Calculating how deep to drive timber pilings (Gulf Coast Area)

[TexastoTexas123]

I am an engineering intern assigned the task of creating a spreadsheet that calculates how deep to drive timber pilings for deep foundations and I was looking for some help on the best way to go about doing this. My goal is the spreadsheet spits out the depth after several variable inputs. The company is in the Galveston, TX area, and typically works with loose, medium dense, and dense silts and sands. We are a windstorm and structural engineering company, so these piling foundations must withstand hurriance and windstorm conditions typically found along the Gulf Coast region. I was working from the Meyerhof method for calculating piling depth using [URL unfurl="true"]http://www.geotechnicalinfo.com/bearing_capacity_technical_guidance.html#deepfoundations[/url]

But I want to make sure I am headed in the right direction. This is my first post of course, and I hope that this is appropriate relative to the forum rules. Thanks in advance for your input.

 

[TexastoTexas123]

Oh, and I must add that I have been told that I can neglect skin friction on this calculation. Not entirely sure the impact of doing so, and maybe someone could shed some light on how that effects the Meyerhof formula. Thanks again.

 

[oldestguy]

A common problem in some soils is that the disturbance apparently weakens or causes excess pore pressure, removing some of the resistance to driving. However, removing that disturbance, as stopping driving, will allow normal skin friction to return, usually.

How you put that in the program is interesting. I usually specify in the specs that when computed depth has been reached, you wait to see if what I say is true and use what ever method you have for evaluating the capacity then. It may take an hour or a day, depending.

Thus, does it make sense to neglect side friction for computation? Sounds weird.

 

[TexastoTexas123]

Oldestguy,

Thank you for your response, and ideally I would like to use the skin friction variable in the calculation. So if you do have a method including Fs, then I am okay with it. Yes, that all makes sense, but remember this is usually all done in quartz dominated sand off the shores of Galveston. They compact the sand before they start driving piles, so its not like they are just driving into the soil as is. Point is, the only value I am concerned with, is how deep based soil type, piling diameter, and actually timber, concrete, or steel piling types. Is it possible to do this without taking into consideration the method for driving the piles?

 

[oldestguy]

Getting complicated now I see. I assume this is soil that likely is saturated, at least for most of the pile length, which seems more likely where piling are used. As to driving method, I would suppose you may need a few different sheets. Vibrating pile drivers and jetting first may take a different sheet than impact hammer driving.

Driving through a well compacted sand zone probably will call for assigning a higher friction there, but is your compacted zone encompassing the full pile depth? I doubt it.

My suggestion a this point is contact a well known coastal pile driving contractor and explain your situation. I find that most contractors are glad to pass on their experiences to an engineer, since many times contractors find they disagree with the engineer and this would be a breath of fresh air. I have found contractor contacts to be very valuable.

 

[BigH]

Don't forget that timber piles are tapered - the tip might be 150 mm and the butt 250 mm. You can look up Nordlund's method of computing the piling capacity of tapered piles (an older version of Tomlinson's piling book which has a nice graph). - or

http://www.timberpilingcouncil.org/manual/timber_manual/timber_manual.pdf

The following link might be useful in showing you various methods of computing piling capacity and the "accuracy" of such

http://www.fhwa.dot.gov/publications/research/infrastructure/structures/04043/07.cfm

.

I might caution about something - that is preparing a "graph" of pile capacity vs depth. I have seen this done by others and you get this - now if I drive to 20 ft I have X tons. But at 21 ft I have X+Y tons - oops, at 22 ft, I have X-z tons - piles won't work that way.

Another source is Flaate and Olsen's papers on pile driving forumulas - with corrections and the use of Gates formula.

Just some additional thoughts.

 

[TexastoTexas123]

Big H,

Thank you for your response. When I come up with something on this I will post in here, and maybe you can take a quick glance at it. I'll take a look at your links.

 

[TexastoTexas123]

Update: I realize now what the engineer here was telling me in regards to neglecting the shaft resistance (skin friction). Basically, we are looking for a really conservative figure on this thing, so if you assume that the toe resistance (end bearing capacity) is the only thing carrying any of the load, then you're more than likely going to assure you have a sufficiently strong piling in place. Any shaft resistance you can get is a plus and just allows for the slight extra piling capacity.

 

[BigH]

The shaft resistance may not be that "slight". In many computations I had done using wood piles and straight piles in "homogeneous" materials - and applying the safety factor, the "working load" was typically in the order of the shaft resistance and the 'end' bearing load was the safety factor. In your computations, you, too, will run into a host of different Nq factors to use - do you use Vesic, Terzaghi, Berentzev, Meyerhoff or ???? and the differences are huge. I've typically stuck with Berentzev.

 

[hokie66]

Not a field where precision comes into play, is it, BigH?

Agree with oldestguy that the OP needs to get some advice from an old hand in piled foundations in the particular area. This is not a task for an intern without an appropriate mentor.

 

[BigH]

Hokie66 - right on the money. it is interesting that all these "analyses" are done, yet, many of the old timers I've talked to just say - "count the blows" -

The old Timber Pile Manual from the 60s had most of its papers written by those from Louisiana - and they might be appropriate.

Some other references that the OP might be interested in:

http://www.fellenius.net/papers/204 Tapered Piles in Sand.pdf

http://www.raymondpile.com/images/Raymond Pile Brochure.pdf

(has one page giving an actual calculation)

http://www.pilelineonline.com/pdfs/IeC_GEO3 tapered piles.pdf

http://krc.cbri.res.in/dspace/bitstream/123456789/1326/1/1976-1980-87.pdf

Now we'll all be befuddled!

 

[jgailla]

You shouldn't neglect the shaft resistance in a tapered timber pile because it is excessively conservative.
Your shaft resistance will be several times more than the tip resistance in soils such as you have described.

 

[oldestguy]

Here is a copy from the Wisconsin DOT Bridge Manual. Note the set-up estimates there. Incorporating something like this in a spreadsheet may do it.

 

[TexastoTexas123]

Thanks for all the responses in this thread. This is at best going to be a rough estimate of how deep these pilings should go, and isn't applicable to the final design. It hopefully gives the contractors a better direction than just a mere guess from past estimates in the same area. We also want to account for the new FEMA regulations regarding base flood elevation, and the lowest horizontal structural member for insurance breaks. Again, this is not going to be the exact number used, simply an effective estimate.

 

[TexastoTexas123]

BigH, I have been using Meyerhof factors of Nq.

 

[TexastoTexas123]

Okay guys (or girls), this is where I am having difficulty: calculating earth pressure coefficients. I found what I was looking for on FEMA's website, but I need something better to get my earth pressure coefficients. Glad I have some Geotechnical folks involved in this thread. Any help is much appreciated. Website I'm using is posted below:

Link

 

[BigH]

@oldestguy - thanks for the chart . . . I notice that there is no adjustments for sands or sands and gravels. If one is driving displacement piles like closed ended steel tubes or timber . . there is a densification around the pile. The OP may wish to look at Poulos and Davis' Piling book which discusses these issues in detail.

 

[TexastoTexas123]

What equation is permissible for a timber piling moment of inertia? Both for round and square tips. I am using (pi/4)(.5)(Dia)^4 for round tips, and (bh^3)/12 for square tips.

 

[jayrod12]

I don't think there are multiple equations possible. It's either a circle or a rectangle. The question is whether the posts are tapered or not and which values to use if tapered.

 

[TexastoTexas123]

Un-tapered. Only solid-uniform piling material.