Our 1250 gal septic holding tank went in about 5 years ago. Before the operator backfilled, i went in and screwed in 6 hand-bomb screw piles around the perimeter, and 3 web straps over the tank tied to the piles to hold the tank down when the soil is saturated and it wants to float up when emptied. It gets wet here in spring, and when empty the tank hasn't budged!One aspect of septic tanks rarely discussed is one floating up out of the ground when pumped out.
If the ground water level is high as in the spring, a pumped tank becomes a boat and the flotation forces will lift it out of the ground.
Well engineered system will have a heavy ballast cap on the tank to prevent the floating.
These caps are formed in place because they are so heavy.
Plastic tanks need to be tied down to a heavy ballast under the tank.
When you see one happen, it looks like a submarine surfacing.