The strength of intact rock decreases with the volume of the tested specimen. Systematic testing shows that this strength decrease is caused by a combination of stored energy size effects and statistical size effects. Related to that is the behavior of jointed rock masses, where the number of joints – hence the size of blocks defined by joints/fractures – affects the strength and deformability of the jointed rock mass.
Li, V.C., Chong, K.P., and Einstein, H.H. (1989). Tension softening and size effects on the fracture determination of geomaterials. In S.P. Shah, S.E. Swartz (eds.): Proceedings of the SEM-RILEM International Conference on Fracture in Concrete and Rock, Houston, Texas, 17-19 June 1987, pp. 255-264. Springer New York 1989. ISBN: 978-0387968803 (Print) 978-1461235781 (Online) doi:10.1007/978-1-4612-3578-1_25
Chong, K.P., Li, V.C., and Einstein, H.H. (1989). Size effects, process zone and tension softening behavior in fracture of geomaterials. Engineering Fracture Mechanics, 34 (3), 669-678. doi:10.1016/0013-7944(89)90129-X
Bellwald, P., Einstein, H. H. (1987). Elasto-plastic constitutive model. In: Proceedings of Workshop on Swelling Rock, Proceedings of the 6th International Congress on Rock Mechanics, Montreal, Canada. Balkema, Rotterdam, 3:1489–1492.
Baecher, G.B., and Einstein, H.H. (1981). Size effect in rock testing. Geophysical Research Letters, 8 (7), 671-674. doi:10.1029/GL008i007p00671
Einstein, H. H., Baecher, G.B., and Hirschfeld (1970). The effect of size on strength in a brittle rock. In: Proceedings of the 2nd International Congress of the International Society for Rock Mechanics, Belgrade, Yugoslavia, September 1970, Theme 3, Vol. 2, pp. 3-2.