You may save a few dollars by using an assumed bearing value, but you take on the liability and when problem arises, and it all get dumped in the court, you become neglect. If you assume a low value, it costs the client, and if you assume a high value, you get movements.
The issue with foundations is usually to much movement. Often we expect movement, and then you have a choice, build, and allow for the movement, or not build. Some of the time, we can switch foundation types, and reduce movement to an acceptable level.
All foundations move. Movement is a response to changing load, and to often, changing moisture regimes. It is limiting these movements to an amount that is acceptable that is the trick.
Our building codes are out to lunch in some respects. It calls for foundation design utilising ULS, which should never control in soils, SLS will always govern. But the building code is written by a bunch of structural engineers, who do not understand soils. They force us soils types to adapt, to take what is right and bend it to fit a wrong code.
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