Determining soil properties for pile foundations

[elperko]

Hi there, I'm trying to design pile foundations. This will involve piling to a depth of around 25m through three different strata with the fourth being end-bearing. I am trying to determine reliable values for undrained shear strength and the adhesion factor in order to estimate my pile bearing capacities, however I have been supplied with very limited information. I have been given results from standard penetration tests for all of the strata which I understand can be correlated to undrained shear strength but should be used with caution (there is also a huge range in the n values found for each stratum). I have also been supplied with data regarding the moisture contents and plasticity indices for some but not all of the stata and the results of a triaxial test for one stratum (not the end-bearing one). Futhermore I have not been given any information relating to the unit density or weight of any of the strata. It is not possible to perform any in-situ tests or gain any other information on the soil than the information I have already been given (apart from finding results from tests on the same soil type in other literature).

Is there any way that I can obtain the values I need from the data that I have been supplied?

Thanks

 

[Ron]

Sounds like a nice homework problem for school.

If not, then get with the geotechnical engineer. If you are not familiar with how to do this, you could easily screw it up and pile foundations cost a lot of money to install....and even more if they fail.

 

[a7x1984]

I recently had a similar issue - mine being with helical piles. I would agree with Ron. That is why God invented Geotechnical Engineers: to provide ALL of the soil data required to perform the foundation design (especially in a large project like this seems to be). Three questions: 1) Is it a compression pile, or tensile? 2) What Seismic site-class are you in? 3) Is there a significant lateral load?

If the answers are 1) compression, 2) A, B, (maybe C), 3) No, then I think you could at least tide yourself over and use the last soil strata for end bearing and friction using the readily available correlations with n, soil type, and phi angle. You will likely have an over-designed pile, but it gets you somewhere while you wait for your lazy Geotechnical Engineer to do more than forward the boring log and Atterberg limits tests to you. BTW, watch our for n-values <5. If those strata are deep, it could indicate compressible peat - ask the geotech guy to provide the surcharge from it's downdrag.

If I got paid for every hour I worked, I'd be a wealthy man.

 

[mikeofBelAir]

Unless things have changed ( doubtful given the state of human nature), SPT results are anything but "Standard". Use them with a large grain of salt. There is a specific procedure for SPT, but very few driller do the test correctly. Remember drillers are paid for the number of feet they drill--Not the accuracy of their logs, nor the accuracy of the SPT. For them, its all about how many feet they drilled. Unless the logs were kept by a trained soils professional 9 Engineer, or tech or geologist) I'd not rely too strongly on them.

 

[hokie66]

There are piles and then there are piles. For some types of piles, and for piles that are end bearing, much of your data is irrelevant. Agree with others...present your issues to the geotechnical engineer or piling contractor on your project.

 

[elperko]

Thanks for the responses guys. It's actually for a final year university project I'm doing and the lecturers are pretty reluctant to give much away. I've managed to source most of the relevant values from literature which I understand they deem acceptable. I'm stuck trying to find a bulk unit weight for the second deepest layer. The layer is supposedly made up of fluvioglacial deposits and as such I cannot find a value for the bulk unit weight. It has been suggested to me that I could extrapolate a value as I know the values for the layers above and below and the relative depths. However I'm struggling to find anything in literature to back this up (I have been told it is somewhere), could anyone shed any light on this?

 

[JennyNakamura]

Wow. Good eye Ron. I had no hint it was "homework."

 

[grabens]

Try the attached. I am a practicing geotech and most of the time we can't get clients to pay for site specific characterization. The attached was produced by ASCE and gives a range of typical values to at least start with. Good luck.

 

[Ron]

Jenny....result of 5 years of teaching in a university!

 

[a7x1984]

Student: Read the eng-tips policy on HW.
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