Design of Friction Piles

[geomtl]

I have a site with 20 feet of very soft silty sands with thin interbedded clay layers (N values of 2 - 4), followed by a compact becoming dense to very dense silty sand with trace gravel. This strata extends to 100 feet where we stopped drilling, assuming we would go with a friction type pile and not a standard end bearing to rock or till.

Water table is at 3 foot depth.

I have determined that the site will not support standard spread footings without undue settlement, and was going to look at the ability to use driven piles. As we normally have rock at less than 50 ft I do not have a lot of experience in designing piles based on friction and end bearing (i.e belled, or even screw type piles.)

The building is a large sports complex with a slab on grade and large spans.

My questions are this, what is procedure I should use to analyse the pile type required given my SPT information, and what are the types of piles that might be appropriate in this situation.

Thanks in advance.

 

[DRC1]

Pile design is as much art as science and tere are not single right answers. I use unisoft to design piles, however there are many metods out here. J.E. Bowles Foundation Design Text gives a good primer on designing piles. Some options are timber piles if your loads are less than 30 tons; Closed pipe up to 14 inch works well. I like taper piles;H-piles work well in dense sand also. I would talk to a couple local pile drivers to get their ideas about the site. I'm sure you would like to do it in house, but I would really sugest getting a little help from an engineer who is familar with pile design.

 

[fndn]

The following link contains information on the design of deep foundations. You should have your final design reviewed by another engineer with pile design background.

http://www.hnd.usace.army.mil/techinfo/UFC/UFC3-220-01A/818-02.PDF

 

[SlideRuleEra]

As DRC1, has stated there is still a lot of "art" to pile driving. The displacement pile types mentioned are typical choices - you want to maximise pile surface area to get a signicant friction support. The H-pile recommended above are usually a good choice, too. The soil contained between the flanges make the section mimic a displacement pile.

A common technique for a large project is to have a preliminary contract to drive a series of marked index piles (say 4 to 10) on the job site to determine pile length and driving conditions. If they will be heavily loaded, perform load test(s) on selected piles. Both the pile type (timber, concrete, steel, etc.) and the driving equipment should be as close as possible to materials and methods intended for production piles. The index piles are often extracted after the test program, but may be abondoned in place and (if successful) sometimes incorporated into the work.

http://www.SlideRuleEra.net

 

[SlideRuleEra]

Forgot to mention, you may want to go to my website (link below) and look at "How To Determing Pile Depth of Embedment" and some of the other (old) ways.

http://www.SlideRuleEra.net

 

[BigH]

I think that you have a lot of choice on the types of piles you can use. Timber piles would be a good choice - but not likely acceptable for the project at hand (not because they would not work but because of perceptions of the design engineer). Other types of displacement piles, I believe, would be the best choice. We drive a lot of closed end steel tube pipe piles. They work fine. Monotube piles would also be a good choice ( monotube (at) raex(dot)com ). Miller Park in Milwaukee was built on the (DFI' Deep Foundations Magazine, Spring 2004, page 67)
But, my likely choice would be the Franki-pile (expanded base piles) as used extensively in the New Jersey area with similar sounding soils. Almost all the casinos in Atlantic city are so founded.
I humbly beg to differ with my friend SRE about the use of H-piles - viz., non-displacement piles. I have seen them driven "forever". One case in point was in Richmond BC. They never seem to "set up". A very experienced structural engineer of great repute once had them driven to over 150 ft - as they never stopped. Chellis' book (original pile bible) talks about this. In such cases, you could put flanges between the webs in order to give some "bearing".
A great book to check out would be M.J. Tomlinson's Pile Design book (Viewpoint Press). Much better than Bowles. Tomlinson was/is with Wimpey and has a wealth of practical experience.
The bottom line is that you don't have to found on rock or till to get a good pile.

 

[geomtl]

thank you very much for all your suggestions. I will look into the problem some more and offer some feedback when I get it setup, and of course this will all be reviewed, my director has extensive experience in these types of problems, I was hoping to go to him with a bit of direction first.

the reference material as suggested is much appreciated also.

regards.

 

[tocoadog]

If you go with a driven foundations, you can download a freeware program called "driven" from FHWA. I don't have a link, but I have the program and use it regularly.

 

[Panars]

Here is the link to Driven from FHWA

http://www.fhwa.dot.gov/bridge/geosoft.htm

It's a pretty good program. However, be careful of using the option to just use the SPT blow counts if you have very fine sands or silty sands. And I think the format of the output is poor.

 

[BigH]

Everyone always seem to talk about "programs". Why not do it the old way - using the formulii, plugging in the values for the various variables yourself? Especially if you don't have a lot of experience, doing it the old fashioned way can help you learn and see what differences each makes. Get proficient - then use the programs - for sure they have their place. But, sometimes we seem to be "searching" for the black box and black boxes give me pause. There is nothing wrong with it; just use programs with good judgment.

 

[GeoPaveTraffic]

I agree with BigH, if you can't do it by hand, then you shouldn't use a program to do it.

 

[DRC1]

I agree that there is a lot to be said for doing it by hand, especally if you are trying to get experience. The difference in H-Piles points out the argument that it is as much art as science and that there is no single right pile for a job. I have seen H-Piles fetch up quickly in fine sands, and in other places they have driven pretty far. A lot is to be said for local experience.

 

[jdonville]

BigH,

Please don't knock the programs - DRIVEN, for example, incorporates accepted procedures required for use on highway projects eligible for federal funds. As such, these programs may not be entirely accurate or flexible, but will get one into the ballpark of the desired answer. Naturally, engineering judgement is required, and a hand check - often made more practical by using a spreadsheet - based on elementary theory is a good idea. Also, as Panars has noted above, the output format from certain applications can leave much to be desired.

geomtl,

I am personally very reluctant to specify driven H-piles in friction applications, due to the low displacement of the cross-section. This can lead to poor fetching at the toe of the pile.

Having just (literally) returned from a short course on augered cast-in-place and drilled displacement piles, I believe that these types of piles might be very effective in these conditions.

Hope this helps,

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com

 

[eric1037]

geomtl,

A great reference is Pile Foundation Analysis and Design by Poulos & Davis. Also the NAVFAC DM7.02 provides easy solutions.

jdonville,

It was a pleasure meeting you at the DFI conference.

 

[BigH]

jdonville - I am not knocking computer programs. This is the day and age of them and even this dinasaur uses them - if I can get ahold of them. What I am suggesting, though, is that many will spend more time looking for "a" program than looking at the practical solution to the problem at hand. For experienced geotechnical engineers as yourself and many others of our forums, one can distinguish whether or not the GIGO syndrome is applicable. But the young engineers who don't have the experience or are looking for a program without understanding it or confirming validity under assumptions, etc. might be lead astray. The opening gambit of this thread was that he usually has rock to drive the pile to; now he doesn't and what to do. In this case, it seems reasonable to me for him to look at the hand-methods first, to get the ball park. Then, fine with the computer programmes. In any event, judgment is needed at the end of the day. Is one going to report an allowable capacity as 66.34 tons because the computer program tells him that? Or will he go with 65 tons. That is a question to be considered as well.
As for Poulos and Davis - I have it and it is a great book but I think that Tomlinson's would be more practical and reasonable for the question at hand. It also has better and more detailed example problems.

 

[VAD]

Good discussion. Can we obtain the sssssssspt blow counts within the compact to dense sand. In my opinion Hpiles or steel pipe piles closed toe or open toe would be suitable for use at this site. In deed one can get piles drifting to deep depths. One should not design such piles in the present conditions for reaching a particular set etc but rather to examine the capacity on static analysis to deterine pile toe elevation. Irrespective of if pile is still moving I would and have on nunerous sites left pile at the depth based on static analysis. Remember driving etc often wekens soil temporarily and soil condition soon reverts to original condition esopecially is sandy soils. Vesic has shown this in his work on pile foundations and one has to consider if the entire load will be taken by the pile in a short time after driving.

We ofter have a FOS of 2 on piles and yes ther is as much judgement as anything that goes into such designs, the cheief of which is familiarity with you area;s geology and past work.

There is still alot to be learned and I agree with Big H the computer solutions despite how admirable are only good to give comparisons, the true understanding of what to do depends on firts can I get my pile to a certain depth, this should be one's first attempt at understanding pile driving and the strata that one is working with.

I have to go - beer time

Regards

 

[jdonville]

eric1037,

Likewise.

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com

 

[vulcanhammer]

All of the above--NAVFAC DM 7.02, DRIVEN, and the FHWA manual on driven piles--can be found at

http://www.vulcanhammer.net/download/

This site also has a calculator for pile capacity, but it's restricted to academic use (uses the Dennis and Olson Method.)

 

[TerraSouth]

Exellent post Gents,

I'm learning alot!

Another excellent reference:
PileBuck

http://www.pilebuck.com/books

Very good text with examples.

Cheers