2) Bieniawski, Z. T.: Geomechanics Classification of Rock Masses and Its Application in Tunneling, Proc. 3rd Int. Congress on Rock Mechanics, Vol. II-A, 1974.
3) Barton, N., Lien, R. and Lunde, J.: Engineering Classification of Rock Masses for the Design of Tunnel Support, Rock Mechanics, Vol. 6, No. 4, 1974.