||06-16-2007 09:11 AM
The resistance to uplift depends to a large degree on the soil type and is not that great for a such a very short pier.
There is a similar situation (reversed) for piling. The soil surrounding piling can settle. The friction between the piling and the soil pulls down and puts more downward pressuer on the point bearing piling.
For deck piers, just put the bottom below the frost line. If you really have bad, bad, bad soil, you can muhroom out the very bottom to gain to bering area and reduce and settlement. - I think Sonotube makes a special base fitting to give you a pretty formed cone.
Sonotubes are far superior to a hand dug/formed pier because of the smooth surface and predictable amount of concrete required.