Texas & Gulf
Coast Clays.
Across much of Texas the ground under a slab is clay that swells when it gets wet and shrinks hard when it dries. The house rides that cycle season after season. It is one of the busiest foundation repair markets in the country, and the reason is in the soil.
Clay that breathes.
Some clay minerals, smectite and montmorillonite above all, pull water into their structure and swell to many times their dry volume. Dry them out and they shrink and crack.
The Texas Water Development Board describes Houston Black, the official state soil, as a classic Vertisol that shrinks and swells markedly with changes in moisture. It covers about 1.5 million acres of the Blackland Prairie, the band that runs from north of Dallas south to San Antonio, and its very high shrink-swell potential commonly limits building site development in metropolitan areas.
Under the Dallas and Fort Worth area, the Eagle Ford Shale weathers into clay with a high potential for expansive movement, and the Taylor Marl carries montmorillonite that expands when wet. Along the Gulf Coast around Houston, thick clay deposits add a second problem: they compact when groundwater is pumped from the sands between them.
Engineers talk about the active zone, the depth over which the soil’s moisture changes with the seasons. Above it the clay is constantly gaining and losing water. Below it the moisture stays fairly steady, and so does the soil.
That is the whole game in Texas. A slab or footing that bears inside the active zone moves with the clay. A repair that stops at the clay it is trying to fix just moves with it too. Lasting support has to come from below the zone that breathes.
Dry edges, wet edges.
The perimeter of a slab sees weather first. The clay under the middle stays more stable, so the edges are what rise and fall.
Center lift
In drought the clay at the perimeter dries and shrinks first. The edges of the slab drop while the middle, on wetter clay, stays where it was.
Edge lift
After heavy rain or with water ponding at the foundation, perimeter clay swells and pushes the edges up while the middle stays put or sags.
Below the active zone
Piers carry the structure past the clay that moves with the seasons, so the building stops riding the cycle.
Water in, water out.
The clay is always there. What changes is how much water reaches it and how unevenly.
Drought
Long dry spells pull moisture out of the clay and drop the perimeter of the slab.
Trees
Large trees near a foundation draw water from the clay under one side of the house.
Plumbing leaks
A leaking line under a slab feeds one spot with water all year and swells the clay around it.
Drainage
Water that sits against the foundation after storms swells the edge it reaches and leaves the rest dry.
The driest year on record.
The drought that began in the fall of 2010 produced the single driest year in recorded Texas history. In 2011 the state averaged only 14.8 inches of rain, and new records for low rainfall were set from March through May and again from June through August.
For clay soils, a year like that means the active zone dries out deeper and farther under the slab than in a normal summer, and the movement that follows can show up for years as the moisture comes back.
Houston’s second problem.
Around Houston the ground has not only been swelling and shrinking. In places it has been sinking for a century.
Under the Houston area, layers of clay and silt sit between water-bearing sands. When groundwater is pumped from those sands and water levels fall, the clay layers compact, and the land surface goes down with them. The Harris-Galveston Subsidence District’s maps show up to 10 to 11 feet of total subsidence measured between 1906 and 2016.
After Hurricane Carla’s storm surge and flooding in 1961, local governments began studying what subsidence was costing them, and in 1975 the Texas Legislature created the Harris-Galveston Subsidence District to regulate groundwater withdrawal. On the coast, lost elevation means water reaches places it never reached before, and a foundation that was fine on day one can end up wet.
Warning signs.
On expansive clay many of these come and go with the seasons. A crack that opens every summer and closes every winter is telling you the slab is moving with the soil.
- 01Diagonal cracks running from the corners of doors and windows
- 02Doors that stick in one season and swing freely in another
- 03Stair-step cracks in brick veneer, or brick pulling away at the frieze board
- 04Gaps opening above baseboards or between the wall and the ceiling
- 05Cracks in the slab, tile or flooring, and floors that are no longer level
- 06Unexplained high water bills or a wet spot that hints at a leak under the slab
- 07Soil pulling away from the foundation with wide cracks in dry weather
- 08Water standing against the foundation after storms
Measure the swell.
On clay the questions are how much the soil can move and how deep the movement goes. Both can be measured.
Borings and moisture
Soil borings recover samples at depth, and moisture content measured down the boring shows how far seasonal drying or wetting reaches.
Atterberg limits
Liquid limit and plastic limit tests give the plasticity index, the standard first measure of how expansive a clay is likely to be.
Potential vertical rise
TxDOT method Tex-124-E estimates how far the ground surface could rise from the Atterberg limits. Some North Texas cities analyze the top 10 to 20 feet and cap post-construction rise at about 4 to 4.5 inches.
Get below the clay that moves.
On expansive soil, a repair has to reach support that does not change with the seasons, and the water around the foundation has to stop swinging the clay.
Foundation Masters
The network’s engineering resource for steel piers and driven pipe piles carried below the active zone, with civil forensic engineering behind the design. Florida CA# 30969.
foundationmasters.com →Florida Karst
A completely different failure: rock dissolving under sand and clay, and soil raveling down into it. Same rule, though. Reach the support below the problem.
Read Florida Karst →Seeing the signs?
Tell us what is happening at the property and whether you already have a soils report or a repair proposal. We reply by email.
Send a question →- Texas Water Development Board, Texas State Soil: Houston Black
- StateImpact Texas (NPR), drought coverage: 2011 rainfall and records
- North Central Texas Council of Governments, Moisture Conditioning presentation: Eagle Ford Shale, Taylor Marl, Tex-124-E, city PVR limits
- Harris-Galveston Subsidence District, Subsidence in Harris and Galveston Counties
- Arizona Geological Survey, Problem Soils: smectite swelling behavior
The Foundation Repair Network publishes general educational information about geology, soils and foundation performance. It is not an engineering evaluation of any specific property. Conditions vary site by site; consult a qualified engineer before making repair decisions.