Concrete settles for a variety of reasons. One of the most common reasons result from the excavation of a basement.
When a basement is excavated, a larger area than the actual basement size is dug out. Excavation
is done to give the workers room to build the basement walls. After the walls are up, you are left with a hole around the basement walls. Workers first fill it with one foot of stone on the bottom (on top of the drain tile) and then backfill the rest of it with the same ground that they previously dug out. This ground isn't compacted, however, and tends to settle in the years to come, taking sidewalks, driveways, and patios down with it. This is why you will tend to see most settling occur near a building.
Settling due to poorly compacted ground also occurs when concrete sits on land that was previously landscaped to a different form. Excavating ground to put in water and sewer pipes will create the same effect.
Settling may also occur if plugged gutters or inadequate downspouts allow for the washing away of soil underneath concrete.
One last reason that you see settling, especially here in Wisconsin
, is the freezing and thawing of ground. This can cause the concrete to heave. On occasion, the concrete will stay settled after Spring arrives.