Expansive clay soils absorb water like sponges. Just like sponges, clays swell up as they absorb water. When a wet clay in a swollen state dries out, it shrinks. How much a clay shrinks depends on how much it swelled up. Some clays will double in volume as they go from a completely dry state to a saturated wet state. That means that when the same clay dries out it will shrink by 50%! I have seen clays in Frisco, Lewisville, and Flower Mound swell up as much as six inches!
When water stands next to your house, it will be absorbed by the soils next to and under your foundation. Over time, if the soils do not get a chance to dry out, they will swell up and lift your home. You might not expect soils to be able to push a home up, but they can. When expansive clay soils swell up, they can push up with pressures of thousands of pounds per square foot, more than enough pressure to lift your home.
To damage your home, water needs to stand in one area for a long time. Sometimes, trickles of water that keep the soil wet can cause problems. For example, if your air conditioning condensate line is dripping constantly, that can supply enough water to lift the side of your home. (As a side note, the best way to deal with the AC condensate line is to have it connected to the sewer.)
To avoid problems, make sure that the soil around your house slopes away from the foundation for a distance of five feet. The water should flow to drains or sloped areas that will carry it away. Your drainage is fine if an hour after a rainstorm, no water is standing around your home.
Question: What is the best way to remove water?
Answer: Grade the surface of the ground. Gravity is free and grading requires little maintenance.
Question: What is the minimum slope that I need for adequate drainage?
Answer: The minimum slope is a drop of one inch every ten feet. Please remember that is the MINIMUM. The minimum slope can easily be blocked by leaves and other debris. A much better goal is a drop of 2 inches every ten feet for swales and a drop of 2 inches per foot within five feet of your home.
Question: What if my lot is too flat to regrade?
Answer: Then you will have to install surface collectors that connect to a drain that leads to a pump. If you are going to pump water to the street, you need to ensure that you have a big enough pump to do the job. A heavy rain can put huge amounts of water on your lot.