PIle Group Densification 2

[kapilgupta]

We are consultants for a pile driving project where contractor is driving 100 feet long piles in dense sands. Pile caps have usually 8 to 12 piles in it. How do I find out it Pile group densification is happening. some of the pile caps have uneven embedment of piles within the pile cap. Even though the contractor is driving piles radially outwards, piles driven later in the pile group are sticking 10-15 ft above the initially driven piles within the same group. I would really appreciate any suggestions.

thanks

 

[eric1037]

This may sound simple, but try doing some soil borings. It should give you an indication.

BTW: What type of piles are being driven? H-Piles, Pipe piles, etc.

 

[jdonville]

kapilgupta,

I just returned from the DFI Conference in Chicago. A paper was presented there by Emre Beringen, a Ph.D. candidate at the University of Wisconsin-Madison on the stiffening effect of driven piles. His research suggests that there is a significant stiffening effect out to 8-10 pile diameters.

If you wanted to measure the amount of densification you could perform before and after CPT probes.

If your contractor is meeting the refusal criteria with shorter and shorter lengths of pile, then you almost certainly have densification happening, although other explanations are certainly possible.

I suspect that you are driving tapered or pipe piles. I would also suspect that the effect might be less pronounced with H-piles.

The case history presented in Appendix C of USACE EM 1110-2-2906 might be helpful to you in this instance. You should contact DFI or Mr. Beringen directly if you wish to obtain a copy of the paper recently presented in Chicago.

Good luck!

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com

 

[kapilgupta]

We are driving 14 inch square Precast concrete piles. Geology is varying throughout the site. in most cases,We are driving piles in dense sands. in some cases, piles are also bearing in deep potomac clays.

thanks jeff, I will contact Mr. Beringen to see if I can get a copy of that paper.

thanks
kapil

 

[jdonville]

kapilgupta,

Also check out the case history in the USACE manual. The fact that you are getting early refusal could be due to denser soils that were not adequately defined during the site investigation. Depending on your response (i.e. cut off at finished elev, pull out and redrive, or let sit and restrike, etc.) you may want to amend your load test program.

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com

 

[fndn]

Usually the spacing of piles comes to play-whether the pile to pile distance is 2.5 B or 3 B. Physically this is what happens-Your first pile is not surrounded by soil that has been densified by the vibrations of pile driving. However, as more piles are driven, more area is densified and future piles have to be driven harder to reach desired elevation. On my last pile job few months back, I only predrilled a 5 foot hole on the first pile and 10 foot on the 2nd pile and 15 foot on the third and so on in pile groups of 4. This got me close to only replacing the driving cushions once on each pile, which increased the efiiciency of the contractor and avoided cutting deformed piles.

The following might help in more detail.

http://www.usace.army.mil/inet/usac...2906/a-c.pdf#search='pile driving efficiency'

 

[kapilgupt]

thanks Jeff and fndn. the link you send is very useful.

 

[fndn]

Sometimes the pile contractor's foreman will give you great advice on what works, remember they do this day in and day out. So when they walk to you and ask how much should they predrill before driving, I usually let the foreman pick a number and just watch. Before the driving starts, I also look at N values, then at anticipated loadings, then at pile geometry and can usually gauge if it is overdesigned or way overdesigned. The soil profiles always vary tremendously and this is the reason for the necessity of overdesign.