Foundation Drainage and Grading in Lubbock, TX

Lubbock has no natural drainage

This is the single most important thing to understand about foundations here, and it is not true of most American cities. The Llano Estacado is one of the flattest large surfaces on the continent. Water that falls on it does not run to a creek and then a river, because for the most part there are no creeks. It collects in playas — shallow closed basins with no outlet, floored with heavy clay, sitting anywhere from ten to fifty feet below the surrounding plain. They average roughly one per square mile across the Southern High Plains.

Lubbock was built across a field of them. A good number of the city's parks are playa basins that were kept as open space precisely because they hold water, and the neighborhoods around them are built on the sloped margin between the basin floor and the flat upland. That margin is where the soil changes character over a short distance, and a short distance is exactly the scale at which differential foundation movement happens.

Two soils, two behaviors

The upland soils mapped across Lubbock County — the Amarillo, Acuff, Olton and Pullman series and their neighbors — are loamy to clay loam, formed in windblown Blackwater Draw Formation sediment. They hold together reasonably well and their shrink-swell behavior is moderate rather than dramatic.

The playa floors are a different animal entirely. They are mapped as Randall clay: a very-fine, smectitic Vertisol. Smectite is the clay mineral that expands aggressively when it takes on water. When Randall clay dries out it can crack to a depth of forty inches, and unplowed playa floors develop the mounded and hollowed surface soil scientists call gilgai — the ground literally churning itself.

Most Lubbock houses are not sitting on a playa floor. But plenty of them are sitting near enough to one that a slab spans across a change in soil behavior, and that is a condition where controlling moisture is not optional.

What actually moves the needle

Grade. The soil against the foundation should fall away from the house, roughly six inches over the first ten feet. In older neighborhoods it usually does not, because forty years of flowerbed soil, mulch, and settled backfill has built the grade back up toward the wall. Cutting that grade back is dull, unglamorous work and it is frequently the highest-value thing you can do.

Downspouts. A roof is a large water collector, and every square foot of it drains to a handful of points. A downspout that discharges at the corner of the slab is delivering thousands of gallons a year to the one place you least want it. Extensions carrying it six to ten feet out, or into a buried line to daylight, solve a lot of problems for very little money.

Irrigation. Heads that spray the foundation, a valve that weeps, a line that broke last spring and has been running underground since. Persistent point-source water beside a slab produces edge lift, and edge lift is one of the most common repair calls in the city.

Beds and hardscape. Curbing that traps water against the house, a patio poured with the fall going the wrong way, a bed of thirsty plants right at the slab edge pulling moisture out of the soil in summer — all of it changes the moisture profile under the perimeter.

Foundation watering during drought

Uncommon advice in wetter parts of the country and standard practice here. During a hard dry stretch, the soil around the perimeter shrinks and pulls away from the slab edge while the covered soil beneath the house stays comparatively stable. That differential is what drops a perimeter or dishes a floor.

The remedy is a soaker hose run at a consistent distance from the foundation — generally around twelve to eighteen inches out, not laid against the concrete — operated on a light, frequent, predictable schedule. The objective is stability, not saturation. Erratic heavy soaking is worse than nothing, because it swings the moisture content in exactly the way you are trying to avoid.

Check your local watering restrictions before setting a schedule, and adjust it seasonally rather than setting it once and forgetting it.

Subsurface drainage

Where surface work is not enough — a lot that slopes toward the house, a persistently wet side yard, a neighbor's runoff crossing your property — a French drain or a surface drain system moves water away below grade. Done properly this means a graded trench, appropriate filter fabric, clean washed stone, perforated pipe, and a real outlet to daylight or a sump. Done improperly it means a trench full of gravel that fills with silt and stops working in two seasons.

In a city with no natural fall, the hardest part is often finding somewhere legitimate for the water to go. That question should be answered before the trenching starts, not after.

Walk your house after the next real rain

• Water standing within a few feet of the slab an hour after the rain stops
• Soil, mulch or a flowerbed sitting above the top of the slab edge
• Downspouts discharging at the corner of the house with no extension
• Erosion channels cut into the yard heading toward the foundation
• A patio, walk or driveway sloping back toward the house
• Irrigation spray hitting the brick or the foundation wall directly
• A green stripe in the lawn during a dry stretch, which often means a broken line