Possible solution exist as following log10(DPI)=-1.05+2.03*log10(SPT), which may be solved for SPT as following SPT=1/3.29034*DPI^0.492611, in accordance to The Relationship Between In Situ CBR Test and Various Penetration Tests, Report Penetration Testing 1988, I. lshai and M. Livneh, ISOPT-1 ...