Single pile's capacity VS Capacity of one row of closely spaced piles

[Elenh]

We have a single pile with diameter D and an estimated ultimate skin friction resistance equal to Qf and base resistance equal to Qb.
My question is: if we have one row of n=20 piles in distances equal to 1.2*D, which should be the total skin resistance and base resistance of the row?
Whould it make any difference if the piles are embedded in sand or clay?

 

[Doc09]

At 1.2D spacing I beleive there will be enough stress overlapp between the piles that a single pile behavior is not likely. By inspection, I would expect this line to behave as a strip fondation with the maximum settlement occuring somewhere near the middle.

Difference beteen Clay or Sand; for short term settlement there will be no difference, assuming both having the same modulus. However, long term settlement is more pronounced for the Clayey soil and should be looked at closely.

 

[Elenh]

Thank you, Doc09, for your answer. I agree with you as it concerns for the settlements, but my problem is the capacity of the row. Is it equal to the sum of capacities of individual pile or not?

 

[PST09]

Elenh, there is a good book called
Pile design and construction practice By Michael John Tomlinson

You can find some good information in that. Some of it is on google books.

http://books.google.co.uk/books?id=...YYnDc&dq=pile group bearing capacity&pg=PA166

the piles would act as a strip foundation rather than and block foundation.

 

[Elenh]

PST09, thank you.
In Tomlinson's book there is an annotation that "the zone of the disturbance of the soil occurs only within a radius of a few pile diameters around and beneath the individual pile, whereas the soil is significantly stressed to a depth to or greater than the width of the group".
Since we have one row the width is equal to pile's diameter.
On the other hand in German practice (din 4014) is noted that "the same procedure shall be followed in the design of axially loaded diaphragm wall panels, with the sum of pile base areas being used as the base area and the envelopping surface area as the skin surface area". A reduction factor, ?, for the point resistance of diaphragm wall panels related to the ratio Length to Thickness of the panels, is used, while the skin resistance is not reduced.
Following the consideration that the row of piles will act as a strip foundation, seems that "diaphragm wall procedure" is the best way to solve my problem, up to now.

 

[PST09]

I would agree with you there....in Tomlinsons book it calculates the width of the pile group but i would not use this approach to model your single 'line of piles'.

The German spec does sound like its the best approach. Essentially what your designing is a ground beam. Please see attached file from the Structural Foundation Designers Manual by W.G. Curtin, G. Shaw, G.I. Parkinson, & J.M. Golding. I don’t have time at the moment to look at depth to see if this document has examples of the calculations involved but it does seem to accurately model your arrangement.

PST

 

[Elenh]

I will see the attached file.Thank you PST09.