You can pour 4,000 PSI concrete with perfect rebar over a bad subgrade and still end up with a cracked, settled slab in five years. You can pour an average mix over a properly prepped subgrade and have a slab that looks the same at year twenty as it did at year one. The dirt under the concrete matters more than the concrete itself for long-term performance.
The subgrade is the prepared layer of soil and base material immediately beneath a concrete slab. On a residential project, that usually means:
Everything that happens to that stack — how level it is, how dense it is, how well it drains — shows up in the slab eventually.
If the subgrade compresses unevenly after the pour, the slab loses support in spots. Concrete is strong in compression but weak in tension. When part of a slab loses support, the unsupported area sags slightly, which puts tension on the top of the slab and creates a crack — often running diagonally from a corner or across the middle of an unsupported span.
On driveways and parking pads, repeated vehicle loads over a soft subgrade cause the slab to flex. Water gets under the slab, then gets pumped in and out with every passing wheel. The slab develops voids underneath, then breaks at the joints. This is one of the most common failure modes on residential driveways in Hixson, Ooltewah, and other areas with clay subgrades.
Greater Chattanooga doesn't have a deep frost line, but freeze/thaw cycles still affect the top several inches of soil. Wet clay that freezes expands and lifts. If your subgrade holds water and the slab edge isn't drained, the edges can lift and crack in winter.
Slabs poured partly on undisturbed soil and partly on uncompacted fill almost always crack along the transition line. The fill compresses, the native ground doesn't, and a straight crack appears within the first year.
Topsoil is 5 to 12 inches deep in much of the Greater Chattanooga area. It has to come off before any base material goes down. Wood, roots, and organic debris go to the burn pile or get hauled off site — never buried under a slab.
For most residential work, a visual evaluation and a feel test is enough — but on questionable sites (filled lots, soft spots, areas with known karst features), getting a soils engineer involved up front is cheap insurance compared to fixing a failed slab.
Fill material gets placed in lifts of 8 inches or less and compacted between lifts. A plate compactor handles small areas; a roller or sheepsfoot is used on bigger pads. Compaction targets are usually 95% of standard Proctor density for structural fill.
For most slabs, a 4-inch base of clean crushed stone (#57 or similar) goes between the compacted subgrade and the concrete. It provides drainage, breaks capillary moisture rise, and gives a level surface to set forms and rebar on. We discuss base material more on the building pads page.
Laser grading the subgrade to within 1/2 inch of plan elevation means the slab thickness stays consistent. Inconsistent slab thickness from a wavy subgrade means thin spots that crack first.
Tearing out a 1,000 sq ft cracked slab, removing it, fixing the subgrade, and re-pouring usually runs 3 to 4 times what doing the subgrade right the first time would have cost. We've been called to do that demo more than once on driveways and garage pads where the original installer skipped compaction. The math is brutal, and the homeowner usually didn't even know it was happening underneath them until the cracks showed.
For most residential work, a heavy plate compactor or roller working in 8-inch lifts until no further compression is visible is the practical standard. For commercial or critical residential work, density testing by a soils lab is the formal answer.
You can, but it's a bad idea. Clay holds water, expands and contracts with moisture, and provides poor drainage. A 4-inch stone base between clay subgrade and slab is standard for a reason.
Soft spots get excavated until you hit firm material, then backfilled with compacted structural fill or stone. Ignoring a soft spot and pouring over it almost always results in a localized failure.
Less than for a driveway, but yes. A patio over a soft, organic-rich subgrade will settle and crack just like a driveway, just under lighter loads.
