Slab foundation repair

A slab in North Texas rarely fails because the concrete is too weak. It fails because the ground under one part of the slab changes volume and the ground under another part does not. The Soil Survey of Dallas County puts it bluntly: the county's soils "are clayey and have a high content of the clay mineral montmorillonite. Soils of this kind swell when they are wet and shrink and crack when they dry. This action creates such movement and pressure that walls and foundations will crack unless they have been adequately designed" (USDA Soil Survey of Dallas County). The engineering tables in the neighboring Tarrant County survey give the numbers behind that sentence: plasticity indexes of 28 to 42 for Houston Black, Heiden and Ferris clays, with shrink-swell rated "very high" — the survey's own scale calls anything above a 9 percent volume change very high (USDA Soil Survey of Tarrant County).

What a crew physically does

Slab repair here almost always means underpinning: transferring perimeter load onto supports that reach below the zone where soil moisture swings seasonally. With pressed concrete pilings, the crew digs a hole against the grade beam, slides a hydraulic ram under the beam edge, and presses precast concrete cylinders roughly a foot long into the ground one at a time, using the weight and stiffness of the house itself as the reaction load (Foundation Performance Association FPA-SC-08-1). Each pile stops at "refusal" — the point at which the driving force exceeds the load being supported — so final depth varies from hole to hole. The same document warns that if a pile "cannot attain sufficient penetration into stable soils, then it may not be resistant to potential movements that occur due to swelling or shrinking of the soils in the movement active zone." Once the piles are in, the crew jacks the beam to the target elevation, sets shims (minimum 1/8 inch thick), releases the rams onto the shims, and backfills in eight-inch loose lifts.

Spacing is not a matter of contractor preference. The FPA's underpinning specification caps exterior piling spacing at 8 feet on center for a one-story wood or brick structure, 7 feet for a 1½-story or a two-story with brick on the first story only, and 6 feet for a two-story brick structure; three or more stories "should have an engineered underpinning design" (FPA-SC-08-1).

When slab repair is the wrong answer

An out-of-level floor is not automatically a damaged foundation. The ACI 117 construction tolerance for slab-on-ground levelness is plus or minus 3/4 inch, "which means there may be a 1-1/2 inch difference in elevation due to original construction," and where there is a difference in floor elevation with no accompanying distress — no sheetrock or brick cracking, no out-of-square doors — "the out-of-level condition is most likely due to original construction and not foundation movement" (Brumley, PTI technical session). Piers cannot fix a slab poured that way, and they cannot resist upward movement at all: the FPA notes a pile "will typically not resist the uplift forces to the foundation." Heave from a plumbing leak or from clay rewetting under a long-dry slab is a different problem with a different fix.

What moves the price

DFW pricing is built from pier count, not square footage. Pressed concrete pilings are published at $300–$700 per pier across regional price lists, and drilled bell-bottom piers at $800–$1,200 (The North Texas Home Guide). A moderate 5–10 pier repair is quoted at $3,500–$7,500 and a typical 8–12 pier job at $5,000–$8,500, with one DFW contractor stating its own company average at $5,284.92 (Stratum Foundation Repair). Full-perimeter work of 15–20 or more piers runs $12,000–$25,000 and up. Interior piers carry a 20–30 percent premium because access means breaking the slab or tunneling, and tunneling itself is billed at "several hundred dollars per linear foot."

Reading a bid

Ask for the pier layout with dimensions, the target lift at each pier, the refusal depth expected, and who seals the design. Most DFW cities force that last answer: Fort Worth states "all foundation repairs or new foundations for residential structures require building permits and engineered building plans" (Fort Worth Development Services), and Arlington has required a permit since November 1, 2011, with two sets of plans sealed by a Texas-licensed professional engineer and an engineer's letter at final inspection (Arlington technical bulletin). Texas does not license foundation repair as a trade; engineers are the ones the state licenses.

Treat these as warning signs: a price quoted per square foot of house, which one Texas source calls unable to account for "foundation elevation measurements, concealed framing damage, crawl-space clearance, soil conditions, or the contractor's selected repair method" (Foundation Compass); spacing wider than the FPA maximums; no stated depth or refusal criterion; and a same-day discount that expires before an engineer could read the plan.