The Hidden Threat of Austin silty clay, 2 to 5 percent slopes, moderately eroded for Homeowners in San Antonio

What is Austin silty clay, 2 to 5 percent slopes, moderately eroded?

Austin silty clay, 2 to 5 percent slopes, moderately eroded is a highly expansive clay soil formation common in San Antonio. It is characterized by a high silica content that causes it to absorb water and swell volume by up to 5.1%. Engineers value it for agriculture but fear it for construction due to its "shrink-swell" volatility.

Why San Antonio Foundations Fail

If you live in Tobin Hill, Government Hill, Midtown, or Saint Mary's Strip, your home is interacting with this critical geology. Unlike stable sandy loams, Austin clay moves.

The defining metric is the Plasticity Index (PI) of 28.0. A PI over 25 is "Severe." The soil acts like a sponge—expanding with tremendous hydraulic force when wet and shrinking away from the slab when dry. This cycle snaps concrete beams.

The "Active Zone" Depth

In San Antonio, the "Active Zone"—where moisture levels fluctuate—extends 12-15 feet deep. Standard builder piers often stop at 8 feet. This mismatch is why we see repetitive failures in 78209.

3 Signs of Austin Soil Failure

• Diagonal Shear Cracks: Extending from door/window corners (45-degree angles).
• Friable Soil Gap: Soil pulling 1-2 inches away from the foundation perimeter in summer.
• Sticking Doors: Specifically on the exterior walls relative to the center of the home.

Engineering Protocol: Correct vs. Incorrect

Neighborhood Watch

We are monitoring elevated PI readings in these areas: