This paper investigates the conditions favourable to the development of a kink zone instability in jointed rock masses in relation with: 1) the rate of deformation and 2) the boundary conditions. Numerical simulations using the distinct element code UDEC have shown that the rate of deformation influences the peak strength, volume variation and post-peak behaviour of kink bands. The model’s peak strength is influenced by the rate of deformation. An increase of the deformation rate leads to an increase up to five times the peak strength. Slow rates show a consistent strength-deformation-dilatancy relationship. Boundary conditions have an important influence on the initiation, orientation and dilatancy within the kink band. Non-coaxial deformation occurs when the boundaries are frictionless, and it does not lead to kink band development. Finally, the numerical modelling results also suggest that lateral freedom of movement of the upper platen is essential for the kink band’s formation.


The stability of rock slopes, underground openings and other works in intensely foliated or jointed rock masses, may be at risk with an unusual failure mode: kink zone instability. This type of instability is developed by buckling of brittle, ductile foliated or layered rock. Jointed and fractured rock masses also show the development of kink zone instability as a mode of failure or deformation. Goodman & Kieffer (2000) classified this mode of failure as “buckling and kink band slumping”. This happens under particular conditions of loading path, joint pattern geometry and orientation relative to the stress tensor and joint spacing relative to their length and stress level. Geomechanical designs of works in jointed and foliated rock masses rarely take into account such a mode of failure, and few studies have considered this problem (Hayashi 1966, Kawamoto 1970, Archambault 1972, Ladanyi & Archambault 1972, 1980, Rizopoulos & Boehler 1992).

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