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.

 

[JedClampett]

I'd explain exactly what you told us. He's probably done wood docks before and has a template to work off of (the dead engineers one page report).
As far as lateral, the wave forces are going to be small. More critical are potential boats pushing against it or drifting debris.
See the attached.

 

[phamENG]

For a small residential dock, I would be okay with (and have used) the ENR formula and others like it. I actually made a spreadsheet to run 5 or 6 different energy method/rule of thumb procedures for comparison.

These things have been built without engineering involvement for hundreds of years. That's not always a good thing, of course - I'm sure there's no shortage of failures out there that could have been prevented by engineering involvement. That said, driving a timber pile for a residential wood dock isn't likely to be a big deal. Pay attention to the detailing of the structural members - braces, beams, decking, rails, etc. Get that right, and use a conservative estimate based on what you and your boss have seen in the area and it should be ok.

If this was a commercial pier, or a boardwalk to be used for big events, or a LARGE residential dock (behind a mansion leading to a boat house that's bigger than what any of us probably live in), I'd be singing a different tune. But for a 3 or 4 foot wide pier leading to a spot to tie up a fishing boat - I wouldn't get too wrapped up.

 

[Ron]

@phamENG....exactly

 

[TLHS]

My only concern would be lack of familiarity with the area and type of work. If you haven't done similar projects what are you going to do if they hit something unexpected, or refuse early, or never hit refusal.

I agree that the design for ultimate capacity isn't that big a deal for something like this, but how do you make the judgement calls in the field?

I can make the judgement calls if I've got expected stratigraphy, maybe a wave analysis, maybe some instrumenting on the piles. I haven't done enough piles in the same place to be comfortable making that kind of judgement call blind.

edit: and yeah, I've done it with limited info with steel piles with low loads. I'm a little less concerned because you tend to have big hammers available, you're not going to break the pile, and can always just splice on some more length. Maybe it's just me and not having a history with wood piles.

 

[r13]

I would:

1) Talk to local geotechnical company, or who is familiar with the local geological formation, to estimate the depth of refusal.
2) Hire experienced pile installer.
3) Have someone, and material ready for splicing the pile, if necessary.
4) Brace the piles in both ways.

 

[SlideRuleEra]

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.

Take Jed Clampett's advice. Call the geotech and tell them that your firm does not have the experience to handle any aspect of either pile design or driving. Ask if they have a simple document (like the deceased engineer's did during his lifetime) that is thorough enough to obtain the permit, but also allows the (assumed reputable and qualified) pile driving contractor to work without further oversight. If so, problem solved; if not, then:

Ask the geotech if they would be willing to witness and document the driving on an index pile in lieu of a geotech investigation... based on successful results, the geotech allows the pile driving contractor to work without further oversight. If not, IMHO, you have two choices:

1) Tell the owner that wastefully expensive geotech investigation and pile driving monitoring will have to be conducted.

2) Find another geotech, with no guarantee that the outcome will be any better.

http://www.SlideRuleEra.net

 

[hokie66]

My first step would be to talk to the contractors in your area who build docks and piers. If there are similar structures on your lake, whoever built them would be my first contact, provided the docks look sound. Sometimes, depending on the site conditions, jetted piles can be more economical than driven.

 

[r13]

Good idea. Always talk to knowledgeable person around.

 

[Simba13]

Thanks everyone I found an engineer that works on a lot of docks in the next county over so I'll give them a call. Hearing that phamENG uses the ENR formula makes me feel better about it. I guess when I first saw it, it struck me as very odd that there is a pile driving formula that only involved energy, and didn't have any specific parameters for soil properties. I have a bit of experience using one of those black box suites (L-pile) at my previous office and they always required soil info. But I guess at this level I'm getting the feeling that experience plays a larger role than any specific formula. But just for curiosity sake, I've looked at a few of the FHWA formulas like the modified Gates equation: Ru= 1.11sqrt(e*Er)*log(10*Nb)-34 for timber piles, e and Er are straightforward but how do you get Nb (number of hammer blows to penetrate the pile one inch) or do you backsolve for Nb with a design Ru?

As for TLHS's comment. I feel exactly the same way. I'll talk to the other engineer and if there is anything I feel is beyond what I can do, I'll tell my boss and we'll tell the Client. On the note of refusal, I talked to the Client on Friday and he said they always drive them to refusal.

Anyway, I'll give that engineer a call, thanks for the advice!

 

[phamENG]

Simba - it's about application. While we should take the possibility of any injury or loss of life very seriously, we have to manage the risk with the reward. An L-pile analysis with full soil profile data will certainly give you a more reliable and accurate idea of what the piles should do. Coupled with a dynamic pile analysis (CAPWAP or similar), you can get a really good idea of how the foundation will perform. You just don't need that in this case. The way the energy equations work is to approximate the static resistance of the soil the pile is being driven into based on the force of the hammer hitting the pile and the distance through which it moves (F*d=Work=Energy). Then they use a range of empirical values to relate energy input static load capacity.

To your question about modified Gates, yes - you need to determine the number of blows per inch or per foot to reach the required capacity.

When using this, it's important to watch the hammer as closely as the pile advancement. There's more than a few pile drivers out there (especially the little rent-a-barge outfits) that will gradually reduce the stroke of the hammer to make sure the pile hits "refusal" where they want it to.

 

[Simba13]

I definitely get it phamENG, sending someone to take samples of soil at the bottom of the lake for a small residential dock to generate a geotech report doesn't seem particularly feasible. I can guarantee if we recommended our Client do that, he would just go to another engineer that could make do without it. I used to work for a larger company that had it's own geotech department, so we always had that kind of info easily available.

I spoke to the other engineer just now. He was nice enough to offer to send his mathcad sheet he uses for pile driving, and an approved plan with his notes. Based on what he said it seems like he does use some soil properties but the assumptions are very conservative and that, as we discussed, it's more a matter of experience of what's always worked in the past. But he does calculate the skin friction and everything.

P.S. there should have been a -34 at the end of that pile driving formula I posted.

 

[SlideRuleEra]

...pile drivers out there (especially the little rent-a-barge outfits) that will gradually reduce the stroke of the hammer to make sure the pile hits "refusal" where they want it to.

I did not know that, and certainly is an elegant solution to overdriving timber piles with a fixed-energy hammer. A pair of great posts, phamENG.

Edit: Simba13 - If you want to read how timber pile design / driving were performed in the past, see some of the documents on this page of my website "Treated Wood"... especially the series "Pile Foundations: Know-How" and "Good Practice In (Timber) Pile Driving". Some of the info remains relevant.

http://www.SlideRuleEra.net

 

[Simba13]

Thanks SRE, I'm reading through it.

As a follow up question to this discussion. With the Modified Gates equation the Nb parameter puzzles me a bit. I'm aiming for a capacity (allowable) of 6 kips for the worst case pile, I use a FS = 3, e =0.75 for a drop hammer, and Er = 3700 apprx. for a 250 lb hammer with a 15' drop. I backsolve the Nb to get 0.42 blows per inch to reach the required capacity. What does this number actually mean? You can't have half a blow (keep it clean fellas) so I'm not sure I understand a number of blows per inch less than one.

 

[r13]

How about the capacity is reached when the pile advances 2.38" per below (1/0.42). But, I doubt there is something wrong with the calculation, the advance seems excessive.

 

[Simba13]

Retired, maybe.. like you said, that seems like a lot, at least intuitively... I solved the Nb value with wolfram alpha as a sanity check, i kept my units in check so I think my math is right. I'll think on it a bit more.

 

[r13]

I don't think it would be the case, but double check the equation, sometimes there is hidden/embedded unit conversion inside.

 

[oldestguy]

OK where is this? If in northern climates be prepared for ice. Where there is any form of tide or other level changes, be prepared for lift in winter. Thus further depth may help, but ice lift is darn strong. My company had significant experiences on Lake Superior. On the marinas had to install ice bubblers to keep water open.

 

[SlideRuleEra]

...Er = 3700 apprx. for a 250 lb hammer with a 15' drop.

Simba13 - I got bad news and good news for you, first the bad news:

That hammer's specs are the typical specs for an "amateur" manual, drop hammer pile driving equipment. Dynamic formulas will, or course, give mathematical results... those results are absolutely worthless. I'll get back to that is a minute.

Now, the good news. Driving conditions at the lake must be "easy" enough that even "amateur" pile driving equipment can deliver satisfactory outcome... if used with care, by a Contractor who knows what he is doing (this Contractor probably meets that requirement). For "easy" driving conditions, with point bearing (practical refusal) all that needs to be done is drive the piles until they stop (but without over driving).

Back to why dynamic equations don't give useful results with a long-stroke drop hammer. The traditional dynamic formulas are based on "professional" level equipment. That is, a single-acting, fixed-stroke hammer, with a very heavy ram, falling a "short" distance, with a high blow count per minute.

A long-stroke drop hammer is slo

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maybe 6 stokes a minute, not an efficient rate of movement for advancing a pile (for comparison a "professional" hammer, more like 50 to 70 strokes a minute).

A long-stroke drop hammer has a high impact velocity, which can "broom" or rupture a wood pile if not used correctly.

If you want to get into more detail, say so. I have spent a life time (not just a career) growing up with, driving, designing, and construction management of pile driving projects.

http://www.SlideRuleEra.net

 

[Simba13]

SlideRuleEra That makes a lot of sense. This guy has done a bunch of these docks, I'm willing to believe that they work, but coming up with a calculation to prove it is a little tricky.

Even though it's bad news, I really appreciate you clearing it up, it would be much worse news to send out a calc I don't really understand.

Oldest It's Virginia, I'll take ice into account before I finish the calc, but I'm fairly certain this lake hasn't frozen over before.