Sandy Subgrade and High Water Tables in Miami-Dade

On a job off Kendall Drive in late September, the crew dug the turndown trench at eight in the morning. By ten there were six inches of standing water in it and nobody had touched a hose. The water table sat about three feet down, and every hole filled from the bottom. My outriggers were already set, and I could feel the machine settle half an inch into the sand.

That is South Florida concrete work in the wet season. The sand under Miami-Dade is not the problem people assume. The water in it is.

Sand is a decent subgrade when it is confined and compacted

Clean sand drains fast and does not swell, which puts it ahead of most of the country. It has almost no cohesion, so it moves sideways under load unless something confines it, and it does not compact by being driven on. Loose sand can lose 10 to 15 percent of its volume once properly vibrated, and all of that shows up as a settled slab.

Strip the organics, wet the sand, compact in 6 to 8 inch lifts. Local practice follows FDOT-style base work: limerock or crushed concrete compacted to about 98 percent of modified Proctor, with a bearing value in the LBR 40 range. Under a residential driveway that means 6 to 8 inches of base, proof-rolled before anybody sets a form. More on why that step outranks everything else in our piece on compaction.

The high water table is what changes the work

The Biscayne Aquifer sits close to the surface across the county, and in Kendall the wet-season water table is commonly two to six feet down. Three consequences:

• You cannot compact saturated sand. Vibrating sand at or above the water line just liquefies it. The excavation gets dewatered first—a sump and trash pump for a shallow trench, or wellpoints for anything deeper, $1,500 to $4,000 on a small residential job.
• Deep excavations are limited. This is why South Florida driveways rarely get the deep turndowns you see in Texas—if a trench fills faster than it can be poured, the design changes.
• Heavy equipment needs a spread footprint. Loose saturated sand can have allowable bearing under 1,500 psf. My pads go on cribbing and I still watch them. Plan for that before the truck shows up.

What the slab itself should be

There is no freeze-thaw here, so entrained air is not required the way it is in Colorado. The threat is chlorides—salt air and salt water in the soil corrode reinforcement, and rusting steel expands enough to spall concrete off the slab.

• 4 inches for cars, 6 inches wherever a delivery truck, boat trailer or dumpster will go.
• 3,500 to 4,000 psi at 28 days, water-cement ratio at or below 0.45 for a less permeable paste.
• #4 rebar at 18 inches on center each way in the 6 inch sections, on chairs, with 2 inches of cover over the steel and 3 inches to the ground. Nearer the water, epoxy-coated bar or synthetic fiber are both reasonable.
• Control joints at 10 to 12 feet, cut a quarter of the depth within 6 to 12 hours.
• Cure 7 days. Humidity helps, but sun and a sea breeze still craze a fresh slab.

Our sand-heavy local mixes pump well—good fines, rounded oolitic aggregate, easy on the hoppers. The bottleneck here is never the concrete. It is getting the machine somewhere it can reach from without sinking, inside the afternoon thunderstorm window. When you are deciding who does the work, a Miami concrete driveway contractor who works here year round schedules around that window instead of hoping.

Drainage is a design item, not an afterthought

Slope the slab 1 to 2 percent away from the house, about a quarter inch of fall per foot. Because so much of the county must keep stormwater on site rather than shedding it into the street, projects often get a French drain or exfiltration trench along the low edge, which works extremely well in sand. Budget $25 to $60 per linear foot.

Permeable options are worth a look for the same reason. Pervious concrete and permeable pavers put water into the sand instead of moving it elsewhere, and the EPA’s stormwater and permeable pavement material is a solid starting point. Pervious concrete needs a clean stone reservoir and periodic vacuuming, and it is wrong under a truck pad.

What it runs

A standard 4 inch broom-finish driveway lands around $8 to $14 per square foot installed here; a 6 inch reinforced slab over a compacted limerock base is more like $12 to $19. Add $1 to $3 if the site needs dewatering, or if base has to be imported because what is there is muck. Removal runs $2 to $6.

Frequently asked questions

Can concrete be poured over sand without a base? It can be, and on light residential work over clean compacted sand it sometimes is. It is still worse. A limerock base spreads wheel loads, gives the crew a firm platform for forms and steel, and keeps the slab from settling when sand under one corner shifts.

Does a high water table mean the driveway will fail? No. It means the base gets built when the ground is workable and the excavation gets dewatered. Failures come from compacting saturated sand, or placing concrete into a trench with water standing in it, diluting the mix exactly where strength is needed.

Is a vapor barrier needed under an exterior slab in Miami? Generally no. Sheet plastic under a driveway traps bleed water at the bottom of the slab and can make curling worse. Vapor retarders belong under enclosed slabs with floor finishes.