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A properly constructed foundation should include proper preparation of the earth beneath the footings, and if that earth is properly prepared (removal of all organic and unsuitable soils and replacement with free draining mineral soils...aka; gravel) and compacted properly, there should be no settling. Settling of a foundation is a sad excuse for what might not have been done completely and properly in the beginning.
[This message has been edited by treebeard (edited February 09, 2004).]
The requirement for free draining and well compacted gravel under foundations , footings, and other poured concrete structures provides a secure footing only if the water has a place to drain to. Picture it this way...a foundation hole is dug in a relatively clayey soil, and that clayey soil holds water. Even if the footing trench is lined with the appropriate gravel and compacted to the extent it can be, the water in the trench still has no place to drain to away from the foundation. If exposed to temperatures below freezing, it will most certainly move as it freezes. And anything built upon it will move, also. If constructed in warm temperatures and backfilled correctly, even then, if everything else is constructed correctly, the structure may move if the wet clayey soils below the foundation and footings have not been removed to natural free draining gravel. The structure will 'settle' or sink under the weight. In circumstances like this where the clays or silts or glacial tills are so deep that removing them is not feasible, the footings can be designed so as to spread the load of the structure over a wider footprint...basically the footings are much wider than normal. Instead of 2-3' you might have 5-6', or somewhere in between.