Why foundations move in Lubbock

Start with the surface: the Llano Estacado

Lubbock sits at 3,202 feet on the Llano Estacado, the southern extension of the High Plains and one of the flattest large landforms in North America. Early Spanish accounts describe crossing it as crossing a sea. That flatness is not scenery — it is the governing fact for anything you build on it.

Flat ground does not drain. There is no fall to carry water off, and across most of the region there are no through-flowing streams to carry it away. What the landscape has instead is playas: shallow closed basins with no outlet, sitting anywhere from ten to fifty feet below the surrounding plain, averaging roughly one per square mile across the Southern High Plains. Water that falls on the plain either soaks in where it lands, evaporates, or runs to the nearest playa and sits there.

Lubbock was built across a field of them. A number of the city's parks are playa basins kept as open space precisely because they hold water, and the neighborhoods around them are built on the sloped margin between basin floor and flat upland.

Then the soil: two very different materials

The upland soils mapped across Lubbock County — the Amarillo, Acuff, Olton and Pullman series and their neighbors — formed in windblown sediment of the Blackwater Draw Formation, laid down during the Pleistocene. They range from fine sandy loam through clay loam. They are not dramatic soils. Their shrink-swell response is moderate compared with the heavy Vertisols of the Blackland Prairie, and they make reasonable building ground.

The playa floors are something else. They are mapped as Randall clay, classified as a very-fine, smectitic, thermic Ustic Epiaquert — a Vertisol. Smectite is the clay mineral that expands aggressively when it takes on water and contracts hard when it loses it. Randall clay can crack to a depth of forty inches when dry, and undisturbed playa floors develop gilgai: a mounded and hollowed surface produced by the soil churning itself through repeated swelling and shrinking.

Most Lubbock houses are not on a playa floor. But a great many sit near enough to a basin margin that the soil changes character across the width of a single slab, and that transition zone is exactly the condition that produces differential movement.

Underneath both: the caliche

Below the windblown sediment lies the Ogallala Formation, which carries indurated caliche — calcium carbonate cemented into a hard, rock-like layer. Several soil series mapped in and around Lubbock County have a cemented petrocalcic horizon within twenty to forty inches of the surface.

Caliche does not swell, and as bearing material it is dense and stable. Its influence on foundation work is mostly practical: it complicates driving piers. A pier driven to refusal on a caliche shelf has not necessarily reached competent bearing underneath it, and drilling through it takes different equipment and more time than punching through soft ground. Anyone quoting a per-pier price without asking about expected depth is not quoting a Lubbock job.

The climate that drives it all

Semi-arid, Köppen BSk, with the wet season arriving as thunderstorms rather than steady rain. The long-term average at the Lubbock station is about 18.6 inches a year. The average is the least useful number in the record.

The record runs from 8.73 inches in 1917 to 40.55 inches in 1941 — a range of nearly thirty-two inches between the driest and wettest years ever measured here. And it is not ancient history. The city recorded 8.81 inches in 2003, the second driest year on record, and then 33.25 inches in 2004, the second wettest. Consecutive years at opposite ends of a century of observation.

A house that stood through 2003 and 2004 experienced soil around its perimeter shrinking away hard, then swelling back hard, inside twenty-four months. That is the mechanism. It is not exotic, and it is not going away.

Which is why the water is the whole story

Every piece of practical foundation advice for this city falls out of the paragraphs above. Keep moisture around the perimeter as steady as you can, because the damage is done by change rather than by any particular level. Get roof water genuinely away from the slab, because a roof concentrates an enormous volume onto a few discharge points. Cut the grade back so the ground falls away from the house, because on flat ground nothing does that on its own. Water during drought — lightly, consistently, a foot or so out from the foundation — because the perimeter dries and the covered interior does not.

And measure before you spend. An elevation survey costs a small fraction of an underpinning job and it is the only thing that will tell you whether you have a real problem or a set of cracks that have not moved in fifteen years.

More on foundation inspections → · More on drainage and grading →