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Photograph of an oil tank floating up out of the ground after area flooding Floating Oil Tanks & Septic Tanks
Causes & Prevention

  • POST a QUESTION or COMMENT about why oil storage tanks & some septic tanks may float up out of the ground and how to prevent the problem

Floating oil storage tanks & floating septic tanks:

This 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 Up 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.

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

Floating up septic tank needs to have been anchored - reply to reader questions

Floating-up lightweight septic tank risks broken pipes & sewage backup (C) InspectApedia.com Roger S@Roger S,

Thanks and condolences for the question and the problem of your floating septic tank.

It is normal for a septic tank to be left empty after pumping, and in fact it's correct; else you may be paying the pumper to do nothing useful (as when they "return the effluent" OR you may be wasting water to fill the tank with fresh water.

Instead, what we have is an improper septic tank original installation: Plastic or fiberglass septic tanks are so light that unlike their concrete older sisters, they will float up out of wet soils in times of wet or flooding weather.

Watch out: because a septic tank that "floats up" like the one in your photo is also likely to have damaged or broken its piping connections, the immediate hazard from a floating-up septic tank is a sewage backup into the building or a failure that spills sewage improperly into the local environment, waterways, or water wells.

The installer of such a tank, at sites where that event is likely, are supposed to anchor the tank to soil using a combination of strapping and driven or buried anchors.

Anchors are required for lightweight buried tanks (oil tanks or septic tanks or water storage tanks) in typical government and model regulations and codes, of which we include examples and citations just below.

"Tank shall be properly supported by a proper foundation or, as applicable, its tie bolts, foundation anchors, or other supporting structure(s)." from the New York DEC guide at https://www.dec.ny.gov/regulations/38102.html

OPINION: I would not blame the septic pumping company for the rising septic tank; the pumper is dealing with a buried tank; she can't see whether or not the tank is strapped down or anchored, and wouldn't know that status unless the pumping company was also the original tank installer.

As to who's responsible for fixing such a problem, probably you, the property owner; and for and who pays, that's an argument about which to ask your lawyer.

Watch out: Some of these float-ups can be quite dangerous, such as the explosion hazard that might accrue if a buried LP tank floats-up from having been inadequately-anchored.

For other readers, the bottom line is that pretty much ALL types of tanks of plastic, fiberglass, or steel that might float up out of the ground under wet or flood conditions require some type of approved and safe anchoring system.

Examples of Codes, Standards Requiring Anchors for Fiberglass, Plastic, Steel Tanks Underground

The installer of a buried fiberglass, plastic or steel tank, at locations where flooding and risk of tank floating-up are likely, are supposed to anchor the tank to soil using a combination of strapping and driven or buried anchors or anchors to a concrete slab..

Some lightweight buried tank anchoring requirements, codes, standards are given below:

On 2021-06-27 by Roger S - septic tanks floated up after being pumped out, in wet rainy weather

Hi, i just had my septic tanks pumped out 3 day’s ago, i have 2 -500 gallon plastic tanks, with air pump, now we had 5 inches of rain and the second tank has floated out of the ground because no water was in it. The first tank was fine, because it was full of water, we’ve had more rain than this a few times before with no problems, system only 5 years old.

They never told me to fill with water after pumping , I didn’t even know this was possible! Who is responsible for this ? Im not an expert in septic they are! No dig caps on tanks my hose was 10 ft away, easy prevention if i would have known this was possible even ? Any feedback, thanks

[See photo above]

On 2020-07-05 by (mod) - can I fix a floating-up abandoned septic tank myself?

Clough

You ask a reasonable question but not one I can answer as we are complete strangers to one another.

In general, there is nothing technically difficult about exposing the top of an old tank, cutting an opening if one is not provided, and filling the tank with stone rubble and sand, perhaps breaking a drainage hole in the tank bottom first.

But there are life-safety hazards that may be present: falling into a tank, especially if it contains sewage, is a quick, nasty death. Never work alone. Hire expert help as needed. Meanwhile keep everyone away as a buried tank of unknown condition is a safety hazard.

On 2020-07-05 by clough1424

I have a floating septic tank that has not been used in decades. Can I fix this myself or do I need to call a professional?

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?

Steve,

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.

And indeed some lightweights septic tank installers might suggest partly-filling the new septic tank with water immediately after installation to prevent a floating upwards septic tank before the septic tank is weighed down by sewage as it will be in normal use.

Really? This is in our view an improper approach. If the septic tank might float up because it's empty when new, it can perfectly as well float up in the future when it has been in use and has been pumped out as part of normal, regular septic tank maintenance.

The right installation, and the right floating septic tank repair is to install the necessary anchoring, not to re-fill the septic tank with water.

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

...


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Reader Comments, Questions & Answers About The Article Above

Below you will find questions and answers previously posted on this page at its page bottom reader comment box.

Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2016-10-31 by Claire - our septic tanks were not tied 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

Claire:

This is a more annoying problem than meets the eye - as we discuss at https://inspectapedia.com/oiltanks/Buried_Tank_Float_Up.php

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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