In order to study the influence of particle grain size on the shearing behaviour of rock joints, a series of tilt and shear tests using a model material has been conducted in the laboratory. The average particle size index has been suggested to be the most appropriate parameter to represent the particle size distribution as compared to the geometric mean particle size. It has been found that the joint roughness coefficient, as well as the peak and residual friction angles increase with the average particle size index while the base friction angle does not. However, it has also been shown that the residual friction angle can differ from the base friction angle with increase in the particle size and, in this case the residual friction angle can be expressed as a function of the base friction angle and the average particle size index.


L'inlluence de la dimension des grains de la roche sur Ie comportement en cisaillement des joints a ete etudie en laboratoire. Deux types d'essais a savoir I'essai de basculement et I'essai de cisaillement en boite ont ete realises sur un materiau synthetique. L'indice moyen de dimension des grains a ete propose comme etant Ie meilleur index pour definir la distribution des grains au sein de la roche, ceci en comparaison avec la moyenne geometrique des grains. Les resultats ont montre que Ie coefficient de rugosite du joint, l' angle de frottement residuel ainsi que l'angle de frottement en pic augmentent avec la dimension moyenne des particules exprimee par I' indice moyen de dimension des grains, tandis que I'angle de frottement de base est independant de ce parametre. L'etude a aussi rnontre que I'angle de frottement residuel peut etre different de I'angle de frottement de base a cause de la dimension des grains. Dans ce cas, il est alors possible d'exprimer I'angle de frottement residuel en fonction de l'angle de frottement de base et de I'indice moyen de dimension des grains.


Urn den Einfluß der Partikelkomgröße auf das Scherverhalten von Kluftflachen zu untersuchen, wurde eine Reihe von Neigungs- und Scherversuchen unter Verwendung eines Modellmaterials im Labor ausgefuehrt. Zur Beschreibung der Partikelgrößenverteilung hat sich der mittlere Partikelgrößenindex im Vergleich zu der mittleren geometrischen Partikelgröße, als der geeignetere Parameter erweisen. Experimente haben gezeigt, daß sowohl die Rauhigkeitskoeffizient der Kluftflachen wie auch der Maximum- und Restreibungswinkel mit zunehmendem mittleren Partikelgrößenindex ansteigen, wahrend der Grundreibungswinkel unabhangig vom Index ist. Mit zunehmender Partikelgrößse kann jedoch der Restreibungswinkel von Grundreibungswinkel abweichen, so daß in diesem Fall der Restreibungswinkel als eine Funktion des Grundreibungswinkels und des mittleren Partikelgrößenindex ausgedriikt werden kann.


The behaviour of a rock mass is determined principally by the shear strength of the jointing system, which in turn is influenced predominantly by the roughness of the joint surfaces. The standard shear tests on joints indicate the presence of a peak shear strength followed by the residual shear strength. This behaviour is predominantly due to the dilation that takes place during the displacement due to joint roughness. In rock engineering practice, the design against failure of a potential sliding surface can be accomplished either by use of the Mohr-Coulomb failure criterion, which combines the normal stress with the base friction angle and cohesion in a linear manner, or by using a normal stress dependant empirical law that corrects the base friction angle in order to take account of the microstructure of the sliding surfaces. In this regard, various parameters have been proposed using different approaches to characterise analytically the joint roughness. One can cite the root mean square of the first slope (Myers, 1962); the roughness angle (Patton, 1966); the joint roughness coefficient, (Barton and Choubey, 1978); the fractal dimension (Carr and Warriner, 1987, Tse and Cruden,1979, Lee, 1988, Huang et al 1992); the modified root mean square of the first derivative and the average roughness inclination (Kulatilake et al 1995). All these empirical laws can be expressed in a general form which resembles to the earlier Patton equation (Patton 1966), where r is the strength, a is the norrnal stress,ø is the base friction angle, and i is the roughness angle of the joint.

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