Foundation cracks are caused by various things including (but not limited to),
Soil that wasn’t adequately compacted before construction. If soil isn’t compacted before construction begins, the heavy structure built on top will sink into the ground unevenly. This causes foundation damage.
Expansive soil. This is soil with a lot of clay in it. It causes problems for construction because it swells a lot when it soaks up moisture and then shrinks by that same amount when it dries out. This creates soil movement, which can cause structural problems.
Weather changes. An example of this would be a house built during the dry season on top of expansive soil. When the rainy season arrives, the soil swells considerably, resulting in damage to the home’s foundation.
Poor drainage around the foundation. Believe it or not, water is the cause of most foundation problems. Either too much or too little of it in the soil around your foundation is a recipe for trouble. Poor drainage around the foundation can cause hydrostatic pressure to build up in the ground and press against foundation walls. If the pressure isn’t relieved, the walls will eventually start to bow inward and even crack.
Soil creep. Homes built on slopes can, over time, develop foundation problems due to soil creep. This is when the soil at the top of a hill eventually makes its way down the hill. Soil creep can cause a foundation to move laterally.
Natural disasters. We probably don’t need to tell you that earthquakes, floods, and other natural disasters can cause foundation problems.
Heavy excavation next to the foundation. This cause of foundation problems isn’t obvious to most homeowners. Here’s a way to picture what happens: Imagine a beach chair sitting on the sand. If you start digging a hole too close to the chair, it will eventually fall into it. Something similar can happen when there’s heavy excavation next to a foundation. The home probably won’t fall into the hole, but the excavation can cause foundation problems.
Whether it’s from expansive soil, improperly compacted soil, or some other reason, all the above can cause foundation cracks because they all cause something called differential settlement.