 cu.ft. = 734 gallons.
A gallon of water weighs 8.34 lbs.
The weight of the water that was displaced is 8.34 lb/g * 734g =6120 lbs.
The weight of water displaced is roughly 6120 pounds but the rough weight of the empty concrete tank is only about 5000 pounds! This means that there needs to be around 1250 pounds of weight on it or in the 500 gallon concrete tank to hold it down (neglecting the coefficient of static friction of the soil around it). If there isn’t any soil or weight on top of it then it will need roughly 150 gallons of water in it to keep it in the ground!