Place a concrete floor on an unstable subgrade and you'll probably experience problems. So, it makes sense to place the slab on strong and stable soil right from the start.
IDENTIFY UNSUITABLE SOILS AND SUBGRADE CONDITIONS
Investigate soil conditions before the floor is built. Needed information may include: types and conditions of soils; soil properties; water table elevation; and drainage conditions. Organic soils such as peat, muck, and organic silts or clays are not suitable subgrade materials.
IMPROVE THE SUBGRADE SOIL
Compaction improves the properties of all but the organic soils by decreasing void content. This increases soil strength and reduces the chances of rutting during construction or settlement later.
INSPECT SUBGRADE PREPARATION METHODS DURING CONSTRUCTION
For floors placed on fills, observe the earthwork operations from start to finish. Use field density tests to verify that specified density is achieved. Supplement field density and moisture content tests with visual observation.