if so can you tell me how to get from an SPT value of 20 from a borehole and relate that into a c value for pile design?

An N value from a SPT test equates rougly to an allowable bearing pressure (qa)of around 210kPa.

This is derived from a chart, but can be represented by (roughly)
qa=10.5N'B where width B<1m

Not sure if this is what youre after?

other than that, this paper is sometimes useful:
http://arsalanghahramani.com/sm12b.pdf

Where are you working, out of curiosity?!

What you want is Stroud and Butler's paper.

Ps Hi! Eng Geologist masquearading as an Engineer.

Mrs Mugsy is a proper Geotech Engineer.

mmm dickydutch Brinch Hansen simplified!

Depends on what material you are talking about. The Stroud paper is more for glacial/over-consolidated clays I think.

yeah it is for tills really. although butlers done one on sensitive rocks i seem to remember.

I am a structural engnieer trying to do some early calcs for a Stage D report and we dont have the full geotech report in.
I am aware of the qa=10N, but the end bearing of a pile uses c

qa=2c

so can I just assume that

c=20N ?

Oh and it is in chalk (with no water)

STW is full of so many informed people! I am never always astounded by the wealth of knowledge out there – thanks everyone so far

mugsys_m8
Overly simplified perhaps!

TimP
Just make sure you state your assumptions and no one can moan!?

if its in chalk, I wouldnt be so sure of the above.
Get the CIRIA document entitled "Engineering in Chalk".
nightmare material!
also, I think theres a document called "Engineering in Weak Rock" or something similar.

CIRIA report PG6 "Piling in Chalk" is worth a look. There are some empircal relationships between shaft resistance and SPT N value by Hobbs and Healy etc.
Some of the charts show that there is quite a variation according to type of chalk and its grade/degree of weathering. That probably doesn't help too much though!

I hate doing SPTs, such a ball ache to perform, and so time consuming in deep holes.

TimP – yep CIRIA C574 2002 is the reference you need. The correlation factors used above are for more cohesive or granular soils not chalk. Chalk is a different beast. Read up Part 4, the mechanical properties of chalk.

Chalk can vary in nature and have deep weathering, and this has profound effects on how it behaves. So much so you may have to change pile type. What solution are you thinking of?

Where abouts' in the country is the site, and have you got a classification and description of the chalk.?. ie "grade DM" etc.

I wouldn't go on just an spt value for the design. Have you got any other test data? Triaxial's, shear box, UCS, did you core it?

To back this up I spent 3 months in a chalk mine in dunstable.!.

It is in Hatfield

we have classification is DM

we are thinking CFA as M+E want to use ground source heating and cooling

Environmental Scientist also masquerading as a geotechnical/environmental engineer.

Can't help though, never done any pile design 😉

I used to live in welwyn garden city, and know that part of the world well… As for the chalk: Grade DM – means it in structureless, very weak to weak if not completely weathered, low density.

not the best ground. Did you know the chalk was mined underneath hatfield. Check GE (Ground Engineering mag) Nov 2007 and did the contractor get a desk study done? no joke an infant school on 'briars lane' fell down a large hole in the late 70's. Mining is a real hazard round there.

as for the pile design CFA is a good bet, esp' with the heat pump solution, however flint is a real problem round there for drilling.

skin friction plays a massive part in pile design in chalk. not so much with cfa piles… but I recommend reading part 8 of CIRIA 574. it gives some calculations that you would need and has a specific chapt on pile types.

without seeing the borehole records I cant be of anymore help – email them over if you want.

http://uncyclopedia.wikia.com/wiki/Geologist

is all i'm saying

As its chalk you may want to design the pile to be skin friction only and ignore the end bearing. Chalk hs a tendancy to be remoulded depending on the type of piling method being used and also if tis a Dm or Dc grade then it is structureless as already mentioned. Reason for this is that as the pile is loaded the ground below the end of the pile will compress and take a certain settlement to fully mobilize. You can account for this by factoring teh end bearing value more or by reducing the safety factor on the skin friction and negating the end bearing capacity altogether. Considering the fact this depends on so many variables and really requires someone with the information in front of them and sufficient knowledge to carry it out I suggest you get a geo engineer to take a look at the data you have and give you a suitable solution.