The shear strength of rock discontinuities subjected to impulsive loading is influenced by loading-rate. The experimentally observed behaviour is simulated with the Distinct Element Method. The comparison between numerical and experimental results gives some insights on the possible use of DEM in the solution of problems particularly influenced by dynamic loads.


La resistance au cisaillement des joints rocheux soumis à des contraintes impulsives est influencee par la vitesse d'application de la charge. Au moyen d'un etude numerique base sur la Metode des Élements Distincts (MED) on cherche d'interpreter les phenomènes observes en laboratoire. La comparaison entre les resultats experimentaux et numeriques permet d'avoir des indications sur la possibilite d'application du MED à des problèmes dans lesquels les contraintes dynamiques deviennent très importantes.


Der Scherwiderstand der Felsentrennflachen, die Impulsbeanspruchungen ausgesetzt sind, wird von der Belastungsanlegensgeschwindigkeit beeinflusst. Mit einer numerischen Forschung durch die Methode der gesonderten Elemente (DEM) versucht man, die experimentell betrachteten Phanomene zu interpretieren. Der Vergleich zwischen den Ergebnissen der Zahlenanalyse und den experimentellen Resultaten erlaubt, Hinweise ueber die Möglichkeit einer Anwendung vom DEM fuer Probleme zu ziehen, in denen die dynamischen Beanspruchungen erhebliche Wichtigkeit gewinnen.


The evaluation of safety of rock masses subjected to dynamic loads (seismic events, blasting, etc.) is one of the most difficult and important problems of rock engineering. In particular, the behaviour of joints under quickly changing stress conditions is of primary concern (Barton, 1988; Bandis, 1990). A comprehensive experimental research on this topic (Barbero and Barla, 1992) has shown interesting results about the influence of the velocity of an impulsive shear load on the strength of saw-cut rock surfaces. The analysis of experimental data showed increasing strength with higher loading-rates, the strength being defined as the shear stress acting at the very beginning of irreversible shear displacements. The aim of this paper is to investigate at first whether the distinct element code UDEC is able to simulate the experimentally observed phenomena. As UDEC performs dynamic stress analyses by using a loading-rate independent friction angle, an "exact" agreement between numerical results and laboratory measurements could allow one to state that the strength increase with the loading-rate is not due to a change of the friction angle, but to inertial effects before sliding. Such an approach, if validated, can also give information on the non homogeneous stress conditions on the joint, and their influence on the sliding history of contact points.


The influence of the rate of shear load applied to smooth planar rock joints has been studied by one of the authors (Barbero, 1992). To this purpose, a new shear test device has been constructed; dynamic tests have been performed and interpreted by means of a simple rigid body model.

2.1 Testing equipment

Tests under dynamic conditions can be carried out on rock samples up to 100 mm in diameter. Horizontal and vertical dynamic loads up to 10kN can be applied within the frequency range of 0.01–3.5 Hz. The maximum allowed displacement is 2 cm in each direction. During a shear test, displacements and pressures are measured by means of transducers and recorded by a data acquisition system for later processing. The values of the pressure in the actuators are transformed into shear stresses acting on the discontinuity surface, by using a calibration curve obtained by comparison with load measurements previously performed with load cells within the shear box.

2.2 Unidirectional impulse tests

The purpose of the experimental program was to study the behaviour of the joint before relative displacements between the contacting surfaces occur.

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