Photograph of  an oil tank floating up out of the ground after area flooding Floating Heating Oil Tanks &
Floating-Up Septic Tanks

  • OIL or SEPTIC TANKS FLOATING UP - CONTENTS: Why oil tanks or septic tanks float up out of the ground or up inside buildings. How to anchor septic tanks or oil tanks & septic tank or oil tank float-up environmental issues. Photographs of float-up oil tanks or floating septic tanks. Risksof Structural or Mechanical Damage due to Floating-up Fuel Storage Tanks During Flooding At or In Buildings
  • POST a QUESTION or READ FAQs about why oil storage tanks & some septic tanks may float up out of the ground and how to prevent the problem
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Floating oil storage tanks & floating septic tanks:

Tthis article explains why some buried oil tanks or septic tanks float up out of the ground, what that event means to the property owner, and how to prevent oil tank or septic tank flotation. Oil tank anchors may be required to prevent empty tanks from floating up out of the ground.

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Why Buried Tanks Float Uup out of the ground or up inside buildings during heavy rains or flooding

Photograph of  an oil tank floating up out of the ground after area flooding

The photograph at the top of this page and this close up show an old abandoned oil tank which has burst up out of its grave in a thicket along a creek in New York.

Recent rains and area flooding brought water level above the top of the ground in which this empty and buried tank had rested for decades.

[Click to enlarge any image]

As water levels rose the buoyancy of the empty tank caused it to lift right out of its burial spot where it had been hidden.

A prior owner had even unscrewed the tank fill and vent valves so that there was no evidence of its presence along this creek.

Oil is lighter than water to begin with, but an oil tank or septic tank which is in use and full is unlikely to rise out of flooded ground even so.

But an empty steel, plastic, or fiberglass tank used for oil storage, such as heating oil, or for a septic tank, is very buoyant. Even a modest increase in ground water can be enough to push the tank up through the ground.

When a buried oil tank or septic tank is lifted by rising water or flooding, any piping connections to the building it serves are likely to be severed, leading to an oil spill or a sewage spill.

The tank shown in these photos should not have been installed by a creek in the first place, which may explain why it was later emptied and "abandoned" in place. Had the tank been properly abandoned it would have been cut open, cleaned, and filled with clean sand.

The fill should have prevented the buried oil tank from rising to the surface despite later rising ground water or floodwaters.

A buried oil tank or a buried steel, plastic, or fiberglass septic tank can be prevented from dislocation caused by rising water or flooding if it is either kept full or is physically anchored to the site using cables or other means.

Risks of Structural or Mechanical Damage or Fuel Leak Contamination due to Floating-up Fuel Storage Tanks During Flooding At or In Buildings

Outdoors or inside as well, heating oil storage tanks that are full or nearly full are less likely to float up out of the ground or to move off of their moorings during area flooding.

During installation of plastic or fiberglass storage tanks for fuel or as septic tanks, the installer can include anchors to help protect these light storage vessels from movement during flooding. But generally installers of heavier steel storage tanks omit tank anchoring systems from their installation.

Outdoors buried steel oil tanks are simply placed into an excavation and connected to fuel fill, vent, and supply piping. And above-ground oil storage tanks both outdoors and indoors are typically installed relying on little more than gravity to hold the tank in place top its legs.

If exposed to flood waters, and depending on the volume of oil that they contain, oil storage tanks of any material, steel, fiberglass, or plastic, may float off of their support or moorings. Even if the tank itself is not damaged, an oil spill is likely as this movement will rupture oil supply piping lines and connections. And movement can also damage the oil storage tank itself or the movement of the tank may damage other building components & mechanicals.

Julie Satow reported in the New York Times (January 2013), during flooding caused by Hurricane Sandy (New York, 2012),

The development 88 Greeenwich ... incurred basement flooding. The water dislodged an oil tank, which hit a ceiling beam and cracked open, necessitating a major cleanup. [14]

Risk of Sewage Backup into Building After Septic Tank has Floated-Up from its Burial Site

On 2017-07-10 by Steve - should I fill up my empty septic tank using water?

I have just had my septic tank emptied and installed a new outgoing baffle. Should I put water in the tank up to the baffle?

This question and answer were posted originally at SEPTIC TANK PUMPING MISTAKES

On 2017-07-10 by (mod) - to prevent a septic tank from floating up or for some other reason, should I fill the septic tank with water after pumping?


If your septic tank is concrete there is never a reason to fill it after pumping.

If your septic tank is thin steel or lightweight plastic or fiberglass, AND if the septic tank was not mechanically anchored to the ground when it was installed, AND if the soil is wet and the area is flooded THEN the septic tank could float up out of the ground. But even then the right repair is to install the necessary anchoring, not to re-fill the septic tank with water.

See OIL or SEPTIC TANKS FLOATING UP where we describe this floating up out of soil problem for other buried tanks.

On 2016-10-31 by Claire - our septic tanks were not tide down and now they have floated because of \rising water table during rain

Our septic tanks were being installed. They were not tied down and not filled with water and now they have floated because of the water table rising from the rain.

What should be done? Does the whole system need to be removed and reinstalled? Would the installer have to wait until better weather or he water table drops? Thank you for any information!!

This question and answer were posted originally at SEPTIC TANK PUMPING MISTAKES

On 2016-10-31 by (mod) - septic tanks floated up out of the ground


This is a more annoying problem than meets the eye - as we discuss at

where we describe floating-up septic tanks or oil tanks.

I think that the septic tanks (presumably fiberglass or plastic) are not correctly installed. The installer probably figured that once the tank filled up with wastewater it'd never float out of the ground during rising ground water. But that's not a safe guess since septic tanks must be pumped out (emptied) regularly as part of normal maintenance.

So a proper installation for a tank that might float up and out of the ground would be to anchor it to the ground using straps and concrete or steel anchors.

Now the worry is that the tank movement will certainly have broken plumbing connections. So at the very minimum those connections need to be inspected and repaired or you risk a sewage backup in the building. Properly the tank is going to need to be anchored lest the problem simply happen again in wet weather.

But yes, the installer may not be able to replace the tank if the destination hole is filled with water.

Watch out: Meanwhile let's worry about where your wastewater is going and about sewage backups.

If the septic tank has floated at all it has probably damaged if not completely broken waste piping near the tank. In turn that means that flushing toilets and running plumbing fixtures in the building risk causing a nasty sewage backup into the structure.


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