H-Piles bearing on shale

[cbosy]

When bearing on shale, should the bearing area be the projected overall area of the H-pile or the area of the steel or shoe? I know it depends on whether a plug forms but unfortunately it is not possible to directly view this. The shale is not considered friable with N-values typically in the range of 100 blows for 1-4 inches. The piles hit practical refusal at about 1 ft into the shale.

 

[GeoPaveTraffic]

In my opinion you would have to treat the pile as unplugged. However, either way you also should consider the skin friction for the remainder of the pile.

 

[cbosy]

It is all recent mine spoil (60 Ft) which is giving huge negative skin friction loads.

 

[VAD]

You should use structural strength of pile. The area of steel will provide a low value of end bearing. The drag load can be equated against the structural pile capacity. Otherwise you will incur a large number of piles than necessary.

 

[BigH]

Try checking out Canadian Geotechnical Journals. They drive a lot of H-piles in Ontario (Toronto) region for supporting highway bridges on shale. Also try to see about any reports from Ontario's DOT equivalent (MTC, now, I believe).

 

[gandersen]

cbosy:

I agree with VAD to use the structural strength of the pile (in other words, to use a capacity for the pile equivalent to a failure of the pile in compression). I practice in Ohio in the U.S. and we frequently drive piles to refusal in shale as you describe. The standard of practice is to refer to the compressive failure of the pile cross-section and to expect settlements that would correspond to the axial shortening of the pile itself. The ODOT website can give design guidance in this respect.

Glen

 

[cbosy]

For all your information, an HP 12x74 experienced a plunging failure at 234 tons with a resultant steel stress of 21.5 ksi (it was 50 ksi yield strength). I can't see how your design using the "structural strength of the pile" comes into play from a design stand point.

 

[ktwannabe]

Like VAD and gandersen, here is PA we typically design a HP 12x74 based on structural capacity, as well, if driven to absolute refusal on bedrock. I caught your last note about the plunging failure at 234 tons and wonder if this shale formation is known for relaxation like the Conemaugh here in PA. Did you redrive any after waiting 48 to 72 hours?

 

[cbosy]

Yes, it was redriven after waiting about 2.5 days (Fri. afternoon until Mon morning) and the capacity theoretically went up to 345 tons based on blow counts and the Wave Equation but was not statically load tested again to check this capacity.

 

[VoyageofDiscovery]

Hi cbosy,

From a structural point of view in this situation, I use the cross sectional area of the pile.

Regards

VOD

 

[VoyageofDiscovery]

or shoe.

VOD

 

[Focht3]

Was the pile pulled, or left as a production pile? I'd be concerned about buckling - although you didn't give us the embedment length. The longer the pile, the bigger the risk.

Buckled piles can seem to be quite "stiff" on redriving, yet still have a diminished ultimate capacity. Proceed with caution -



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

 

[cbosy]

The pile will not be used as a production pile. The length was 60 ft and there was no indication of buckling.

 

[Focht3]

Pull it and look at it. Let us know what you find.



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

 

[geonet]

Perhaps your hammer was not large enough to overcome the driving skin friction and drive the pile to its full structural capacity. Just a thought. Although piles are often designed based on their structural capacity there are a variety of reasons why it might not be appropriate for a particular design.