Drill Concrete Pier - Bells 1

[jchi]

Designing a job on drilled concrete piers in Texas (expansive soils). What is the typical diameter (or most cost effective) and maximum bell diameter.

Looking a 12" diameter aprox. 10'-15' deep -- what can I use for a maximum bell diameter?

Jeff

 

[MiketheEngineer]

Typically the bell is hand dug. At 10' this would be difficult. Most often used in piers 3'-5' deep.

To answer your question - few inches on either side would be normal.

 

[rowingengineer]

Bells are hardly ever cost effective.

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it

 

[frv]

Bells are not that uncommon in Texas- especially in expansive clays..

The geotech report should provide you with some guidance on this, but if I'm not mistaken, the bell is usually somewhere between two and three times the diameter of the shaft.

 

[nutte]

Hand dug? Not cost effective? That has not been my experience. Rather, I agree with frv.

12" diameter is pretty small, at least for a commercial application. Would this be for a residential application?

 

[JAE]

Usually the geotechs (those I've worked with in Texas) recommend a maximum bell-to-shaft ration of 3:1.

When you get shallow, (such as your 10 feet) the bell may create an unsafe situation where the height of the bell might be susceptible to cave-in around the shaft at the surface. Also, at only 10 feet deep you aren't really getting the uplift anchorage from the bell anyway and the expansive clay uplift is also limited.

Bells are usually dug out with special drill cans that are exchanged on the Kelly bar for the typical drill "bit".
These have rotating cutter wedges that slowly extend out from the can as downward pressure is induced on the can. They cut into the surrounding ground and form the bell excavation at the bottom of the shaft.

 

[BAretired]

Belled piles are fairly common in my area. The bell diameter must not exceed three times the shaft diameter. Usually, 16" is the minimum shaft diameter, so 48" is the maximum bell in that case.

The depth of pile is usually about 20 feet, but it can be much deeper if soil conditions dictate. Often the allowable bearing pressure for a belled pile is in the order of 8,000 to 12,000 psf in glacial till.

BA

 

[rowingengineer]

I guess I have fallen for the old horses for courses. In my area I only know of a few machines that can dig a bell, this is different in Texas and other parts of the world. I shall give myself a upper cut and remember this is an international forum, so local rules of thumb don’t apply.

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it

 

[jchi]

Thanks!

3X is what I thought, but it has been awhile.

Jeff

 

[JLNJ]

3x shaft is the max because the belling mechanism fits inside the shaft (obviously) and then is flared out once the belling device is lowered to the bottom of the hole. I try to keep it under 3x so that I'm not pushing the limit of the equipment.

A 12" shaft is pretty small in diameter for a drilled pier. That's more the auger-cast type (which cannot be belled). I have not asked for a bell in anything small than 30" diameter. In fact it's been my experience that most geotechs don't like to recommend shaft diamters for drilled piers which are less than 30". Smaller shafts are difficult to inspect.

 

[JAE]

Here’s an article of one project using bells - with a photo of the bell bit:

http://www.structuremag.org/article.aspx?articleID=811

 

[ishvaaag]

And just a determination of the main characteristics of a pile with bell for anchorage in expansive clays, a port of what said in Peck, Hanson, Thornburn. Gives the required bell diameter for the case.