The purpose of this Technical note is to draw attention to an important result that is the consequence of testing specimens of rock large enough to approach the rock mass properties rather than the rock material properties usually determined on smaller specimens; it is based on some recent work carried out at the Transport and Road Research Laboratory.

The designer of an underground excavation needs to know the stress-strain characteristics of the rock in which the excavation is to be made if he is to be able to predict the loads and displacements in the rock around the opening (Hoek & Brown 1980). Using the apparatus shown in Fig. 1., the stress-strain behaviour of large (165 mm diameter, 270 mm length) specimens of mudstone has been studied in the triaxial compression test. The specimens were prepared from vertical and inclined ZF cores obtained from close to the Kielder Experimental Tunnel in County Durham, England.

All specimens were tested under effective stress conditions; some multi-stage and some single-stage tests were carried out; and the cell pressures (up to 1300 kN/m2 total pressure) used were consistent with the stress conditions developed around underground excavations. In addition to axial strain, volumetric strain was measured. It is considered that the large specimen size in relation to the spacing of bedding plane partings and joints in the rock means that the properties of the rock mass that are important from the engineering point of view, rather than the properties of the rock material

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