Building a pool in clay soil in Northeast Ohio

Mostly glacial till: a dense mix of clay, silt, sand and broken rock left when the Wisconsinan ice sheets retreated. On top of it, many soils have a thin cap of wind-blown silt. The USDA's official soil series descriptions for the region read like a list of our service area, because several were first described here.

• Mahoning silt loam, first described in Mahoning County in 1917, covers more than 450,000 acres of northeastern Ohio and northwestern Pennsylvania. It formed in low-lime till derived mainly from shale and siltstone, is classed as fine and illitic, and is somewhat poorly drained.
• Canfield silt loam, first described in Trumbull County, is moderately well drained above a dense fragipan that starts 15 to 30 inches down.
• Ellsworth, another Mahoning County soil covering more than 190,000 acres of northeastern Ohio, is fine and illitic with moderately low to low permeability below the surface.
• Wadsworth silt loam averages 27 to 35 percent clay in its subsoil, sits over a fragipan 18 to 30 inches down and is somewhat poorly drained.

Your own yard may differ, especially if it was graded during construction, when topsoil is often stripped and the builder's fill left behind. But if you dig a hole in most Summit, Portage, Trumbull, Mahoning or Medina County lots, you will hit dense, sticky subsoil within a foot or two.

Three reasons: water, movement and handling.

Water. Clay and fragipan layers slow water moving down, so rain and snowmelt pile up above them. The USDA describes a perched seasonal high water table 6 to 12 inches below the surface in Mahoning soils from October through June in most years, 10 to 21 inches in Canfield soils from November to April, and 11 to 24 inches in Ellsworth soils from November to May. A pool excavation that goes 6 or 8 ft down in that ground will usually collect water, and the water keeps coming.

Movement. Clay changes volume as it wets and dries, and in Ohio it also freezes. Saturated clay behind a pool wall or under a deck freezes, expands, thaws and settles. Over years that can push on walls, crack concrete and lift pavers.

Handling. Wet clay is miserable to work. It smears instead of compacting, it clings to equipment and it turns a backyard access route into a rut. Timing and protection of the lawn matter more here than in sandy regions.

Not with the clay that came out of the hole. Fiberglass shell makers and experienced installers favor clean crushed stone for the base and backfill in cold, wet climates. Thursday Pools, a fiberglass manufacturer, notes that in the Midwest and Northeast, freeze-thaw cycles call for crushed stone backfill that drains naturally to keep the shell from shifting. Installers such as River Pools describe the same approach: clean stone drains well and its angular pieces lock together, which helps hold the pool walls in place.

The stone does two jobs in clay soil. It gives the shell firm, even support that does not turn to paste when wet. And it creates a path for groundwater to reach a sump pipe at the deep end, where it can be checked and pumped if the pool ever has to be lowered. We explain that sump in our guide to high water tables. The native clay goes elsewhere: hauled off, or used for rough grading away from the pool where settling does not matter.

The same soil logic applies with different details. An ICF concrete pool has poured, reinforced walls inside insulated forms, so it is rigid and heavy, but the backfill against those walls and the base under the floor still need to drain and compact properly. A vinyl liner pool has panel walls and a floor built on site, and its wall bracing and backfill need the same attention so the walls stay straight and the liner stays smooth.

No pool type makes the ground irrelevant. Fiberglass is our core product partly because a one-piece shell with the right stone backfill handles Ohio freeze-thaw well, but on any build the base, backfill and drainage decide how the pool looks in ten years.

Most deck problems here start under the concrete, not in it. Clay that stays wet under a slab freezes and heaves in winter and settles in spring. The fixes are simple but have to be done during construction:

• Strip the soft topsoil and organic material under the deck area.
• Place a compacted aggregate base, built up in lifts, rather than pouring on clay or loose fill.
• Slope the deck away from the pool so water does not pond at the coping or run under the slab.
• Collect water at the low edge of the deck with grading or a drain and carry it somewhere it can go.
• Keep downspouts from the house from discharging onto the deck area.

We stress excavation, compaction, drainage and structure on every job because in Northeast Ohio soils those steps are what separate a deck that lasts from one that cracks in its second winter. The 4 ft concrete wrap deck in our fiberglass packages is built on that kind of base, and extra patio is priced to the same standard.

When the ground is least wet, which usually means late spring into fall once the perched water table has dropped. The USDA's water table months for local soils explain the pattern: the wettest digs tend to fall in late fall, winter and early spring. A June or September excavation in the same yard is often far drier than one in April.

That does not rule out a spring start. It means planning for pumping during the dig, protecting the access route, and being ready to pause through a heavy rain. Dewatering, when a site needs it, is priced as a site item rather than built into the package, because some yards need none and some need a pump running for days.

Glacial till is not pure clay. The USDA notes that the rock fragments in Mahoning soils are glacial erratics, mostly shale and siltstone with some limestone and crystalline rock, and that cobblestones and boulders show up in some places. Canfield soils carry sandstone and shale fragments too.

For a pool dig that usually means an occasional boulder the excavator has to work around or lift out, not a reason to move the pool. Some lots also have shale bedrock within reach of a pool's deep end, and hitting it slows the excavation and can change the equipment needed. We look at local soil maps and the lay of the land before we quote, and we tell you plainly if rock is a real possibility on your lot.

For most residential pools in the region, a formal geotechnical report is not required, and the local soil data plus a look at the site tell you most of what you need. A few situations call for more: a lot that was heavily filled, a steep slope where the pool will be partly out of the ground, a known history of springs or wet basements, or a pool very close to a house foundation.

You can look up your own soils for free on the USDA's Web Soil Survey by drawing your lot on the map. It lists the soil series, drainage class, depth to water table and depth to bedrock. If it shows a somewhat poorly drained till soil with a high seasonal water table, that is normal for this region, and it tells you drainage and backfill deserve a real line in the budget.

Heavy equipment on wet clay compacts it, and compacted clay grows poor grass and sheds water. After the pool and deck are done, the disturbed areas need loosening where possible, a layer of real topsoil, and seed or sod. Plan the equipment route to cross as little lawn as practical, and expect the access path to need the most repair. A dry-weather build helps here as much as anywhere.

Clay rarely stops a pool, but it can add cost around the edges. The items that most often move are dewatering during the dig, hauling off spoils that cannot be reused, extra stone, and drainage work around the deck. Our fiberglass packages start at $68,400 installed and include the Pentair equipment, a 4 ft concrete deck and startup. Spoils haul-off, dewatering and extra decking are priced after we see your site, and we would rather show you those numbers on the first quote than surprise you later. Financing is available through lenders we work with if you want to cover the whole project together.