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 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 rock mass properties which are important from the engineering point of view, rather than the rock material properties have been determined. The measurement of volumetric strain during the test allowed the results from one specimen to be analysed using critical state theory, in particular to see if the measured strain vectors complied with the condition of normality of the associated flow rule. (A)

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