Deep concrete drilled shaft in soft clay

[ClarksonEng]

An outside engineer firm is proposing a 10' diameter by 49' drilled shaft in soft clay soil. Using the geotechnical report, the axial load resistance is 266 kips. The pole supported has an axial load of 58 kips, but the foundation alone weighs 577 kips. This raised some flags for our engineering staff so we asked they provided supporting calculations, they answered with this:

"Note that the mass of concrete is displacing the in situ soil that will be removed and therefore only the net increase in mass need be considered. This net difference plus the axial load imparted by the pole can resisted by skin friction."

Is this an acceptable assumption? Other resources I've been able to find are for piles, and don't specify the type or diameter.

 

[jayrod12]

I agree with their stance. You are removing a relatively large amount of soil. Although it personally seems ridiculous to have a 10'x49' pile resist 58 kips. Is there something else at play? Lateral loads etc? If axial load alone, I would expect the pile diameter to be closer to 30" diameter which is much more reasonable.

 

[fattdad]

it's logical, but it's not dollar for dollar. Displacing soil with 110 pcf with concrete at 150 pcf is not awash. It's a 36 percent increase.

Otherwise, I'd agree.

f-d

ípapß gordo ainÆt no madre flaca!

 

[ClarksonEng]

Yes there are other loads that justify the depth. 7500k-ft moment and 89 k shear. Their supplemental calculations do show the increase in weight for the new foundation, and with safety factors it still barely passes.

Is this industry practice? Are there any resources that you know of that go into this?

 

[oldestguy]

There must be a less costly and complicated way. Has piling been ruled out for some reason?

 

[Bobby46]

I tend to disagree. The skin friction is a bond stress which would not be effected by the removed weight of soil. The base resistance is determined by a limiting value that is empirically based on load test data.

I believe strength should be evaluated considering the weight of the shaft. FHWA Geotechnical Engineering Circular 10 considers the weight of the drilled in axial resistance calculations. However, you can consider the buoyant weight of concrete which may reduce you shaft weight considerably from 577 kips.

 

[BridgeSmith]

Generally, a cap on 4 much smaller shafts would be much more economical. It takes a massive specialized drill rig to turn a 10' diameter auger.