Engineering and Construction

Figure 5.2 shows various types of mat foundations. Based on economic considerations, mat foundations are often constructed for the following reasons (NAVFAC DM-7.2, 1982):

1. Large individual footings. A mat foundation is often constructed when the sum of individual footing areas exceeds about one-half of the total foundation area.

2. Cavities or compressible lenses. A mat foundation can be used when the subsurface exploration indicates that there will be unequal settlement caused by small cavities or compressible lenses below the foundation. A mat foundation would tend to span over the small cavities or weak lenses and create a more uniform settlement condition.

3. Shallow settlements. A mat foundation can be recommended when shallow settlements predominate and the mat foundation would minimize differential settlements.

4. Unequal distribution of loads. For some structures, there can be a large difference in building loads acting on different areas of the foundation. Conventional spread footings could be subjected to excessive differential settlement, but a mat foundation would tend to distribute the unequal building loads and reduce the differential settlements.

5. Hydrostatic uplift. When the foundation will be subjected to hydrostatic uplift due to a high groundwater table, a mat foundation could be used to resist the uplift forces.