This paper investigates the possibility of interpreting the shear band evolution in hard soils and soft rocks as the result of shear propagation from pre-existing natural defects. This is done through the application of the principles of Fracture Mechanics, a Slip-Weakening Model (SWM) being used to simulate the non-linear zone at the tips of the discontinuity. Experimental observations have been carried out on prismatic samples of Beaucaire marl through biaxial compression tests in plane strain conditions. A numerical simulation confirms the validity of the proposed approach.


Cet article presente la possibilite d'interpreter l'evolution des bandes de cisaillement dans les roches tendres et les sols raides comme le resultat de la propagation pour cisaillement à partir d'un defaut preexistant. Le Slip-Weakening Model (SWM) a ete employe pour modeliser la zone non lineaire a l'extremite d'une discontinuite. Nous presentons des observations experimentaux obtenues à travers des essais de compression en deformation plane sur des echantillons prismatiques de marne. La simulation numerique d'un essai confirme la validite de la methode proposee.


Die vorliegende Arbeit versucht die Ausbreitung von Scherzonen in (bindigen) Böden und im Lockergestein anhand der Entwicklung von Scherrissen ausgehend von existierenden Schwachstellen zu erklaren. Die Umsetzung erfolgte mit Hilfe der Bruchmechanik, fuer die Modellierung der nichtlinearen Zone an den Rissenden wurde ein Slip-Weakening-Modell (SWM) eingesetzt. Es wurden zweiaxiale Druckversuche an prismatischen Probekörpern von Beaucaire Mergel unter der Bedingung eines ebenen Verzerrungszustandes durchgefuehrt. Die numerische Untersuchungen bestatigten die Annahmen der untersuchten Theorie.


Several observations in laboratory compressive tests indicate that somewhere near the peak load, rock deformation becomes localized and, after the peak, evolves into a fracture, fault, shear band, rupture zone or simply a failure plane (Waversik & Fairhust, 1970; Labuz et al., 1996).

Complete and accurate experimental results are given by a series of tests carried out at the Laboratory 3S in Grenoble. For such tests a biaxial apparatus, developed by Desrues (1984) and modified by Charrier et al. (2001), has been employed in plane strain conditions. The results of the tests, carried out in undrained conditions and without stress confinement, has been elaborated by means of a stereo-photogrammetric technique (Desrues, 1984, 1995). Such analysis allows to obtain a complete and quantitative representation of the specimen deformation and explicit data relative to the formation and propagation of shear bands.

Microscopical studies based on Fracture Mechanics (Reches & Lockner, 1994; Horii & Nemat-Nasser, 1985), show that a shear fault nucleates by local interaction among a few micro-cracks and that it propagates into the unfaulted region by inducing micro-crack growth at its front. This micro-structural process of breakdown near the crack tip can be interpreted by assuming that it gives rise to cohesive stresses, which oppose the action of applied loads. Thus, a Non Linear Fracture Mechanics approach, based on a cohesive-zone model, seems promising for an appropriate macroscopical representation of shear crack propagation in rock. Palmer & Rice (1973) first developed such model (Slip-Weakening Model, SWM, also analyzed by Li, 1987).

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