Lab tests and possible contamination? 2

[DaveVikingPE]

Any assistance/thoughts/etc. appreciated, re:

Boring logs indicate silty sand and sand from the surface to about 15 ft, thence a very soft, fat clay all the way down to the bottom of the bore hole, +50 ft.

Triaxial tests of sample taken from, according to test report, 25+/- ft, sure do look like a clayey sand. Two of the Mohr's circles look like a sandy clay, but the third skews everything, giving a high phi (I expected phi = 0 or less than 10 degrees at most) and higher cohesion values (expected c to top out in the 500 psf, results are above 1000 psf).

Unconfined compression for another sample, from 40+/- ft down the hole behaves like a soft clay.

My gut instinct is to toss out the triaxial test results, look to the unconfined compression test and rely on conservative book values based on what the boring logs tell me is actually in the ground. This is for a timber pile foundation.

Thanks in advance for any responses!

 

[BigH]

Dave - too bad they didn't do any field vanes or miniature vanes in the end of the thin walled tubes on extracation from the ground. That would help considerably. Maybe your clay isn't as "soft" as they describe - what evidence do you have it is "soft" - your testing doesn't support it - so how do you come by describing it as "soft"? Yet, I can understand your delemma - if normally consolidated with a water table about 3 ft below ground level, you would expect plus/minus 500 psf undrained shear strength. How sensitive is the clay?

 

[oldestguy]

Will you have some control of the pile driving? It is highly possible that your static type lab tests won't tell you much about the dynamic condtions during driving and you may find a call from the contractor that "we didn't reach bearing at the time the pile was fully driven".

Do you have some way of measuring pile capacity well after driving, to see if static conditions are better than during driving? If nothing else, careful measuring for the first inch may tell a lot, say an hour or two after driving.

It may well be that a pile type, other than timber, is better. One which can be "lengthened" to allow for the unexpected experiences during driving gives you some breathing room.

Being concerned with test interpretation is fine, but you may have bigger problems to solve on the job. This site looks like one such a job.

 

[DaveVikingPE]

Thanks BigH!

The boring logs have "soft clay" written down (also, in terms of N-values, they're the order of weight of hammer to 3 or 4). The test reports say "soft, fat clay," in writing. The project area is somewhat widely understood as being one of marine clay, so my preliminary, conservative pile capacity calcs (for which I used the DRIVEN program - another thread in the Geotech other forum) reflected a soft clay - I was hoping that that would be the worst case and that the tests would come out less conservative. Perhaps the tests did come out less conservative, but the results, as I indicated above, didn't make sense to me.

I did not witness the sampling nor have I seen/touched the samples physically. I don't know the clay's sensitivity, that's one I have to ask the geotech's about, though it's fat.

I'm not a geotech and am working with a full-time geotechnical engineer on this. My concern is for the geotechnical pile capacity, his is for the integrity (and thus usefulness) of the tests. I asked for the tests, but didn't spec them out; the geotech did with some basic ("what do you want to do with the results?") input from me.

I'm always ready to learn and if I'm wrong about the soils it's fine with me... but I do want a solid foundation (pun intended) for being proved wrong.

 

[DaveVikingPE]

Thanks, oldestguy!

We are requiring test piles and drivability analysis prior to drivign production piles. I am quite concerned that timber piles won't be adequate.

 

[BigH]

Dave: What was the relative density of the sand? Loose, compact? What is the nature of the structure? Are you adding any fill to the site? Some thoughts, and my alumni colleague can add to or delete from, are:
1. If you only investigated 50 ft and the "N" values are only 3 or 4 to that particular depth, I'd be very careful about founding the piles as friction (adhesion) piles within the clay - wood piles will only go to some 40 ft (typical lengths, now). This may spell trouble. Fpr example - assuming you can drive 25 ft into the clay, the clay adhesion capacity is something like 0.7 (Tomlinson's alpha)x0.7xpix25xSu = 40xSu. For Su = 600 psf, you get approximately 8 tons (SF=3) for the clay portion (neglecting the small amount of taper)contribution and presuming no fill addition to the site.
2. If you need to increase the shear strength of the clay - you might try preloading with pvd wick drains - to cause an increase in the undrained shear strength - you might end up with 12 tons as the clay portion. If you are going to have any "capacity" at all, you will be counting on the 15 ft. of sandy soil.
3. Clearly, the clay is adding very little to any real capacity (other than for small buildings). You didn't go deep enough.
4. I suppose you could also improve the clay properties by lime mixing, etc. - but you would need a more extensive soils investigation than what you have.
3. If you are NOT adding any fill, you might be able to found on short timber piles (or geopiers) within the upper sand - and suitable spaced so as not to have 'overlapping' pressure bulbs. I did this once where I had a 10 ft sand layer sandwiched between to soft clay layers. We used timber piles bearing in the sand layer - at 5 or 6 ft pile to pile spacing along a grade beam type pile cap - it worked fine. We got our bearing and didn't overstress the underlying clay (it was a 2 storey school addition).
4. You might also want to consider putting in a basement to unload the soil - then keep your building load within the weight of the soil removed (well documented case history from the Demarara Sugar Factory in Guyana).
I think that you should consider taking a hard look at the type of structure you have, the tolerances to settlement and come up with a better solution that an assumed timber piling into the clay.

 

[DaveVikingPE]

Thanks again, BigH, for your kind assistance. Today is a good day to stay at home and think about work, then...

The pile foundations are for a non-builiding, outdoor structure subject to lower-volume highway traffic with a max live load of HS-20 = some other smaller live loads, a construction loading, etc., etc.; I have a confidentiality issue and that's the most I can say.

The sand is loose, as I recall. We will be excavating all of the sand, adding no fill, and driving piles from an elevation of about 20 ft below grade (that is, based on the boring logs, entirly in clay). For my preliminay analysis, I assumed an alpha of 1.0 with therefore alpha X c = a = 250 psf, based on conservative values from NAVFAC DM 7-02 for my hand calcs and computer anlysis (my understanding about the DRIVEN program is that it uses Tomlinson's method). I computed an ultimate capacity - assuming no point bearing contribution - of about 21 kips and used a factor of safety of 2, for an allowable capacity of about 10.5 kips or say 5 tons (I've always been a "kips" person, so tons sometimes throw me off). I used FS = 2 since I'm specifying pile load tests prior to construction (and can make adjustments... hopefully not...) and per AWPA's 2002 Timber Pile Design and Construction Manual. I also assumed that a = 250 psf would be overly conservative as proven by the test results, once we got them in. I also assumed 40 ft timber piles.

What is of most concern to me is that while my prelim assumptions were conservative, the test results might indicate that I was too overly conservative... that is, we will be too confident in what the geotechnical capacity will be.