Timber Pile Driving 8

[Simba13]

Afternoon all,

I've been tasked by my boss to do a bit of research on timber piles for a Client. The Client is trying to build a small timber dock (residential) on a lake. We've designed and drawn up the framing for this and other projects for this client but we've noted on our drawings "pile design by others", my boss and I are structural, not geotech so we left that to someone else. The Client got another engineer (structural, geotech hybrid) to do a one page calc a while ago, using the Engineering News Record Formula saying that the barge and everything to drive these piles is A OK. That engineer has recently passed away and this guy isn't able to get a permit passed using that old calc. So we're trying to look into it, so we might be able to make a recommendation (this isn't a big town and thus geotechs aren't setting up shop on every corner). I've looked at the timber pile construction guide and it seems like a geotech report is non-negotiable since you need the SPT results or undrained shear strength to get a capacity. I'll probably call the one geotech in town and talk it over but I'd like to at least have enough info for a basis of a conversation. So have you had any experience with this sort of thing? What are the main design concerns? I'm in a low seismic area, are lateral forces a consideration here (very small dock projected area)? I imagine I would need to make sure bearing and skin friction aren't exceed when loaded, but also that skin friction isn't exceeded through buoyancy when unloaded right?

Any tips or tricks of the trade would be very much appreciated.

 

[r13]

I've looked at a few of the FHWA formulas like the modified Gates equation: Ru= 1.11sqrt(e*Er)*log(10*Nb) for timber piles

Simba13,

Per the linked paper, your equation is off. I don't know if it affects your calculation or not. Link (p.5)

Modified Gates Equation (Olson and Flaate): Ru= 1.11sqrt(e*Er)*log(10*Nb)- 34
FHWA-Modified Gates Equation (USDOT): Ru= 1.75sqrt(e*Er)*log(10*Nb)-100

 

[SlideRuleEra]

Simba13 - In this case, instead of calculations see if you would be allowed to have the Contractor drive an index pile, at this project site, using this equipment. (I mentioned an index pile the other day but did not explain).

An index pile is the very first production pile. Driving is fully and carefully monitored (most likely by you or the geotech, preferably both). Keep accurate, thorough records on blow count, time, and pile embedment... for the entire driving process. Doing this over water may be a little tricky, but it can be done. Results are used a baseline for driving the remaining piling. Submit the results as required... a little late, but better than meaningless calcs.

Index piles are not often used, but they are a good supplement to test piles. An index pile calibrates a Contractors chosen equipment to a project site. A test pile may or may not do that. Contractors sometimes do this themselves (driving a temporary index pile outside a project's footprint), at their own expense, if no test piles are required. Can be performed quickly (maybe an hour or two), the Contractor gets an idea of what to expect for production piles, and the Contractor gets his (temporary) index pile back. Often a temporary index pile is painted like a zebra, with marks every foot of length all around, for easy visibility and monitoring during driving.

http://www.SlideRuleEra.net

 

[Simba13]

retired13 Yep, I realized when I posted it, I forgot the -34 at the end. But I'm using it in the calc.

SlideRuleEra Okay, I talked to the dock guy but I think at this point I'll call the geotech in town and see if they have any experience monitoring pile driving. Thanks!

 

[Simba13]

Quick follow up on this conversation.

SlideRuleEra You mentioned the differences between an amateur hammer and a professional level one (the one I'm looking at now is the montana 1500E). In your experience, do the results of the modified gates equation make realistic sense for this type of pile driver?

I'm also getting what feels like an unrealistic answer for the Janbu formula when I run this hammer. I think the issue is that despite the hammer being much heavier than the amateur one, since the drop is so small, the energy ends up being less. Have you used this formula before?

 

[SlideRuleEra]

Simba13 - I took a look at the Montana 1500E. That is what I might call a professional "miniature" hammer.

Note the blow rate: 400 to 800 blows / minute. To achieve that rate, the unit has to be "double-acting"; i.e. for this machine, the hydraulic system forces the ram both "up" and "down". That is a perfectly acceptable way to drive piling, but none of the common dynamic formulas (including ENR, Gates, and Janbu) apply to double-acting hammers.

The dynamic formulas were originated strictly for "single-acting" hammers. The hammer's power system lifts the ram "up", but gravity is the only energy source for "down". Thanks to Sir Issac Newton, energy output of a single-acting hammer is totally predictable.

A ram having a short drop is exactly the hallmark of a professional hammer. It seems counter intuitive, the most efficient way to drive a pile is to have a very heavy ram impact the pile at the lowest velocity practical. A couple of examples:
A common professional hammer is a Vulcan #1... 5000 lb ram falling 3 ft = 15,000 ft.-lb.
A larger model is a Vulcan 020... 20,000 lb ram falling 3 ft = 60,000 ft-lb
There are other types and hammers, and more to hammer selection, but that's the basics.

Bottom line, IMHO, a Montana 1500 should be fine for driving "small" wood piles to practical refusal in "easy" driving conditions, but the dynamic equations are of no value.

http://www.SlideRuleEra.net

 

[Simba13]

SlideRuleEra

That clarified a lot, thanks for taking a look at it. Glad I asked you rather than chasing my tail for a while. So how would you go about proving to the reviewer that the Montana can get you to the required capacity? An index pile? Or wave analysis (never done this before, just read about it)

Again thanks for all the advice.

 

[SlideRuleEra]

Simba13 - Start by asking the Contractor if he has any driving records from similar nearby jobs using the same hammer. Maybe the reviewer would accept those records... but chances are the Contractor does not have suitable records.

I would not spend money on wave analysis... probably would not provide usable data from a "post driver" anyway.

That pretty much leaves an index pile. Let the Contractor make all decisions on when to terminate driving of a pile. Just observe and record the results. Despite wider use of pile driving analyzers (PDA), driving wood piles to refusal remains more art than science... wood piles, unlike steel or concrete are easily damaged by even a few seconds of over driving. With a "post driver" probably not much chance of over driving, but don't take the risk.

http://www.SlideRuleEra.net