Joint shear strength dependence on shear direction and sense is of major importance during cyclic shear. In fact, cyclic tests are particularly useful to simulate dynamic loads or shear load reversals so frequently observed in nature. Some simple laboratory tests have been performed at constant normal load on a massive serpentinitic rock. Results, in accordance with previous authors, are presented with particular regard to shear strength, shear stiffness and asperity degradation.
La dependence entre la resistence au cisaillement a une grande importance pendant un cisaillement ciclique. En effect, les tests cicliques sont particulierment utiles pour simuler les charges dynamiques et les inversion du charge de cisaillement, qui sont frequentes en nature. On a fait des tests de laboratoire avec des roches serpentiniques massives sous un charge normale et constante. On presente les resultes, qui sont en accord avec les precendes auteurs, avec particuliere attention pour la resistence et la rigidite au cisaillement et la degradation des asperites.
Bei wechselnder Scherbeanspruchung hangt die Kluftscherfestigkeit in hohem Maβe von der Scherrichtung und dem Schersinn ab. Zur Simulation von dynamischen Belastungen und der Scherspannungsumkehr, die haufig in der Natur beobachtet warden, sind Scherversuche mit wechselnden Beanspruchungen besonders geeignet. Unter Normallast wurden an massivem Serpentinit einfache Laborversuche durchgefuehrt. Die Ergebnisse werden, in Übereinstimmung mit anderen Autoren, unter besonderer Beruecksichtigung der Scherfestigkeit, der Scherungsteifigkeit und der Rauhigkeitsverluste vorgestellt.
The behaviour of rock joints during cyclic or reversal shear has been recognised as anisotropic because of its dependence on both the shear direction and the shear sense (Celestino & Moodman, 1979; Hutson & Dowding, 1990; Muralha & Pinto da Cunha, 1990). During shear, the strength and deformability of rock discontinuities are in continuous evolution and the mechanical properties of the discontinuities dominate the response of the entire rock mass. Present and previous normal and shear loads, shear displacement, shear direction and shear rate, compressive strength of intact rock and joint, together with some scale-dependent parameters as joint roughness, waviness and infilling material, are the main factors influencing joint behaviour under a shearing action. Other parameters influencing joint behaviour are joint asperity degradation and the initial aperture (gap width). The accumulation of shear displacement in laboratory tests by changing the sense of shearing but not the direction represents one of the best ways to model seismic loads, cyclic shear loads or the unloading of previously sheared discontinuity, reaching amounts of displacement large enough to mobilize the entire range of discontinuity shear strength, from the peak up to the residual shear strength. In fact, examples of reverse shear movement can be imagined as a consequence of pressure release, due to surficial or underground excavations, where shear displacements were previously induced. In spite of the enourmous importance that cyclic shear and stress-path have in the, stress deformation behaviour of rock joints few data have been published (Celestino & Goodman, 1979; Barton, 1988; Hutson & Dowding, 1990). In this work, laboratory tests have been performed to obtain some information on the behaviour of serpentinitic rocks under cyclic joint shear.
To study the directional dependence of joint friction, a complete series of cyclic shear tests were performed on air dried artificially opened joints with a surface area ranging between 80 and 120 cm2 without actually examine any scale effect. Rock samples consist of green schistose serpentine (greenstones) of a massive variety (chloritic schist) and were collected from a quarry in Val Malenco (Italy) where the material is extracted for construction and craftworks. Artificial joints were created by opening massive blocks along a schistosity plane and hence joint roughness changed from one sample to another.
