Determination of Cn for SPT Overburden Correction

[JHorton]

I'm attempting to correct some SPT values (derived from beckers) for overburden pressure. While there are numerous different equations available they all cover up to 500kPa at best and my data extends considerably below this. Even at the 500kPa level there is considerable varition between the curves for different types of sand (Cn of 0.12 to 0.56).

I would like to derive a site specific correction curve if possible but cannot find what method was used to determine Cn for a specific material.

Regards,

Jay