Let me begin by prefacing my experience or lack there of. I have been a Construction Superintendent/Project Manager for 16+ years and have been in and around the construction industry my whole life. I am not a Structural Engineer, Achitect or any other kind of Design Profesional.
Expansive soils do seem to be common in and around San Antonio. I myself just finished a 200 unit complex on the west side where the soils had a Plasticity Index(PI) of 34. This very high. From my experience a none expansive soil would have a PI of 17 or less. As a reference sand would have a PI of 0 or slightly more depending on the loom of the soil. Not all areas of San Antonio have soils with these conditions. There are areas where the soil is thin and mostly stone. In Amarillo where I was building previously they called this type of soil “Fat Clay” which was just a way to describe how the soil reacted when it got wet. Which is to say it expands when wet and shrinks when it gets dry. Ever see pictures of soil when it is very dry that looks Ike there are cracks through out, that is because the clay has shrunk.
The effects of expansive soils on a foundation are:
When the soil gets wet from periods of extensive rainfall, the soil will expand and push up (heave) on the outer edge of the foundation. This will cause doors that were previously operating correctly to suddenly stick shut or the doorknob strikes to not latch any longer with big gaps at the top corner of the door. You may see cracks above the doors and windows inside on the drywall and outside of the house in the facade. Depending on the type of roofing system you may see the drywall on the walls separating from the ceiling in the interior walls. If you pull the carpet/pad back or have a bare slab you may see cracks in the slab. Sometimes you can look outside at the ridge line of the roof and see a sway in the middle of the roof.
There are ways to minimize the effects of this, but I have never seen nore heard of the problem being completely cured. One way to minimize this is by over excavation of the affected soils and replace it with a low PI soil (calichi, sand, limstone) and compact to the required psi as designed by a structural engineer. When designing the foundation for expansive soils, structural engineers typically will require a post tension (PT) slab (you can look this up on YouTube or just google it). In addition SE will design a “honeycomb footing” this just means there are footings running the width and length of the slab criss crossing perpendicular to each other the entire area of the foundation. Additionally, they will sometimes drape PT cables in the middle of the footings to pull up the middle of the slab and push down the outer portions of the slab. After the fact you should slope the soils around the perimeter of the foundation to allow water drain away from the house. You may also employ soaker hose around the perimeter of the foundation to keep the soils at constant moisture conditions.
In my opinion I would not be too concerned with soil expansion, but settling is another story entirely. One way to tell the difference is to get a laser level from your local hardware store as someone else mentioned. Place it in the middle of the room, measure the height of the beam up close to the laser and then at the outer edges of the room being measured. If the measurements at the outer edge is less than at the level this would indicate that the soils are expanding (heaving). If the measurements at the outer edge is greater then this could indicate settling in the foundation. Of course there are exceptions to every rule. You should always consult with a credible contractor. As to how to tell if they are a credible contractor or not is much tuffer. I have been doing this along time and I still get fooled from time to time. My advise would be to get and CALL references.
Sorry for the long post, hope this helps.
