Burnet sits on the eastern margin of the Llano Uplift, where Precambrian basement rock is exposed inside the surrounding Cretaceous Edwards Plateau. The USGS mapping for the county reflects that mix: Walnut Clay covers about 21 percent of the surface, Glen Rose Limestone about 11 percent, and Town Mountain Granite about 5 percent, with gneiss, schist and Ordovician limestone units besides.
Each behaves differently under concrete. On granite terrain you get decomposed granite, often called granite gravel, and the depth to solid granite bedrock has been reported to range from a few feet in some places to nearly 100 feet in others. It bears well and has low shrink-swell, but it erodes.
On limestone terrain you get shallow clayey soil over hard rock, and the shallow series in this region hit limestone anywhere from four to forty inches down. Rock excavation is the constraint, not heave.
In between there are clay flats that do move seasonally. We do not publish a single answer for what is under a Burnet property, because there is not one. We look.