Stiff clays forming part of natural slopes contain rough cracks. Very little is known about the influence of the roughness of the cracks on the strength of these materials and on the way the cracks propagate in them. This study reports the results of a laboratory investigation on fissured clays subjected to uniaxial compression and direct shear stress conditions. For the uniaxial compression tests, prismatic samples of kaolinite clay measuring 7.6 cm in height and width, and 2.54 cm in thickness were used. For the direct shear tests the prismatic samples tested measured 12.70 cm in width, 11.4 cm in height, and 3.17 cm in thickness. The cracks in the samples were made by inserting and removing thin metallic sheets that were either planar or have a stepped form. The laboratory test results indicated that: (1) the samples with rough (stepped) cracks had higher uniaxial compressive and shear strengths than the samples that have planar cracks; and (2) the higher the amplitude of the rough cracks, the higher were the uniaxial and the shear strengths measured in the samples. Thus, the roughness of cracks has a large influence on the compressive and shear strengths of fissured clays.
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