Uplift resistance of underreamed drilled shafts 1

[eric1037]

I am having a bit of trouble determining the uplift resistance of the underreamed (belled) portion of a drilled shaft in cohesive soils.

In Fang's Foundation Engineering Handbook, a reference to Kulhawy says that "based on empirical studies for shafts with D/Bs > 10, there is little apparant uplift resistance because of the bell, and therefore the design B cand be approximated with Bs. For shafts with D/Bs less than 5, the bell influence is significant. A convenient assumption is to use a design B equal to [Bs + (Bb-Bs)/3]. For D/Bs from 5 to 10, a linear interpolation between the above limits can be used." D = Depth, Bs = diameter of shaft, Bb = diameter of bell

However, in the FHWA Drilled Shafts, Reese and O'Neil say that ther is a "Base Resistance", qmax. Their equation is

qmax(uplift)=su*Nu

where

Nu = bearing capacity factor for uplift = 3.5 Db/Bb<=9
su = average undrained shear strength of the cohesive soil between the base of the bell and 2 Bb above the base.

This qmax is applied to the projected area of the bell.

In the past I have used the Reese and O'Neil method. However, I am concerned that this is a non-conservative approach since the uplift resistance compared to the Kulhawy method is vastly different.

Can anyone give me any guidance based on their experience or the standard of practice in their area?

Thanks!

 

[eric1037]

I'm suprised I haven't received a response on this question.

Is it a stumper?

I'm still stumped.

 

[jdonville]

eric,

For my part, there is not much call for drilled shafts resisting uplift. However, let's look at the situation of the belled drilled shaft. Please note that I am not cribbing from FHWA, so bear with me.

Uplift resistance comes from 3 possible places:

1) Effective weight of the drilled shaft itself
2) Shaft friction
3) Mechanical resistance from the projecting area of the bell

1) is straightforward to calculate.

2) is usually taken to be some portion of the shaft friction resisting compressive axial loads. Thinking about how the strains work out, it seems to me that the drilled shaft would have to displace vertically to the point where the side friction resisting a downward load becomes zero and then travel upwards further to mobilize the uplift side friction.

3) is a function of the projected area of the bell. Conceptually, I think of this as mobilizing a portion of passive earth pressure on the top face of the bell. I would think that for most drilled shafts, by the time you mobilized full passive pressure on the top face of the bell, you would be risking relatively large displacements (on the order of several inches of upward displacement).

Additional Discussion

(Remember eric, I'm just talking off the top of my head here)

As the strata that are usually belled are also usually very stiff to hard cohesive soils, I have a difficult time imagining the local failure mechanism in the soil that would negate any contribution of (3). However, if the column is unreinforced, a sufficiently large load/displacement might rip the bell off the bottom of the shaft. Could this be why the "ultimate" resistance is based on Bs for Kulhawy?!

At the other extreme, at some point, the bell, assuming it stays attached to the shaft, will begin to fail the soil above the projected area - either the stiff belling soil is pushed up into a weaker stratum or the entire soil column around the drilled shaft and above the bell begins to move up (implying a cylindrical block shear failure of the entire soil mass above the bell). In this case, the uplift resistance of the drilled shaft would then be skin friction (soil on soil) the diameter of the bell all the way to the surface PLUS the combined effective weight of soil + shaft.

I suspect that I have merely rambled and not answered your question, but hopefully have provided some ENTERTAINMENT at least, and maybe some useful discussion.

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com

 

[eric1037]

jdonville:

Thanks for the response.

I have had some of the same thoughts as you. I actually think it may be more appropriate to use neither of the approaches and use one based more on passive pressures.

I guess I assume that the structural design of the shaft will be sufficient to ensure that the bell stays attached to the shaft. Otherwise, you wouldn't be able to count on the bell for any resistance (or at least only up to the point of failure).

I don't really buy the skin friction-only approach since the bell is not cylindrical in shape. I think the resistance of a bell would be much higher than an average equivalent cylinder. Plus, Kulhawy uses the friction between the concrete and soil, not the soil to soil friction (or adhesion in this case).

I tend to agree with the Reese & O'Neill method because if the bell is sufficiently deep, it should almost act as an upside-down bearing pressure. It could be thought of as a bearing pressure on an inclined plane. It would probably not apply for drilled shafts that are relatively shallow.

Maybe I need to formulate my own expression based on mobilizing a portion of the passive resistance. Sounds like a good research project.

 

[eric1037]

Here's a good paper that I just discovered:

http://gemsoft.us/Underreamed Footings in Jointed Clay.pdf

Another aspect that was reviewed is the affects of suction on the bottom of the shaft for uplift resistance.