Drilled pier uplift friction values

[HankTexas]

Several years ago I did quite a bit of foundation design for the structures I designed but haven’t done a lot lately. I recently designed a large structure that is being erected in Australia and I was asked to review the foundation design that was done by a local company. Since there were large uplift forces on some of the columns, they used drilled piers for the foundation. They gave two schedules that depended on the “shaft roughness class”. Apparently they were doing something to the sides of the soil to increase the friction value. The pier diameters were 30” and the depth was for example 12’ for one roughness and 30’ for the plain drilled shift. I’m used to surface uplift friction values of 200-600 psf. That value was about right for the deep shafts but worked out to be up to 1800 psf for the shallow shafts. Has anyone heard of this ‘roughening’ of the shaft sides to produce considerably more skin friction for uplift?

 

[ATSE]

You can roughen or enhance the irregular profile of the augered pier sidewall to increase the concrete to soil interface friction, but the effective skin friction "interface" capacity should not be when it exceeds the shear strength of the soil (Mohr Coulomb).
Tripling the strength seems unlikely.
Clay (cohesive) or sand (granular)?
Su or phi provided?
Do the sidewall friction values depend on effective stress (like fs = beta*sigma'), or constant with depth,or a combination?

 

[hokie66]

Are these bored piles to be drilled into rock? That might be one explanation, but I don't know. Why not have a conversation with the Australian engineer? Just remember the time difference.

 

[HankTexas]

Thanks for the replies. They give me something to work with. There have already been a couple of glaring mistakes that I’ve had to address but even though I’ve tried to be tactful with my comments, no one likes it when mistakes are pointed out in front of the customer. Now I’m not getting much cooperation and I can’t get a soils report. Since I am just reviewing the design it’s not my responsibility but it is my responsibility to point out anything I see that looks hazardous.

Thanks again for the replies. This will help.