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Abstract To investigate the dependence of the characteristics of rock mass fragments upon the two joint geometrical parameters，i.e. joint inclination angle and joint continuity factor，sieve tests were carried out for fragments of gypsum specimens with one set of discontinuous open joints after uniaxial compression tests. The fragments are classified into four size groups，i.e. large-sized，medium-sized，small-sized and fine，whose diameter are larger than 10 mm，5–10 mm，0.075–5 mm and smaller than 0.075 mm，respectively. The mass percentages of fragments in each size group，numbers of fragments in each size range，the specific surface area and fractal dimensions of size-mass distribution are calculated. It is found that mass percentage of large-sized fragments first decreases and then increases when the joint orientation angle increases and the minimum value appears at around 45°. On the contrary，mass percentages of other fragments，numbers of fragments in each size range，the specific surface area and the fractal dimensions first increase and then decrease when the joint orientation angle increase and the maximum values appear at around 45°，which is similar with peak strength and Young?s modulus vs. joint orientation angle. Though the peak strength and Young?s modulus decrease gradually with the increase of joint continuity factor，the variation of the above parameters of fragments vs. joint continuity factor is more complicated. In general，for specimens with joint orientation angles of 0°，15°，75°and 90°，the mass percentage of large-sized fragments is larger than that of the intact specimen while the mass percentages of other fragment groups，the numbers of fragments in each size range，the specific surface area and the fractal dimensions are smaller than those of the intact specimen. Therefore，the existences of joints in this joint orientation angle group cause more cracks and more energy dissipation. For specimens with joint orientation angles of 30°，45° and 60°，it is versus. This can be attributed to the different failure modes of the two joint orientation angle groups. Axial cleavage failure which observed in the intact specimen，combined with crushing and rotation failure in the former group，will create larger number of new fractures and continuous failure surfaces，and dissipate higher energy than the shear failure in the later group. Among all specimens，the specimen with joint orientation angle of 45° has the least number of continuous failure surfaces，which only fractures along the diagonal plane.
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Received: 27 December 2011
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2012, Vol. 31 
