Belled Caisson Construction

[SirAl]

For belled, end bearing caissons, I am interested in obtaining opinions concerning the taper angle from the shaft (at the top a of bell reamed at the base of a pile boring) to the bottom (bearing surface)of the bell. The soils of interest are clays and clay tills with working end bearing capacities of 200 to 500 kPa. I have seen various structural details showing differing tapers. Belling tools used by most local contractors will produce a taper of about 1V:1H to 1.5V:1H when the wings are fully extended.

 

[cbosy]

Here in Chicago, it is typically specified that the bell diameter can not exceed 3 times the shaft diameter.

 

[PEinc]

FHWA-HI-88-042, Construction Procedures and Design Methods, 1988, Pages 39 & 40, authored by Lymon Reese and Michael O/Neill, says the maximum bell diameter should not be more than 3 times the shaft diameter and that the angle of the bell's sides (underream angle) is usually 45 to 60 degrees below horizontal in order to control tensile stresses in the bell.

 

[Focht3]

The 3:1 rule addresses two problems: shear failure of the unreinforced concrete in uplift (as indicated by [blue]PEinc[/blue]) and shear (punching) failure during compressive loading as well. Remember that the bell ("underream&quot is not reinforced.

In my practice, I recommend a maximum 45 degree angle (usually 3:1 bell:shaft ratio) unless I am concerned about the bell collapsing. Under those circumstances, I will limit the bell to a 60 degree angle - or recommend the use of straight shafts.



Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[BigH]

Just a point - sometimes, the bell is reinforced if the bored caissons are primarily tension piles. First job I did in Ontario was inspection of battered bell caissons for very large power line towers for a major steel mill. Cages for the caisson went all the way to the bottom and steel rebars were then fanned out from the cage into the bells.

 

[SirAl]

Thank you all for your valued comments.