It is common for soil adjacent to foundation walls to settle, or consolidate . When a foundation is constructed soil is excavated from the area approximately four feet wide adjacent to the wall to provide access to construct the footings and foundation wall. After the foundation is completed the soil is back-filled, but is often not compacted.
Over time the soil settles into the voids, which is called consolidation. Although the weight of the soil causes some consolidation to occurs at the deeper portions of the excavated area, clayey soils do not consolidate to the level of compaction recommended to support foundations or floor slabs.
It is relatively common to finds voids below floor slabs as the soil consolidates. It the floor slab is supported by a foundation wall or is wedged against a foundation wall, and if the width of the unsupported portion of the slab is not very wide, slabs will often remain in place with no apparent adverse effects. However, slabs-on-grade are typically not designed to be unsupported.
The garage slab shown in the photographs below remained in place for over 15 years, even though it was not properly supported for the entire time and cars were parked in the garage on a regular basis. Then one day the portion of the slab adjacent to the basement gave way and dropped about one foot. You can see the refrigerator and freezer are leaning and the white shelf that used to rest on the floor is now hanging from the wall.