In the practice of foundation engineering, one always works with incomplete data, since a full understanding of the subsoil conditions can never be obtained. Much is unknown about soil behavior and the alteration of soil properties with time, change of environment and loading. Risks, which have been described as "calculated risks," are selected by the engineer in the formulation of his recommendations. With each selected risk for a project, there is an associated construction cost. The owner ultimately makes decisions based on the advice of the consulting engineer who has defined the risk-cost alternatives. Obviously, the greater the risk the owner selects, the less the cost of the project, and vice versa. Neither the engineer nor the owner wishes to bury money in the ground. However, it is usually an easy matter for the engineer to be safe at the owner's expense, which is a disservice.

Successful application of the risk-cost approach to foundation engineering requires that the engineer be involved in all phases of the project to assure himself and the owner that the actual soil-structure interaction will be compatible with that predicted, and within the risk selected.

A very large part of the engineer's decision is based upon his experience in working with similar soil conditions. The results of the laboratory testing for each lot are certainly considered, but the overall soil profile is looked at. The volume of soil tested in the laboratory is very small when compared with the volume of soil that will affect the completed house, or subdivision. So the big picture must be evaluated.

Things to be considered when planning a foundation design include:

1. Results of swell-consolidation testing
2. Moisture-Density testing
3. Type of soil - i.e., clayey sand or clay, etc.
4. Depth to expansive material below conventional depth excavations
5. Depth to bedrock
6. Depth to ground water
7. Type of basement (i.e., unfinished full depth, or finished walkout)
8. Previous experience with slab-on-grade construction in the immediate area
9. Level of risk which is acceptable to you, the homebuilder, based upon your experience and warranty program.
10. Appropriate selection and placement of void materials (WallVoid™, SlabVoid™), to ensure a properly voided construction.

Keeping in mind that anyone can build a "battleship" - a very conservative engineering design. But, who could afford them? So, there has to be a balance.

Figure 10 is a comparison of swell percentages under different loads (surcharges). The comparison is based on the opinions of a number of geotechnical engineers in the area. Some firms use a surcharge of 500 psf, others use 1,000 psf. There is no standard. The surcharge loads are roughly about what would be the condition if the house was supported on a conventional spread footing - 500 psf for the garage wall and 1,000 psf for the basement.