An experimental study was conducted to determine the influence of specimen size on the mechanical response of rock. Specimens of Charcoal Black granite (Cold Spring, Minnesota) and Indiana limestone (Bedford, Indiana) ranging in size from 2-in. dia to 36-in. dia (32-in. dia for the granite) were tested in triaxial compression. Test data included axial and circumferential strains at up to 30 locations on the largest specimens, and axial and radial pressures. Data for loading, unloading and reloading conditions were collected. The loading data were fit to models describing bulk modulus and shear modulus, from which other moduli were determined. The reduction in strength over the size range and confining pressures employed was approximately 50 percent.


One of the problems in mining and underground construction is that little is known about the mass behavior of rock. It is recognized that the rock mass behaves differently from small specimens that can be tested in the laboratory, but the effect of scale is not well understood, particularly under generalized stress conditions. Since large scale tests are difficult and expensive even when they are possible, investigation into the influence of specimen size helps to establish a relationship between the behavior of small specimens in routine laboratory tests and the mass behavior of rock in the field.

Several investigators have attempted to determine the effect of size on strength. Salaman and Munro1 have summarized some studies on coal. Other empirical relations have been given by Greenwald et al2, Steart3, and Protodayakonov4. Statistical relations dependent on the Weibull theory5 have also been suggested. Grobbelaer6 has expanded on the work of Epstein7 , Bieniawski8 and others to develop relationships based on the "weakest link theory". Glucklich and Cohen9,10 have indicated, however, that effects other than statistical need to be considered, and these have been examined by Baecher11. Most studies on confinement of rocks do not consider size, although considerable research has been conducted12-15. Several contributions to the understanding of brittle failure in rock 8,16-21 have been made in recent years.



Four triaxial cells were set up as shown below:


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The general configuration for these cells is shown in Fig. 1. One region contained the specimen, and was pressurized to the desired confining pressure. Axial load was transmitted to the rock by the sliding piston. The maximum axial stress in the rock depends upon the ratio of the rock and piston areas and the difference in the confining pressure and the axial chamber pressure.

The three smaller test cells are incorporated into a single system with centralized controls, instrumentation, and pumping systems. The tests conducted in this system consisted of initial hydrostatic loading up to the desired confining pressure, followed by triaxial compression at constant s 3 above that pressure. At least one load-unload- reload cycle was observed for each test. Provision was made for pressure cross-connections between the confining pressure and axial pressure chamber volumes to insure hydrostatic conditions during the hydrostatic test phases

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