Drilled belled shaft piles

I think generally about 2.5 to 1... bell to shaft with the sides of the lower 6" of the bell, being vertical. The shaft had to be 30 or 32" dia, for a person to enter for 'hand cleaning'. That was common in this area.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

Depends on local practice, but bell to shaft ratio of 2 to 2.5 is fairly common. Bell shape also varies with local practice. Some times the bottom portion of the bell is vertical as stated previously, sometimes all of the bell roof is sloped. Minimum shaft diameter also various, but a min. of 24-inch-dia is needed if hand cleaning is to be done.

Thanks, Geo... I think locally (and it may be in this jurisdiction) for safety reasons the shafts have to be 30" for a person to go down, but I'm not sure... I'll check on Monday just to be sure.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

@GeoPaveTraffic on a sort of related note. Based on your experience, is there a conservative rational method for determining the uplift skin friction value for concrete cast-in-place piers without having to perform a soil test, if the soil type is known? And if so, how many feet of the top of the pier would it be applied too? Additionally, I have heard of an "alpha method" to aid in the determination of the resisting skin friction. Are you familiar with this method?

@antimatter,

There are methods, the alpha method is one. Should be able to find info on the method in any good pile foundation design book. There are also methods discussed in various FHWA publications.

As for the length of the shaft capable of developing resistance, for straight shafts, I always use the full shaft length. For belled shafts, just the portion a couple of bell diameters above bell. Many times the capacity is controlled by the cone of soil weight that can be mobilized. Doesn't do you any good to develop the skin friction if there isn't enough soil weight to resist the friction.

I actually didnt think that belled shafts were used anymore!! Thought they were resigned to textbooks. Good to know.

Not common, but are still used in these neighbourhoods... another thing like spread boreds is the 'big foot' foundation used for light loads, with excavated foundations.


Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

They still existed in Alberta, Canada when I left but not overly common. Screw piles, driven steel piles or h piles and straight shaft piles were more common

One of the first jobs I had some 47 years ago was inclined belled caissons for very high power tower lines at a Southern Ontario Steel Mill. Good for uplift. The one good thing I remembered was that it was cold outside and since the bells were 12 to 16 ft in diameter we'd ride the the bucket down to the bottom (for final clean-up) as it was 5 degC outside but 15 degC in the bell. It was a good first job!!