Clay East, Limestone West: The Ground Under Austin Houses

Austin straddles the Balcones fault zone. To the west, thin soils cover limestone hills; to the east, roughly from I-35, the deep clays of the Blackland Prairie begin (Texas A&M AgriLife). Houston Black, the classic shrink-swell clay, has its official type location in Travis County, and the USDA calls its very high shrink-swell potential a common limit on building sites.

What is the Balcones fault zone?

It is a band of faults that runs northeast through the west side of Austin. The Bureau of Economic Geology, the Texas state geological survey, maps the Mount Bonnell fault and many smaller northeast-trending faults there. Most are downthrown to the east, with total displacement across the zone of about 1,000 feet, and the land west of them is highly dissected hill country. The result is the line you can see from Mount Bonnell: hills of limestone on one side, flatter prairie on the other.

Why do Austin clay soils move so much?

The prairie clays east of the faults are built to swell. Houston Black, which the USDA names the Texas state soil, covers about 1.5 million acres of Blackland Prairie from north of Dallas to San Antonio. Its clays are the classic Vertisols, soils that shrink and swell markedly as their moisture changes. Water enters fast when the soil is dry and cracked, and very slowly once it is wet. The state geological survey's description of Austin's clay units is blunter: weak, unstable and impermeable, generally needing special engineering designs.

Is limestone ground safe from foundation trouble?

Safer, not immune. The same survey describes the limestone units as generally stable and resistant. Houses on the west side can still move where a lot was cut into a hill and built up with fill, where a pocket of clay sits on the rock, or where runoff from the slope above collects against the foundation. Rock close to the surface also makes digging for piers or drainage slower.

Weather does the pumping

Austin's rain comes in bursts. Camp Mabry averages 5.04 inches in May and 3.91 in October but 1.96 in July, with about 11 days a year bringing an inch or more (NOAA 1991–2020 normals). Storms can be extreme: on October 31, 2013, Onion Creek at US 183 crested at a record 40.15 feet, far above its 27-foot major flood stage (National Weather Service). Clay that is cracked and dry in July can be saturated in a day.

What to do with this

Look up the soil under your lot in the USDA Web Soil Survey. Wherever you are, keep water moving away: working gutters, soil sloping away from the house, and no beds that trap water against the slab. If you plan to water near the foundation in a dry spell, check Austin Water's current watering rules first. Then read about slab repair on Austin clay or re-leveling a pier and beam house, and compare prices in the Austin foundation cost guide.