I've seen them specified in few soils reports through the years. Seems like that was situations where they had possible heave from clay soils. I remember calling one local pier-drilling company and talking to one of their estimators. His summary: "I hate belled piers with a purple passion."
Thanks. I live in central Oklahoma and have not seen them used in the 15 years that I've been practicing. Tried to specify belled piers when I first started as an engineer and found out quickly that drillers around here did not like the bells. Have not tried to spec since. Geotech reports do not specify bells, at least in the reports that I receive for my projects. How are the bells inspected?
I have used them many, many places in the upper midwest. Use minimum straight shaft diameter of 30". Maximum bell diameter 3 times the shaft diameter. Very common for building construction!
Here in NW Florida all our foundation piers at our site have bell hole bottoms. Most are 36" dia with a minimum 48" dia bell at 90' deep. Our soil here is sand and hard pan and the piers are setting on 100 or so ft of hard pan.
Anecdotal:
The best thing our construction people could see was the drilling company's drilling rig setting up on site at the close of a job. Several offered to buy them a trailer to camp out on site.
when belled piers are used in expansive clays, is the bell acting at a lug in the ground? How is the capacity of the uplift resistance determined? Any recommendations on texts for this problem?
Yes, in expansive clays the bell serves to anchor the pier in the ground since the clays will, upon expanding, grab the pier and try to lift it.
Most of the south Texas expansive clay projects I've worked on had belled piers with the geotechnical engineer providing guidelines on the required amount of reinforcing in the pier based upon pier diameter/surface area.
calculating the required reinforcing is not a problem, as I've done that with straight shaft piers socketed into shale or sandstone using skin friction. I am not certain how to determine the uplift resistance provided by the bell itself.
