2) Fujita, K. (1966): "Standard Penetration tests," Japanese Society of Soil Mechanics and Foundation Engineering, pp.29-76 (in Japanese).
3) Gibbs, H.J. and Holtz, W.G. (1957): "Research on determining the density of sand by spoon penetration test," Proc. 4th International Conference on Soil Mechanics and Foundation Engineering, Vol. 1, pp.35-39.