Large settlement of driven pile

[designrider]

I recently had a driven pile (PP24.00x0.500 open-ended) that settled 10 inches. Yes, 10 inches, when loaded with a temporary load. Pile was driven to a depth of 120ft achieving 550kips ultimate capacity using the gates equation. Pile log verifies capacity. Maximum load onto pile was 220kips. Fortunately the incident didn't have crippling effects on the project as it was only a temporary works pile. Contractor has continued using the pile with lighter subsequent loads. No continued settlement has been observed. Nonetheless, the issue is disconcerting and understanding what happened would be preferred.

My initial theory is that the pile pushed through dense layer of sand into a loose density soil seam or layer below. If so, pile bearing is less reliable potentially settling more with even light subsequent loads.

Nearest test boring data (~60ft away):
0-15: Fine sandy silt, ML medium dense
15-45: Clayey Silt with some sand
45-75: Sand, SP, wet, medium dense with occasional silt seams
75-110: Silt, ML medium dense to dense
110-130: Sand, SP, very dense

Any other thoughts?

 

[motorbiketocrank]

Would be helpful to know the length of time between pile installation and the application of load and also how long it took for the settlement to occur.

First, I think it's a mistake to believe the equation along with the pile log are a confident indicator of the capacity achieved. The equation is a very approximate tool. Plus even if the equation were perfect, relying on the pile log assumes the correct energy was going into the pile. Problems with equipment, etc could significantly affect results (i.e. giving higher blows than you would have otherwise). And I've seen pipe piles where you got a small bulge in the steel near the top and it was basically just acting like a spring (i.e. blows were just compressing the steel rather than being transferred down the pile). Point is that the equation and log can be misleading. If you had monitoring with a pile driving analyzer and analysis with CAPWAP, you'd have a good feel for capacity...but not with your current data.

But lets say your log and equation are correct....what then? Well, really not sure. Could the plug that formed in the end and gave resistance during driving have stopped acting like a plug (i.e. casing slid past inside material giving basically no end bearing). Could there have been a layer that was significantly disturbed by driving and resulted in soil settlement and downdrag? Just a couple quick guesses and I'm sure there's lots of other possibilities.

 

[designrider]

Geobdg, thanks for the comments. The pile was installed about 10 months before the loading and settlement occurred. The load was applied for approximately 12 hours.

I agree with the comments concerning variability/reliability in the capacity equation. My gut feeling is that it was the result of construction error, misreporting, or equipment performance. I don't think enough time has passed for downdrag to develop to these force levels. Soil plug failure is possible.