N-SPT values observed during testing are not utilized directly in the assessment of soil properties. These values need to be corrected to account for hammer efficiency , borehole diameter,sampling method, rod length and the overburden pressure. Current tool sets the rules to allow the calculations for the correct SPT numbers, N60 and (N1)60.

Friction angle together with cohesion defines the shear strength parameters of soil. Its definition is derived from the Mohr-Coulomb failure criterion. It can be determined in the laboratory by direct shear test or triaxial tests or from theoretical correlations with density or N SPT number.

The current tool sets the rules to calculate friction angle for granular soils based on N SPT number.

\(x = {-b \pm \sqrt{b^2-4ac} \over 2a}\)This is an updated text