Thoughts on cast-in-place pile resistance

[bman99]

Hey,

Young engineer here, first post by the way,

I wanted to gather a few opinions on the way other industry professionals are handling cast-in-place pile resistance recommendations. Specifically, I want to know why some engineers in my local area are, in the design of belled concrete piles, providing shaft resistance values AND end-bearing resistance values, versus some engineers preferring to provide only end-bearing resistance values, not allowing shaft resistance to be included in the computation of the pile resistance.

I have read many mixed texts on the subject, some referring to long-term dissipation of skin friction, thereby transferring the load to the bell. And as well as other well known texts, such as "Pile Design and Construction" by Michael Tomlinson, who allow the contribution of both shaft and end bearing resistance values, so long as certain zones (the top 1-2 m of "fill" or superficial clay, and one bell diameter above the bell top) are discounted.

Just eager to hear thoughts, references appreciated.

 

[BridgeSmith]

According to a widely used work by O'Neill and Reese (Published in 1999; the full title is escaping me at the moment), side skin friction can be combined with end bearing, but unless the end bearing area is cleaned and inspected, one value or the other is dramatically reduced. As I understand it, it has to do with the skin friction being mobilized at a much smaller amount of movement than the end bearing. In other words, when the shaft has settled enough to fully mobilize the end bearing, the soil along the sides has 'failed' in shear and lost most of its resistance.

Since we don't want to specify the required inspection, after going through a fairly arduous calculation, we typically ended up 100% end bearing and 5% to 10% of the skin friction, or vice versa. Even with the inspection, for most soils, you have to discount the skin friction for 2 diameters of the shaft (or bell, for belled shafts) at the bottom. We also ignore 5 ft at the top for frost heave effects. For shallower shafts, that sometimes doesn't leave much effective length, anyway.

Some of what is in the O'Neill and Reese work, including the end bearing + skin friction equations, have been contradicted by more recent empirical data, i.e. the reductions when combining the resistance are not nearly as large as the O'Neill and Reese equations predicted.

 

[Geosubhtech]

According to my view, i will design the pile as fully friction pile in case of soil.
If pile terminates in clay strata; having completely clay above founding level, then friction contribution will be more as compared to end bearing so it will be friction pile.
The contribution of sand layer in end bearing is very high. but requires more care during termination. questions to be asked like, how much penetrating to the sandy layer to get the contribution ? is really the sandy strata continues along the stretch or in that area?
Rock the end bearing will be very high but same termination needs more care.

And while pile drilling also if the bottom is not cleaned properly, the end bearing will be far less as calculated in design.

So by keeping all the things in mind the design and capacity calculations should be done.

 

[HENRYZAU]

There are other pile types: driven cast-in-situ and CFA piles. The OP is after bored piles terminating in soils? The answer could be different for bored piles terminating in rock.

 

[bman99]

Terminating in soils.