Pile Capacity - SPT - N values or In-situ Undrained Shear Strength

[EireChch]

Hi all,

Here is the context of my question. I am working as a civil engineer doing drainage design, flood assessment etc for a new school development. The building is a 4 storey structure. We are also doing some earthworks design. As part of the earthworks design I had to review to geotech report to get an idea on what soil types im dealing with. This is when I came across the foundation section of the report.

The borelogs show SPT testing and shear vane testing every 1.5 - 2m. SPT then 1m deeper SVT then SPT and so on.

There is a layer of made ground to a depth of roughly 2m, broken concrete, glass etc, so definitely not suitable to support foundations. From 2m depth the soils are silty clay and clay. Uncorrected N values are as follows:

10 - 15N from 2m to 10m Cu of 40 - 60kPa from 2 - 6m
20 - 28N from 10 - 15m 80 - 90kPa from 6 - 10m
100 - 110kPa from 10 - 15m

The geotech have gone for bored piles since we are near some existing buildings and vibrations from pile driving would be an issue. In particular CFA piles over bored-cast in place as with bored pile you would need temp/permanent casing thorough made ground. I agree with all of this so far.

When assessing pile capacity they have used Qb base capacity = Nc * Ap * Su (with Nc = 9)
and,
Qs shaft capacity = As(area of shaft) x fs (shaft Friction in kPa)...........fs = α * Su

They have used the Stroud correlation of 4.5N = Su.

Surely using Su or Cu from the in - situ shear vane test would be the most appropriate to use?

 

[oldestguy]

This is no place to ask for criticism of a report apparently prepared by a professional. There may be other factors involved not mentioned in the post. Discuss sny questions with the report author. If you are not satisfied, go to another professional for their comments.

 

[EireChch]

Thanks for the response OG. Maybe the phrasing of my post was a bit harsh but I am not trying to critise the author but simply wondering what other engineers would do if they have SPT and Vane testing. May be i should have just asked "what is better to use in foundation design undrained shear strength from in situ vane testing or undrained shear strength correlated from SPT N value", something along those lines. I thought giving the full context of the project would be helpful.

I know my boss wouldn't like me approaching them as he steers clear of geotech since it is not our specialty. If i managed to get into a meeting and the geotech was there I could maybe ask them informally about the design method but i dont want it to come across as me challenging their methods (if you get what i mean.

 

[oldestguy]

OK, OG here again. For a general discussion of the plus and minus evaluations of exploration methods, this may be a place to start. I, for one, can find good or bad factors for each, but would not want to apply them to your specific job. As for the N values, that has really been kicked around and now take any text or reference and it's use thereof for finding soil properties and you need to look at correcting or not as a beginning. As to vanes, what sort of "standard" method was used, etc. The list goes on.

 

[oldestguy]

Here goes on the SPT. Having spent many years observing the "test", a few comments. Supposedly the original method was properly evaluated when the correction took place to show how inefficient or efficient the energy was imparted. However, even that original method had many variables, such as number of wraps on the cat-head, height of hammer drop, borehole clean out, if any, damage to the spoon, where to count the N in the penetration length, etc. I once inspected a contractor doing work for the Tri-state roadway around Chicago and elsewhere. Many times the opening to the spoon was buggered into less than half the ID of spoon, Bent spoons were common, etc. Weight contact with the drill rods varied from steel to steel, steel to wood block, etc. How many of these original oddities were considered to "correct" supposed better techniques? The newer methods probably also can e affected by methods actually used. It goes to show how rough the"test is.

 

[Mccoy]

EireChCh, I think with his latest post OG answered pretty eloquently although not explicitly.

On my side, I would sure use a test which gives a 'direct' measurement of a soil property (thru the medium of the vane geometry) rather than a test with such a massive initial uncertainty like the NSPT (with more uncertainty on the correlation scenario). Moreover, the vane test can be used to measure Su residual as well.

Of course, I too would wonder why the original engineer used the NSPT derived values notwithsanding the existance of a more logical choice of data....Any specific reason he knew that we don't know?

http://www.mccoy.it

 

[EireChch]

Thanks McCoy/OG for the responses. Yes i agree with you both that there are so many uncertainties with SPT and to add human error with it as well just adds to it.

I dont know if there was any other reasons that author used a correlation form N to Su, maybe there is. Like i said above I am not in a position to approach him but if i get an informal chance I might try to pick his brain. I dont want to criticise him in any way, I purely just want to try and learn something that I might not know! I also wondered would using Meyerhoffs end bearing capacity method for SPT testing from pg 895 of Bowels - Foundation Analysis and Design (cant remember if its fifth or seventh edition)be a another option since this uses direct N values but i think this method was derived from SPT testing in sands and not clay so may not be appropriate.

I suppose one aspect of my question, although this may be another thread topic, is also why is SPT just the default in a borehole? In my limited experience in the UK and Overseas I have seen this. I have worked in an area where the soils are 9/10 clay and silt mixtures and yet SPTs would be always be done without vanes. On occasion we would go out with the driller and do our own vane tests with Pilcon vane and 1m extension rods. Typically we would do up to 8m, after that i think rod length becomes an issue.

I accept that there are times when vane testing might not be an option and due to limited resources SPT is the only option so you just have to use what you have got an apply some engineering judgement (aka best guess )

 

[Okiryu]

You can doble check your undrained shear strengths with its correlation with the effective stress (su/sigma' = 0.22 ~ 0.25).