In this work the effect of sample size on the compressive strength of Cedar City quartz diorite is investigated. Results are reported for laboratory unconfined compression tests on cylindrical samples, 25 mm (1 in) to 146 mm (5.77 in) in diameter with 1:2.5 diameter to length ratio. Stiffness compensators were inserted in the machine to simulate the in situ load frame stiffness of previous field tests. Each sample was strain-gaged at three positions around the cross section. Nonuniform sample deformations were observed during the tests. The maximum strain in each sample was calculated and a maximum stress was evaluated based on combined compression and bending loading conditions. The results indicate a slight decrease in nominal compressive strength, sc, with an increase in sample diameter, D, given by the equation(mathematical equation)(available in full paper)

Furthermore, the effect of load frame stiffness on the measured strength is negligible. Weathering effects, on the other hand, seem to cause a reduction in the strength of the material. Finally, a maximum strain failure criterion for the tested rock was found to fit most of the data obtained in the present investigation and in previous work [Pratt, et al. (1975)]. The maximum allowable compressive strain is 0.74 percent with a standard deviation of .07 percent.


Measurement of the resistance of materials to catastrophic failure and knowledge of their load carrying capacities are essential for the safe design of engineering and geological structures. In these measurements it has always been assumed that the fracture strength is an inherent material property. This assumption implies that the size of the structure has no effect on its material strength. Consequently, the common practice is to evaluate the strength in laboratory experiments on small size samples or prototypes. The measured values are then applied to large-scale components. However, such practice for geological materials became questionable when experimental investigations on rock materials revealed a change in measured strength with size of test samples. Briefly, the reported effect of the size on "measured strength" is a reduction in resistance to failure with an increase of part size [c.f. Gaddv (1956), Evans and Pomeroy (1958), Bieniawiski (1968), Pratt et al. (1972)]. However, confining pressure tends to suppress the effect of size on the failure strength of rocks [Habib and Vouille (1966)]. Furthermore, the size seems to display its strongest influence on the initiation of failure and not on the maximum stress attained in a triaxial compression test [Huck (1972) and Hunt (1973)]. Huck (1972) also observed that smaller samples fail more catastrophically than the larger ones which have a tendency to fail very gradually. On the other hand, some experimental investigations revealed little or no effect of size on strength, especially for hard competent rocks [Hodgson and Cook (1970)]. Moreover, while testing several rocks, Hoskins and Horino (1969) observed an increase of strength with part size followed by a reduction with further increase in specimen diameter. Several authors have attempted to establish a phenomenological size-strength relationship to represent some of the available experimental results. The phenomenon however proved to be very complex. Theoretical reasoning to explain the experimentally observed "size effects" have been developed along two main lines of thoughts.

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