interpreting skin friction graph from geotech 1

[delagina]

I usually just get one value of skin friction from geotech that I multiply with pile circumference and length.

This time I got a graph instead.
Am I right that for every feet there are abrupt changes in skin friction values.

Say at around 25 feet, I'll multiply 1 feet of the 25 feet by 600 psf and another 1 feet by 250 psf, and so on then add them?
This looks like simple math but I'm surprised with the abrupt changes in skin friction per feet.

 

[jayrod12]

I think most geotechs just take a conservative average. And in fact, I've never known one around here to take detailed enough samples to provide a graph in 1ft increments.

 

[Lomarandil]

Agreed, I've always seen geotechs smooth out the curve into a few strata (at least 5'+ thick each). I'd be surprised if yours has the testing data really necessary to provide the level of confidence that graph implies.

But, maybe they really did test the heck out of it.. or they took a bore exactly where you plan to put your one pile.

 

[oldestguy]

Someone is making a relationship between blow count and skin friction, generally wishful thinking. I'd just average the numbers in long lengths.

 

[Ron]

This looks like a printout from a piezo-cone or similar rig. Have the geotech interpret and give you a reasonable average value. Not your job, so make him/her do it.

 

[GeoPaveTraffic]

I agree that this data was likely derived from a cone penotrometer. The cone provides data on a fixed interval usually every 0.2 feet or so.

As for what to use for design, depends on what you are designing and how much settlement/movement you can handle. Only you and the geotechnical engineer working together can determine that.

Mike Lambert

 

[delagina]

so basically i should ask for the "average" skin friction value that i can use for the whole length of the pile?

but can i use this data though instead of the "average"? it's not complicated to assign a skin friction value per feet or 2 feet from this graph?

 

[SlideRuleEra]

I agree with oldestguy, the graph has the look of being derived from pile blow count. If so, when considered along with information in the OP's other recent posts, the graph has a shape that I'm very familiar with.

delagina - Sit down with geotech (per GeoPaveTraffic's comment). Find out all the details of the soil at the 18' to 25' depth. If my hunch is right, you may be able to get point bearing on some lightly loaded short piles.

Note: I'm not saying that this will work... I am saying that a meaningful discussion with the geotech is well worth the effort.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[BigH]

I personally think these kind of graphs are a bit on the dishonest side - insofar, as others have indicated, they have been based on what appears to be N values which, as we know, are not reliable. to have 10 values of side friction in a 20 ft zone seems very odd. Even if developed by piezocone I would think it a bit "much" to imply we know the side friction to the accuracy shown.

I have seen this by others for footings and even for end bearing on piles and that gets even more weird. For spread footings at 1.5 m you can have 325 kPa bearing, but at 1.7 m you can have only 250 kPa, yet at 1.9, it is back up to 350 kPa. I doubt very seriously that they took into account zone of influence.

 

[hokie66]

This is just laziness on the part of the geotech. He has a program into which is plugged the N values, and out spits a graph. No interpretation. Is B-34 just a boring number? Does he give a number of these graphs, expecting you to smooth the results?

 

[KootK]

Speaking from the structural side of the table, I expect geotechnical engineers to supply me with the actual parameters that I will use in my design, not intermediate "data" that I'm left to interpret. Sure, nine times out of ten I probably can interpret appropriately. However, there's a) that one time and b) my firm's imperative to not take on liability that doesn't make sense. I like my geotechnical recommendations "no assembly required". And most geotechnical engineers seem happy to provide this kind of service as long as I'm clear about my expectations up front.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[BAretired]

I have never before seen a graph like the one above. I agree there should be some interpretation on the part of the geotech.

BA

 

[AdityaDeshmukh]

I am not very experienced with this, but i feel there is a strata between 20 and 100 ft that is not very good. Either it is a mineral formation or fractured soil. Rather go for changes in design, like adding bulb or a kink in a pile to accommodate low skin friction. As far as values are considered, a factor of safety of 1.2 and mean of the values can be used for skin friction.

 

[Teguci]

This is a bit confusing -
He cites FHWA - HIF-07-03 which I couldn't find. I did find HIF-07-039 but that doesn't relate to geotech reports (its for augered pile design)

FHWA NHI-10-016 is for LRFD Design of Drilled Shafts and does have info regarding the geo report (and I highly recommend it as a free resource for anyone who designs drilled pier foundations):
"Once the data from the field investigation and laboratory testing program have been obtained, the
geotechnical engineer is responsible for reduction and interpretation of these data,..." - pg 2-3

"A geotechnical design report, also called a foundation report, typically provides an assessment of existing
subsurface conditions at the project site, presents geotechnical analyses, and provides appropriate
recommendations for design and construction..." - pg 2-24

"For each geomaterial layer established in Step 11-1 and for each limit state identified in Step 11-2,
assign the appropriate geomaterial properties needed for evaluation of axial resistances." - pg 13-4

If you haven't reviewed this guide before, I suggest you give it a glance through before completing a deep foundation design.

http://www.fhwa.dot.gov/engineering/geotech/library_listing.cfm?sort=FHWA_PubNo