Pile Driving-tip elevation, bedrock or cobble? 4

[Silty]

Hi All,
I am involved in a project that piles needs to be driven to bedrock. There is a 6 ft thick layer of cobbles and boulders just above the bedrock. Piles are H-piles with shoes and the QC include recording number of blows per 10". The drawings saying using Hiley formula for ultimate loads, and there is no PDA testing.
Question is:
How we know we reached bedrock?
I should mention that the contractor stopped 1 m above the geotech report bedrock level saying they hit refusal.
The second question is what should be criteria for refusal to make sure we reached bedrock and at the same time do not cause damage due to the over driving.

Thank you very much,

 

[r13]

What is the option/action when refusal is reached in a depth much shallow than anticipated?

 

[PEinc]

It depends on the depth of the minimum pile tip elevation specified by the pile designer or structural engineer. Short piles might or might not be a problem.

http://www.PeirceEngineering.com

 

[MTNClimber]

how will you know if the pile is refusing on bedrock or big boulders and cobbles? You can't.

I would disagree if you have the right conditions. If you have a tight group of piles (say 15 or 20 in a pile cap) and piles are driven to a pile tip of -100' to -103' and a couple random piles are at -94', I think you can make an educated assumption what happened. This is assuming bedrock variation is small across the pile cap and you have a boring at the pile cap (typical for bridges but not for buildings). Also, depending on the shape of the boulders and cobbles and how they are naturally seated, you might see the blow counts "slowly" creep up as the boulders/cobbles shift and find their new position, where if you drive the pile to hard rock it just literally stops within a few blows. But there are so many variables in this scenario is hard to predict what will actually happen.

What is the option/action when refusal is reached in a depth much shallow than anticipated?

That should be up to the GEOR. They may be fine if you have the blow counts and correct hammer energy. They might want testing to confirm capacity.

 

[jayrod12]

If you have uplift on your piles, sometimes early refusal can be detrimental, even if the refusal criteria is met in full. Because uplift on driven piles relies on skin friction, so shorter embedment equals less capacity.

 

[r13]

Have you ever heard pile slipped from the boulder (was suspected too close to the edge of a large boulder) and disappeared? I think it could occur.

MTNClimber and jayrod, got you. Thanks.

 

[PEinc]

An excellent point that had not yet been mentioned, jayrod12.

http://www.PeirceEngineering.com

 

[BridgeSmith]

Have you ever heard pile slipped from the boulder (was suspected too close to the edge of a large boulder) and disappeared? I think it could occur.

You mean like it's driven to refusal and then shifts under load in service? Not unless there's erosion below the pile tip elevation, or a large earthquake.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[BridgeSmith]

What is the option/action when refusal is reached in a depth much shallow than anticipated?

As long as there's not uplift capacity required, the operator gets to knock off early, and we don't have to pay them as much.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[PEinc]

..... unless the pile hangs up (refuses) on an obstruction above a known soft or compressible stratum.

http://www.PeirceEngineering.com

 

[r13]

Rod,

No, the pile hit the edge of a large boulder at a distance above the hardpan, the driving is terminated, but the pile, due to slight inclination, slipped away from the boulder, and disappeared...