calculating capacity of hammer driven piles

[ketanco]

in six minute solutions for geotechnical for civil pe exam, question 72, why did we use no soil information given in the question, to calculate the capacity of the pile? For the people who doesnt have the book, it is a question that asks pile capacity, which gives the hammer drop distance per blow, , plie length, striking weight, hammer energy rating, pile weight etc.. and the only used the formula of

Q = 2E / (S+0.1)

In the question, all soils data were given too but none used. Why? They gave all gamma, Teta, and chesion for instance. But they only used above formula to calculate the capacity. Why didnt we used any soils data here?

 

[Lomarandil]

One thought -- did they give enough information on the pile profile to be able to solve the problem using a skin friction analysis?

Most of my experience has been with drilled shafts, not driven piles.. but it seems that for piles the energy approach is more commonly used in practice than test borings and calculations. I don't know what the reason is -- maybe that has to do with non-homogeneity of soil affecting driven piles more, or something entirely different. It seems the common mentality is "if in doubt, tap/retap, and see what it's good for".

Lo

 

[Badger]

Hi,

The hint was to use the dynamic driving data.

This is calculating the the actual capacity developed on site. The geotechnical data I beleive is used to determine wheter or not piles should be used.

In my limited experience the design engineer determined a number of blows required per foot to develop the needed capacity using the ENR equation, and selected a pile type and size.

This was usually around 42-48 using a simple drop hammer driver.

The last job I observed they were driving piles for for bridge abutments using four piles per abutment. This was for a remote forest road bridge, where no formal soil testing was obtained. Seven of the eight piles reached or exceeded capacity at 26 to 31 feet. One pile didn't reach capacity at about 35 feet requiring them to weld another section of pile, I beleive it reached capacity at about 56 feet.