Drainage correction and French drains

Drainage work is the only foundation service that addresses the cause rather than the symptom. In Blackland Prairie clay the damaging variable is moisture change, not water itself: the county soil survey describes soils that "swell when they are wet and shrink and crack when they dry," creating "such movement and pressure that walls and foundations will crack unless they have been adequately designed and carefully installed" (USDA Soil Survey of Dallas County). Those clays also drain badly by nature — the Denton County survey lists "very slow permeability" among the main limitations of its Sanger, Slidell, Ferris and Vertel map units (USDA Soil Survey of Denton County). Water that ponds beside a slab does not soak away quickly. It sits there and wets one side of the house while the other side dries.

What the work involves

Drainage correction is a sequence, and a good contractor works it in order rather than selling the last step first.

Grading comes first: soil is added or cut so the ground falls away from the foundation for several feet, then compacted, since loose fill settles into a new low spot within a season. Second is collection — gutters, downspout extensions that discharge well away from the beam, and area drains set in the low points of a yard, piped to daylight or a curb. Third, where surface work cannot solve it, is a subsurface French drain: a trench, a filter fabric liner, perforated pipe laid to a consistent fall, washed aggregate backfill, and fabric folded over the top before soil is replaced. Slope is the whole job. A drain laid flat is a buried gravel trench that holds water next to the foundation, which is worse than doing nothing.

Around a slab in this clay there is a fourth option worth naming: nothing at all on one side. Where a downspout is the entire problem, a $200 extension solves what a $6,000 trench would also solve.

Why the clay changes the design

Houston Black clay opens cracks "1.3 to 10 cm (1/2 to 4 in) wide at 30 cm (12 in) depths during dry periods" (USDA Official Series Description). Those cracks are direct conduits: a heavy August storm on a shrunken profile puts water several feet down at the beam almost instantly, and the clay swells hardest where it has been driest. That is why the goal here is stability rather than dryness. Texas engineering practice defines expansive clay as soil with a weighted plasticity index above 15 and flags sites with over 1.0 inch of movement potential for special design (Texas Section ASCE). Drainage that eliminates ponding while irrigation keeps the perimeter from going bone dry does more for a slab than either measure alone.

Local watering rules cut both ways, and one city addresses the conflict directly: Denton limits watering to twice a week and requires June–September irrigation to happen between 6 p.m. and 10 a.m., "except for hand-watering and the use of soaker hoses for foundations" (City of Denton). Fort Worth runs twice-per-week watering year-round with no sprinklers between 10 a.m. and 6 p.m. (Fort Worth Water). Drought is a documented local driver of foundation damage — an NBC 5 report on a South Arlington homeowner quoted an Arlington repair firm saying drought conditions were "the worst in years" (NBC 5 DFW).

Prices

Per-foot pricing tracks depth. A shallow exterior yard drain runs $10–$35 per linear foot, a deep exterior perimeter drain at footing level $30–$90 per foot, and a crawl space interior drain $20–$40 per foot; whole exterior jobs span $500–$8,800 (HomeGuide). DFW contractors publish $1,500–$8,000 for drainage correction performed alongside foundation repair (Level Home Foundation Repair). The interior-basement drain tier in national data is largely irrelevant here.

Reading a drainage bid

Ask where the water goes. A bid should name the discharge point — daylight at a slope, a curb, an alley — and state the fall in inches per foot or as a percentage. Ask for pipe type and diameter, whether the trench is fabric-lined, whether the aggregate is washed, and how deep the trench runs relative to the grade beam. Ask what happens to the sprinkler heads and irrigation lines in the trench path. Then ask the question most bids avoid: what is the observed water problem, and how was it observed? Suspicious signs include a French drain proposed without anyone having looked at the gutters, a perimeter drain quoted for a yard that already drains away from the house, drainage bundled into a pier price with no separate line item, and a promise that a drain will stop foundation movement. It reduces one driver of movement. Nothing installed in a yard removes expansive clay from under a slab.