Geotechnical: Prevent 500-Gallon Septic Tank Floating in Houston

[Shackleford]

What capacity should be maintained to not float a 500-gallon plastic septic tank under groundwater-caused buoyancy? What other external installation steps should be taken to prevent floating. It rains a bit in Houston. Thanks.

 

[CarlAK]

If the tank is right at the ground surface, and flooding may occur to the ground surface, then it would need to be kept full to not float.

I assume the top of the tank is 4 feet or below below the ground, This Earth cover helps prevent flotation, even when entirely submerged. to determine likelihood of flotation, an engineering approach is to determine the safety factor; the resisting forces divided by the uplift forces. You would like to have a SF of at least 1.1 under worst conditions.

Uplift forces:
-tank outside vol x 62.4 pcf (if fully submerged)

Resisting forces:
-weight of soil over the tank (if submerged, effective soil unit weight is approx 100 pcf - 62.4 pcf)
-weight of tank & contents

To increase resisting forces, you could pile soil higher over the tank, construct a concrete slab over the tank, use concrete or other deadmen below/along tank with straps over the tank, etc. Helical anchors & straps another option.