Pile skin friction during an earthquake?

[dirtsqueezer]

I have a question about a project I never felt comfortable with.
A few years ago, I inspected pile driving for a building built entirely on timber piles, a number of which simply didn't meet the criteria upon initial driving. It was only after we ran into this shortcoming that the engineer agreed to re-driving the piles- it just seemed flimsy. If the engineer was competent enough to recommend pile criteria in the first place, why didn't he include re-drive criteria along with it? Not only that, but if competency depends 75% on skin friction, what happens when the case of an earthquake, when those particles are put in motion? I was in good-old-boy territory over there, so that may have had something to do with my questioning. Is this all due process for piles engineered mostly based on skin friction?

 

[Focht3]

Where was the site? What were the soil conditions? Groundwater conditions? Pile size (length)?

The only condition that I really worry about friction in earthquakes is when the soils are 'cohesionless' or 'semi-cohesive' and may contract under shearing. Liquefiable silts and fine sands are the principal problem soils, at least in my view. I guess that I would add quick clays to that list, although I wouldn't design them for friction in an earthquake prone area. Heck, they couldn't be counted on for much friction anyway.

If the sands are dense and dry, I don't worry at all. If the soils are moderately strong clays, ditto.

What is the failure mechanism that concerns you?



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?"