Hydrostatic Pressure and Pool Popping: How Pools Float Out

An empty in-ground pool behaves a lot like an empty boat, and it sits in ground that may be wet. If the groundwater around the shell stands higher than the water inside, the shell gets pushed upward, the way a bucket pushed under a lake tries to come back up. That upward push is the hydrostatic pressure pool owners are warned about before a drain, and it can tilt a pool, crack it, snap its plumbing and sometimes lift it a foot or more above the deck.

So don't drain a pool when the ground is wet, and don't leave it empty a day longer than the work needs.

What hydrostatic pressure is in plain terms

Soil holds water. Below a certain level, called the water table, every gap between the soil grains is full of it, and that level rises after rain and snowmelt. Water in the ground pushes on anything buried in it, and the push grows with depth, by about 0.43 pounds per square inch for every foot of water above the point you measure.

When the pool is full, the weight of the water inside more than balances the groundwater pushing from below. When the pool is empty, nothing presses back, and what holds the floor down is mostly the weight of the shell itself. In wet ground that often isn't enough.

Why an empty pool is a boat

The numbers here are rough, for illustration only.

Take a 16 x 32 ft pool, a footprint of about 512 square feet, and treat it as a simple box. Suppose a wet week has raised the groundwater to 2 feet above the floor. The upward force equals the weight of the water the shell displaces, so 512 square feet x 2 feet x 62.4 pounds per cubic foot, or roughly 64,000 pounds pushing up.

Full, that pool holds about 18,000 gallons at 8.34 pounds each, close to 150,000 pounds of water sitting on the floor. Empty, you have the shell alone. A concrete shell is heavy but weighs well under that, and a fiberglass shell of that size typically weighs a few thousand pounds. Once the groundwater rises high enough, the push from below beats the shell's weight.

Which pools are most at risk (fiberglass, then concrete, then vinyl)

Fiberglass is the most likely to float, because a one-piece shell is light and rigid. Makers generally say these pools shouldn't be drained except by a professional, who checks the groundwater and pumps it down first, often through a sump pipe set beside the pool, and may brace the walls.

Concrete pools (gunite or shotcrete under plaster, pebble or quartz) are heavy enough to stay put in dry ground and are the type most often built with a hydrostatic relief valve. They still pop, mostly after heavy rain or on low lots, and a concrete shell that lifts usually cracks too, which is why the repair is rarely cheap.

Vinyl liner pools have steel or polymer walls set in the backfill, a sand-and-cement or vermiculite floor and the liner as the waterproof skin. The pool itself rarely lifts, but groundwater can push up under the liner and float it. With the water out, the walls can bow and the floor can wash out, and an exposed liner shrinks as it dries and may tear on refill. The draining a vinyl liner pool guide explains why these pools are drained only partially, and only with a plan.

Where the hydrostatic relief valve is and how to open it

Concrete pools usually have one or more hydrostatic relief valves in the floor, most often inside the main drain sump at the deep end. The valve is a threaded plug over a pipe that runs down to a gravel bed under the shell. It lets groundwater flow up into the pool instead of lifting it, and while there is water in the pool, the pressure inside keeps it shut. Valves can stick or clog, so for a full drain, contractors commonly open them by hand.

1. Drain the pool until only a few inches are left over the main drain.
2. Shut off the pool pump at the breaker so it can't run dry.
3. Take off the main drain cover.
4. Find the plug in the sump floor and unscrew it, chipping it free first if it's set in plaster. Have a new plug ready.
5. Watch what happens. Groundwater flowing up through the opening means the relief is working and the water table is high. Leave the opening clear while the pool is empty, and pump the water somewhere it won't drain back under the pool.
6. Before refilling, thread the new plug in and put the drain cover back.

If you can't find a valve, assume the pool doesn't have one and treat it as higher risk. Drain only in a dry spell and keep the empty time short. The complete draining guide puts the valve step in sequence with the three draining methods.

Warning signs: rain, water table, season

Hold off on draining this week if any of these are true.

• It rained hard in the last several days, or heavy rain is forecast before the refill.
• It is early spring and the ground is still taking in snowmelt.
• Your basement sump pump has run at all this month.
• The lot is low, near a creek, lake, canal or wetland, or the yard stays soggy after rain.
• Water appears in the main drain sump when you open it, or the deep end floor has ever had a damp patch or a bulge.

You want a dry forecast through the refill, usually a window of a few days, so book the refill before the drain starts.

How long a pool can stay empty

Keep it as short as you can, and how short depends on the shell. A concrete pool in mild, dry weather can sit a few days for repairs, an acid wash or a replaster with the relief valve open, and the acid wash and replaster timeline shows where the empty days fall in each job. A vinyl liner pool should be refilled as soon as the liner work allows, ideally the same day. A fiberglass shell shouldn't sit empty at all without the installer's say-so.

Draining is the quick part (the how long draining takes guide compares pump times), and the risk is in the days after. By garden hose at 5 to 10 gallons a minute, refilling a 20,000-gallon pool takes 33 to 67 hours, a day and a half to three days with the shell only part full. By tanker it's 3 to 4 loads of 6,000 to 8,000 gallons, usually done in a day once the work finishes. Our pool refill after drain service times the first load to your contractor's schedule.

If a pool has already shifted

Stop draining. Water is what holds the pool down, so a contractor may tell you to put some back to stop it rising further. That won't push a lifted shell back into place, because soil and gravel shift in underneath it, and if the shell is cracked or the plumbing broken, the water just runs into the ground. Ask before you add any.

Switch off the pump at the breaker, since the plumbing has probably pulled away from the shell. Photograph everything, then call a pool contractor who does structural work, plus an engineer for a concrete pool. Talk to your insurer early, and expect a hard conversation, because standard homeowners policies usually exclude damage from water below the surface of the ground and the usual wording names swimming pools.

Frequently asked questions

Can a pool pop out of the ground when it is full?

Very rarely. A typical in-ground pool holds well over 100,000 pounds of water, so a full shell almost never rises. The usual exception is a vinyl liner lifting off the floor when groundwater climbs above the pool's own water level, in a flood or with the water lowered for winter.

How do I know if my pool has a hydrostatic relief valve?

Look in the main drain sump at the deep end, under the drain cover. A round threaded plug in the sump floor is the valve, and some pools have more along the floor. Fiberglass pools usually have a sump pipe beside the pool instead, and many older and vinyl liner pools have none.

Can a popped pool be fixed?

It depends on the shell. A fiberglass one can often be lifted out, the ground under it rebuilt, and the pool reset and re-plumbed. A concrete shell that lifted has usually cracked and often has to be broken out and rebuilt. Either way it's a contractor and engineer job.

Should I open the relief valve for a partial drain?

Not if the water stays well above the main drain. Several feet of water still holds the shell down, and opening the valve under water just lets pool water out into the ground. Open it only when the pool is nearly empty, and close it before the refill.

The less time a pool sits empty, the safer it is, so book the refill loads before you pull the plug. Get your refill price.