Increased Bearing for H-Pile

[dirtdobber]

Chellis is recommended plates be welded 8-10' from pile tip to increase bearing in sands. Why not just weld a plate across the tip? Are the upper plates desired over the tip plate so the pile will drive straight? I have a problem in which design engineer estimated pile length at about 40' in clayey sand; we drove 65 and 100' and still do not have required capacity. We are not waiting 10 days to restrike. However, I would like to shorten length of piles in adjacent foundation and had thought of plate at tip.

Thanks,

Dirtdobber

 

[SlideRuleEra]

dirtdobber - I'll offer some comments, for what they are worth:

Based on your description even the 65' to 100' piling are NOT point bearing.

Download the book "Bethlehem Steel H Piles" from this page of my website:

http://www.slideruleera.net/Steel-Piling.html

See "Section 3, End-Bearing Piles" for a description of how point bearing piling behave.

Also, take a look at "Section 4, Friction Piles".

Whatever loads the piling can support sounds like it is coming from skin friction. For practical purposes, a steel plate at the point would turn an H pile into a displacement pile and destroy what skin friction is currently available, see page 42. Also, a steel plate at the point may not even survive the (impact) driving process. Damage to the plate would likely distort the pile, too, making things worse.

IMHO, taking steps to increase skin friction is worth investigating, see "Section 10, Increasing Capacity With Lagging".

You are doing the right thing to look at various ways to address the existing conditions... the design engineer may have just been wrong and it is now time to move on.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[BigH]

Th\anks SRE - I think that you hit the point correctly about the lagging - which is along the lines of Chellis' remarks. The question I have, though, is more philosophical. Why are they driving H-piles in sand deposit(s)? It is like pushing a knife through butter. It has always been my contention that displacement piles are the way to go in sands as the driving will offer increased bearing and friction as the sand is displaced by the pile. Tapered piles would be better than straight-shafted piles. Expanded base piles, in my view, are the "cat's meow."

I was peripherally involved in the same situation as the OP a long time ago. The designers came to me after they found the H-pile was being driven with no "set-up". I looked at the situation and determined that they drive to a particular "set" - by Gates formula - or to a specific depth (by geotechnical principals) whichever was less. Had they used a displacement pile, there would have been no problems.

 

[Jalthi]

My 2 cents.
If the groundwater is located at a shallower depth, there is a chance that the sandy soil liquefies during pile driving due to excessive vibration. If this is the case, you may have to wait until the excess pore water pressure dissipates and soil regains its strength.

The PDA testing is also an important factor. Make sure the firm running PDA is experienced enough on similar soil conditions. Otherwise you can get any results you like with PDA.

 

[DRC1]

Agree with PDA testing. Static analysis is based on many assumptions. Capacity may be based on hammer formula or wave equation which involves different assumptions. Thus there may be a disconect. This is especially true if the blow count you are driving to is based on hammer formula. Use PDA, drive to 40 feet wait 2-3 days and restrike with PDA. 10 days may be needed, but it seems like a lot. Then drive the rest of the piles to the tested pile criteria. Test a few along the way.

I have never seen plates welded anywhere but at the tip. Pipe piles and taper piles work well in sand as do timber. Prestress also works well in sand

 

[BigH]

PDA wasn't "readily available" back in 1986. It was a small job - bollards for ships docking at a river port for offloading imported cars. For 15 or so piles to be driven, waiting 10 or even 2 days would have been a bit much. In and out. Criteria given seemed reasonable at the time; still does. For lateral loads - we provided "full" crushed stone granular fill within the passive resistance zone.

 

[dirtdobber]

I would like to thank each of you for your comments. Restrike of piles at 10 days indicated they had required capacity. Subsoil profile at opposite bent is very similar to that at this bent.

For contractual reasons we are going to drive piles at opposite bent to 65' and restrike after 10 days...anticipating same results obtained at this bent.

I plan to share your comments with original designers.