1. B. H. Fellenius and B. B. Broms, "Negative skin friction for long piles driven in clay," Proc. VII Int. Conf. Soil Mech. Found. Eng., Mexico (1969).
2. L. Bjerrum, I. J. Johannessen, and O. Eide, "Reduction of negative skin friction on steel piles to rock," Proc. VII Int. Conf. Soil Mech. Found. Eng., Mexico (1969).
3. M. Endo, U. Minou, T. Kavasaki, and T. Shibata, "Negative skin friction acting on steel pipe pile in clay," Proc. VII Int. Conf. Soil Mech. Found. Eng., Mexico (1969).
4. T. Ocabe, "Large negative friction and friction-free pile methods," Proc. IX Int. Conf. Soil Mech. Found. Eng., Tokyo (1977).
5. E. Horvat and C. Van der Veen, "Negative skin friction and safety analysis of piles," Proc. IX Int. Conf. Soil Mech. Found. Eng., Tokyo (1977).