footing for post

[schmism]

The guys says he watches the pressure gauge on the lift to let him know when he hit the solid clay.

this is how they drive pile for foundations. The soils engineer specs out what the movment per hit should be. once they reach that they stop

 

[CurlyDave]

Haven't been on for a bit.....

... Problem is the slab is pretty thin, so I'm going to cut a square in the slab, dig out the dirt and fill it flush with concrete to strengthen the area for the posts. I'm thinking 12"x12" about 6" deep should do it. Anybody have any spicific parameters for depth and size of the footing?

I have never seen a footing for a post base, designed by an engineer, which was less than 12" thick.

I have read a few soils reports, maybe a dozen or so, and the lowest load bearing strength I have seen for any soil is 2000 psf.

Bottom line is I think you want a minimum of 24" on a side and 12" thick.

 

[CurlyDave]

this is how they drive pile for foundations. The soils engineer specs out what the movment per hit should be. once they reach that they stop

Pilings work on an entirely different principle than footings.

A footer transfers force to the ground through simple down pressure, a piling works through friction on the sides of the piling.

 

[schmism]

this is how they drive pile for foundations. The soils engineer specs out what the movment per hit should be. once they reach that they stop

Pilings work on an entirely different principle than footings.

A footer transfers force to the ground through simple down pressure, a piling works through friction on the sides of the piling.

ummm entirely diffrent principle? ahhhhhh the ground is still supporting the load of the building.

how you transfer the load to the ground can occur though diffrent methods i agree, but at the end of the day, (how ever you do it) the ground still holds it up.

 

[CurlyDave]

ummm entirely diffrent principle? ahhhhhh the ground is still supporting the load of the building.

With footings the ground directly under the footing is under a compressive load.

With pilings, the ground around the piling is under a shear load.

These are entirely different principles and the design calculations are entirely different.

Many people are under the mistaken impression that the bottom of a piling rests on something solid and this is what supports the load. This is usually untrue, although there are rare special cases where the piling, more or less accidentally, hits a big rock.