1. Bergstrom, B. H., Sollenberger, C. L. and Mitchell, W. (1962). Proc. 5th U.S. Symp. Rock Mech. 155–172.
2. Birkimer, D. L., (1971). Proc. 12th U.S. Symp. Rock Mech., Vol. 573. Baltimore, Maryland.
3. Bjork, R. L., Kreyenhagen, K. N. and Wagner, M. H. (1967). NASA Report CR-757.