Building a pool with a high water table in South Florida

Limestone and groundwater under South Florida

Much of eastern Miami-Dade and southern Broward sits on the Atlantic Coastal Ridge, where Miami Limestone, formed roughly 125,000 years ago, comes close to the surface. Digging a pool there is closer to quarrying than to scooping sand, and the excavator often carries a hydraulic hammer to break it up.

Right under and inside that rock lies the Biscayne Aquifer. It is shallow and unconfined, held in porous rock and sediment, so water moves through it with almost nothing in the way. Many pool digs here hit groundwater. How deep it shows up changes from lot to lot and from month to month, which is why we look at your yard before promising anything about it. Low-lying parts of Coral Gables are a well known case.

How we keep the dig dry

We handle groundwater in a pool excavation by pulling it out faster than it seeps in, using a method picked for that lot once we see the soil and the water on day one.

• Sump pump. We cut a pit next to the deepest part of the hole and a pump lifts the water out while the crew works.
• Well points. Small pipes driven around the perimeter feed a vacuum pump that draws the water down before it reaches the hole.
• French drains. Gravel trenches along the floor lead seepage toward the pump.

Somebody checks the water level every day, and a backup pump stays on site, because one heavy afternoon storm can raise the table fast. Rock and groundwater are not part of the base pool price. They are priced after the site visit and before work starts, as explained on what a pool costs and what is extra.

Where the pumped water goes

Pumped groundwater gets discharged well away from the excavation so it doesn't run straight back in. In Miami-Dade, temporary dewatering from an excavation falls under a DERM Class V permit, which exists to keep sediment and cloudy water out of storm drains and waterways and stays valid for up to 365 days.

The City of Miami adds its own step. Sending the water into the storm sewer needs a Public Works dewatering permit, which is only issued alongside the DERM Class V, after a site location review, with turbidity at the outlet kept below 29 NTU. Pumping uncontaminated groundwater also calls for a state generic permit. Some sites have been told to keep the water on the lot instead, in pits filled with stone. We sort this out with the pool permit, not after the hole is open.

Rain and a rising water table

Heavy rain lifts South Florida's water table quickly. During the wet months, from late May to mid October, that means longer pump runs and some lost days while the hole and the steel wait. Weather and groundwater are two of the things that stretch a schedule, covered stage by stage in how long a pool build takes.

Protecting the shell from groundwater

An empty concrete shell sitting in high groundwater can be pushed upward. Builders call it floating or popping out. To relieve that pressure, a hydrostatic relief valve can be set in the floor. It opens and lets groundwater into the shell rather than letting it lift the whole structure. The signed design for your lot says if the pool gets one.

Valves can clog or fail, so a shell should not sit empty longer than the work requires, and nobody should drain a pool right after a storm. Industry standards also call for at least 7 days of curing before backfill goes against the walls. How the shell is built and cured is covered in our concrete shell work, and the order of every step is on our build process.

Why you never drain a pool before a hurricane

Water in the pool is what holds the shell down. Storm rain raises the groundwater around it, and an empty pool loses that weight right when the push from below is strongest. Before a storm, do this instead:

1. Leave the water in.
2. Super-chlorinate it.
3. Shut off the pool equipment at the breakers.
4. Cover or remove the motors when flooding is possible.

Frequently asked questions