In my opinion there is no good relationship. If all I have is SPT data, I use N=1 is equal to 200 psf.

Mike Lambert

I haven't seen any relationship.

Experienced geotech around my area use Su= SPT-N/4, and c = SPT-N/8.

Su=4 to 6 N (kPa), please check Figure 2.4 from Book Piling Engineering.

There is a "general" relationship given in almost all geotechnical books - to be used with caution and modified for local practice, plasticity, etc. For preliminary estimates.

very soft: 0-2 (to 12.5 kPa); soft: 2-4 (12.5 to 25 kPa);Medium (firm in Canada): 4-8 (25 to 50 kPa);Stiff: 8-15 (50 - 100 kPa); very stiff 15-30 (100 - 200 kPa); hard >30 (>200 kPa)

The above is as per Terzaghi Peck and Mesri, Table 12.2 page 63 (note I changed unconfined compressive strength to undrained shear strength). George Sowers in his book Soil Mechanics and Foundations, 4th Edition has shown the approximate relationship given the plasticity (CL vs CH soil) Fig 7.10. - see attached.

BS 5930-1999 doesn't give a relationship (that I could find on a quick glance) for N value vs Undrained Shear Strength but interestingly they have a different take on the naming of consistency relative to undrained shear strength:
very soft: 0 to 20 kPa; soft: 20 to 40 kPa; firm: 40 to 75 kPa; stiff: 75 to 150 kPa; very stiff: 150 - 300 kPa; hard: >300 kPa.

As most will say - use with caution - and use better obtained values if one can. Best, in my view, to develop your own best estimate based on your own site conditions. See also, attached one I did for a project in West Bengal.

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