Caisson Pile with rock socket

[PT999]

Consider a caison steel pile, concrete filled with a rebar center core running from top of pile to bottom of rock socket.

The steel casing extends a small distance into the socket, after that, only concrete and rebar core.

Design Capacity
My view is that the capacity is limited to the allowable capacity for concrete plus allowable capacity for rebar stress, but not the casing, because right at the point that the casing ends (at the top of the socket), only concrete and rebar is working.

Finally, the socket depth is a function of allowable adhesive stress x perimeter of concrete x rock socket depth.
Socket depth is a separate limiting factor, not related to pile capacity stated above, except that it has to be the same or greater

Others have suggested the pile capacity includes adding the casing capacity to the concrete plus rebar.
Are they right?

 

[Michel60]

Good afternoon PT999,
I haven't had the occasion to need to include the casing as a part of the design capacity. For your application how much capacity, relatively, would it add?

In my area casings are provide by the driller on an "as needed" basis. I'd be sticking my neck out a bit if I assumed there was a continuous casing present. Use of a casing pretty much a given for you?

regards,
Michel

 

[FixedEarth]

The geotechnical capacity will most likely control. Your geotechnical engineer can provide you the required rock socket depth for axial load and the embedment needed for shear and moment loading. If your piers are spaced 6B or more, there is will be additional reduction due to group efficiency.

 

[PT999]

Steel casing is continuous, and would add alot of capacity, but even though I have seen others include its value additive to the concrete and rebar, I dont think it is correct.

Depth of rock socket is not in question.

 

[Michel60]

I think I'm on your side with neglecting the casing, in part since it just hasn't been my experience to include it and in part since it would seem to bring along a lot of unnecessary assumptions about how the system all works over time, for example what happens if the casing corrodes? I've been of the mind that foundations just aren't the place to get overly aggressive. They are really difficult and expensive things to repair. My vote would be to add more rebar if you need it and let the casing be primarily a concrete form and provide reserve capcity should it ever be needed.

regards,
Michel