Back when we were courting lots, one of the factors we considered before making an offer was the surface geology. What lies beneath your lot has huge implications for your foundation. Ultimately a geotechnical firm will come out to drill a few boreholes before you build to see what lies beneath, but you can get a hint of what’s under those weeds before you buy the lot. And what’s under those weeds could save you, or cost you, a lot of money.
The key words I used above were “state geologic survey”. These are the folks that have pulled on their hiking boots and figured out the types of rocks and where those rocks are. As a result of this geologic mapping, they’ve also put together geologic maps, colorful renderings of what lies beneath. In some cases, particularly urban areas, the geologic survey may have even put together special maps of environmental geology geared toward, among other things, building and what the geology means for foundations. Most potential homebuilders aren’t aware of these resources. As it turns out, I have a degree in geophysics and worked, in my deep dark past, at the state geologic survey for a number of years. That gave us a leg up in these geologic matters.
Ideally, you want to stay away from clay and shale. Clay typically shrinks and swells depending on its water content. Shrinking and swelling means the ground beneath your foundation actually moves up and down. This would be fine if the ground moved the same amount at the same rate, but this is rarely, if ever, the case. All that shrinking and swelling winds up cracking your foundation if you are on a slab. Depending on how the ground moves, walls may crack and pipes may break. And messing with broken pipes in a slab is an expensive and miserably messy experience. Note that not all clays and shales are the same. Some move much less than others, but most move. Solid rock is best for building.
Austin sits squarely on a geologic transition point, and this has important implications for foundations. Millions and millions of years ago, there was a mountain range here, an ancestral extension of the Appalachians, that ran through Texas via Dallas through Waco through Austin through San Antonio and then out yonder to big Bend (where you can still visit remnants of these old mountains). The mountains are long gone, eroded and buried by other sediments, but they strongly influence the present lay of the land.
When the Rocky Mountains pushed out of the Earth some 65 million years ago and again 23 million years ago, they also lifted up much of Texas up to and including the old Appalachians. This caused sediments to the east and south of the old mountain range to sluff off toward the Gulf of Mexico. This created the Balcones Fault Zone, which gives the central spine of Austin its geologic complexity (don’t worry: this fault zone has been dead for millions of years). Because of all this faulting, you can have solid rock here and, ten feet later, mushy clay there.
Fortunately, Austin has a report on environmental geology (“Environmental Geology of the Austin Area: An Aid to Urban Planning” published by the Bureau of Economic Geology. Yeah, it was published in 1976 but, trust me, the rocks haven’t changed much since then…). Included in this report is a simplified geologic map that shows the location of clay, limestone, sand and gravel, and basalt (Austin had a volcano pop off back in the good ole days!). The report also includes the more detailed geologic map; however, this requires more interpretation for a non-rock person. If your area doesn’t have such a report, I reckon you could talk to someone at your geologic survey about the rocks in the area or even try talking to a geotechnical firm to get a heads up on what to look for.
One thing we avoided when looking for lots was the Del Rio Clay. Living in a house on this clay is like riding a slow motion roller coaster, including the screaming… Del Rio Clay was a deal breaker for us. If you’re looking to build close to a creek, then you’ll probably have to deal with alluvium, sand and gravel in the creek bed, which can extend for quite a distance from the present location of the creek. Friends building a house nearby and close to the creek had to install piers 30 feet deep on one side of their house to hit competent rock for their foundation. Another think we looked for was faulting. Building over a fault can be a problem, especially in California! Here in Austin it’s a potential problem if there’s different geology on the other side of the fault. For example, limestone on one side and shale on the other can be a challenge if your foundation has to straddle the fault. Same rocks on both sides? No problem.
So what did we find under our lot, at least according to the geologic maps? Austin Chalk. Good solid fine-grained limestone that is great stuff to build on. Nice neighbors around the corner from our lot that just built a house with geothermal (more on that later…) were kind enough to show us their geotechnical report. Thin topsoil, 10 feet or so of weathered (tan) but competent chalk, and then many feet of unweathered blue chalk. It’s a beautiful sight (and not just because I did my dissertation on the Austin Chalk south of Dallas).
One thing to note: geologic maps are often interpretive. Geologists get clues of the geology from where the rock is exposed, from boreholes, and from the vegetation growing on the surface (certain plants prefer certain geology). In other words, they have to fill in the blanks by connecting the geologic dots. That means that the maps may not be accurate, especially down to the resolution of a city lot. For example, if we had seen that a fault had Del Rio Clay on our neighbor’s lot but Austin Chalk on our lot, we would have been a little nervous. That fault probably wasn’t mapped at a lot-level of accuracy, in which case we might have Del Rio Clay under our lot or, even worse, Del Rio Clay and Austin Chalk. The other thing to note is that these maps show surface geology. For example, that limestone you see may only be a few feet thick or it maybe a hundred feet thick. If that the limestone is underlain by shale, a far less competent rock (as is the case with the Austin Chalk underlain as it is by the Eagle Ford Shale), thickness is important. However, if your property is in the middle of a lot of stuff of a certain preferred geologic flavor (as we are), you are probably OK. Ultimately, your geotechnical contractor will confirm your situation.