Soil Conditions and Pool Construction: What Every Builder Needs to Know

Most pool builders spend more time thinking about the shell than what the shell is sitting on. That's a mistake. Soil conditions are one of the primary drivers of pool cracking, shell movement, and structural failure — and they vary enormously from one job site to the next, sometimes even within the same neighborhood.

Expansive clay soils

Expansive (or 'reactive') clay soils absorb water and expand significantly in volume — up to 10% or more. When a pool is surrounded by expansive clay, the soil that was disturbed during excavation and later backfilled around the shell will move as moisture content changes seasonally.

• Expansion puts lateral pressure on pool walls and can crack or bow plumbing lines
• Shrinkage during dry periods creates voids under the pool deck and around the shell
• Common in Texas, Oklahoma, parts of California, the Southeast, and much of the Midwest
• In high-PI (plasticity index) soils, an engineered plan is typically required
• Mitigation: proper compaction of backfill, drainage design that keeps moisture level consistent, and in severe cases, a structurally deepened shell

Caliche

Caliche is a hardened calcium carbonate layer found commonly in the American Southwest — Arizona, New Mexico, West Texas, Nevada, and Southern California. It can occur as a thin lens a few inches thick or as a massive formation several feet deep.

• Caliche requires jackhammering or even blasting to excavate — adds significant cost to the dig
• If caliche is present under the pool floor but not fully removed, it can create drainage issues and restrict the pool from 'floating' in the event of hydrostatic pressure
• Disclose the possibility of caliche to homeowners before signing a contract in affected areas — excavation overages due to caliche are a legitimate change order item

High water table

A pool shell is a vessel. If the groundwater table rises above the bottom of the shell, hydrostatic pressure can push the shell up — a phenomenon called pool flotation or 'pool pop.' A dewatered empty pool in high water table conditions can float out of the ground in hours.

• Install a hydrostatic relief valve in the main drain sump to allow groundwater to enter the pool rather than push it up
• Never empty a pool in high water table conditions without first determining the groundwater level
• Some pools in extreme high-water-table areas require a dewatering system during construction
• Advise homeowners in these areas never to empty the pool without consulting a professional

Sandy or loose soils

Sandy soils are generally easier to excavate but provide less lateral support for the pool walls. In very loose, dry sand, the excavation walls may not hold their shape, requiring shoring or a modified excavation approach.

Sandy soils also drain freely, which reduces hydrostatic pressure — but backfill in sandy soil can settle over time, creating voids under the pool deck. Compact all backfill thoroughly and consider a granular backfill specified to minimize settlement.

When to involve a geotechnical engineer

If you encounter unexpected conditions during excavation — water, rock, highly expansive clay, or poor-bearing soil — stop and call an engineer before proceeding. The cost of a geotech consultation is trivial compared to the cost of a cracked shell or a warranty dispute over a pool that moved.