Can a fiberglass pool pop out of the ground?

A pool shell in the ground behaves a lot like a boat hull. Groundwater pushes up on the floor and in on the walls, and that pressure grows with depth. When the pool is full, the water inside pushes back with roughly equal force, so nothing moves. Take the water out and the only things holding the shell down are its own weight, the backfill and friction from the soil.

A fiberglass shell is light for its size. That is part of why it can be craned into a backyard in a day, and it is also why an empty one can lift. An engineering bulletin from PE Group, a forensic engineering firm, puts it plainly: when a pool is empty or partly empty and the surrounding soil is saturated, the upward force can make it float.

What surprises most people is that the amount of water in the ground is not the point. In an AQUA Magazine piece on hydrostatic failures, an industry engineer explains that what matters is the height of the water outside the pool compared with the water level inside it. A thin layer of water around the shell can lift it if the ground keeps feeding that water while the shell rises. That is how a spring rain can pop a pool someone drained the day before.

Because of our soil. Much of the region sits on glacial till with a dense, slow-draining layer underneath, and the USDA's official soil descriptions are blunt about what that means for water.

• Ravenna silt loam, named for the Portage County seat and found across northeastern Ohio, is somewhat poorly drained. The USDA puts the top of its perched seasonal high water table 7 to 11 inches below the surface from October to June in normal years.
• Mahoning silt loam covers more than 450,000 acres of Northeast Ohio and northwestern Pennsylvania. Its seasonal high water table can sit 6 to 12 inches down from October through June in most years.
• Canfield silt loam, which drains a bit better and sits on higher ground, can still hold a perched water table 10 to 21 inches down from November to April.

A pool is five or six feet deep. On a lot with soils like these, the deep end of the shell sits well below where the groundwater lives for much of the year. Add snowmelt in March and April, a wet spring, and downspouts that dump next to the deck, and the ground around a pool can stay saturated for weeks. None of that is a problem while the pool is full. It becomes a problem the moment someone lowers the water too far.

The story is nearly always one of these:

• An owner drains the pool to clean it, repaint something or fix a leak, often after reading advice written for a dry climate.
• The water is lowered too far for winter, then a thaw and a few days of rain raise the groundwater.
• A slow leak drops the level over a few weeks and nobody notices until the shell has shifted.
• During construction, water collects in the dig or along the old excavation ramp before the shell is backfilled and filled.

That last one catches builders, not owners. The AQUA article describes shells popping on dry sites because rain gathered where the excavation ramp had been and ran in around the pool. The ground was not wet. The construction scar was. This is one reason we treat drainage planning as part of every build rather than an afterthought.

We install Liberty Composites shells, which are made by Latham. Latham's current Liberty warranty is clear that the pool is designed to stay full. Among its conditions, the pool must remain full of water at all times, and the warranty is voided entirely by failing to keep the water level above the skimmer outlet or by allowing groundwater to build up under or around the pool.

It also excludes damage from improper or unauthorized draining, from the groundwater table rising above the pool water level, and from runoff that is not drained away from the pool and deck. In other words, the manufacturer treats a popped or bulged shell as a site and maintenance problem, not a defect. We would rather you knew that before the pool goes in than after.

Most of the work happens before you ever see the shell. On a wet site, groundwater is pumped out of the excavation so the base can be set and the shell leveled on firm, dry material. The pool is filled with water as the backfill goes in around it, which keeps the pressure balanced on both sides of the walls while the soil settles.

Many installers also leave a way to deal with groundwater later. The PE Group bulletin describes a well point: a pipe set in gravel beside or beneath the shell, reaching below the floor, that can be pumped to pull groundwater down before a pool is ever emptied. Some pools also have a hydrostatic relief valve in the floor that lets groundwater into the pool instead of letting pressure build under it. Valves can stick, though, which is why a well point you can actually pump is the more reliable tool on clay.

Then there is the surface. Grading the deck and yard so rain runs away from the pool, not toward it, and routing downspouts elsewhere, keeps the soil around the shell from turning into a bathtub. When a site needs dewatering during the dig, we price it as its own line item after the site visit rather than burying it, because it depends entirely on what the ground is doing.

Very little, as long as you respect one rule: do not drain a fiberglass pool. For everyday care there is no reason to. The gelcoat does not need resurfacing and it does not need to be dry to be cleaned.

• Keep the water at its normal level, around the middle of the skimmer opening. If it drops quickly, look for a leak.
• For winter, lower the water only as far as your closing procedure calls for, typically just below the skimmer, and never more.
• Keep downspouts, sump discharge and neighbors' runoff away from the pool and deck.
• If a repair ever truly requires lowering the water, have a pool professional check the groundwater first, pump it down through a well point if one exists, and refill the same day.

If you hire a service company for openings and closings, ask how far they lower the water on a fiberglass pool. Some closing routines for other pool types drop the water well below the skimmer, and a crew working from habit can take a fiberglass pool lower than it should go. It takes one sentence to check, and it is worth asking even when the crew has done your pool before.

A pool that is starting to lift or bulge usually tells you. The waterline stops being level along the tile or coping. A gap opens between the coping and the deck on one side. A wall looks bowed inward, or the steps seem to tilt. You may see new cracks in the deck right at the pool edge.

If you notice any of that, stop adding or removing water and call a professional. The PE Group bulletin warns that a pool that has already popped should not simply be refilled, because the added weight can crack a shell that is no longer sitting on its base. The fix depends on how far it moved, and it starts with getting the groundwater down.

Every inground pool can be pushed by groundwater. Fiberglass shells are lighter, so an empty one lifts more easily. Concrete pools are much heavier but can still float or crack when drained on saturated ground, which is why they are typically built with relief valves. A vinyl liner pool has a different weak point: the liner itself can lift and float off the floor when groundwater rises behind it.

So the honest answer is that pool type changes how the failure looks, not whether groundwater matters. What makes the difference is a build that controls water from day one and an owner who keeps the pool full. On those terms fiberglass does very well in Ohio, and it is still the pool we recommend most often.