An important aspect of the behavior of joints in rock masses is damage and degradation of mechanical properties due to progressive fracturing of the rock bridges or cemented materials existing in the joint. The damage can occur in both normal and shear modes. In this work a coupled normal-shear damage model is developed based on the mechanism mentioned here above. The joint parameters decrease as a function of a damage scalar variable "D" which increases from zero for non-damaged state to one for a totally damaged joint. This results to a coupled evolution of stiffness in normal and shear directions as well as equivalent cohesion and tensile strength of the joint. The damage variable D is a function of relative displacement in the joint. The damage criterion delimits a domain in stress space whose size decreases with the damage parameter increase. The model is implemented in CESAR-LCPC which is a general Finite Element code for civil engineering and geotechnical applications. The numerical applications carried out show that the model is capable to represent principle aspects of geotechnical damageable joint's behavior.
Existence of joints and discontinuities in geological structures modify dramatically the stress and deformation fields. This is the case also for engineering structures particularly for concrete joints and rock-concrete interfaces. Rock joints in underground constructions, mining, rock slope stability, geothermic sites and faults, concrete joints in massive structures like concrete dams and interface of concrete structures founded on or constructed in rocks are examples of this problem. These types of quasi-brittle materials reveal in general a damage behavior by stiffness softening which is recognizable especially under cyclic loading. Consequently the joints existing in these materials could show similar behavior. Damage may occur in the joints containing some continuous segments as rock bridges or cemented material as well as in the partially cemented rock-concrete interfaces. Some authors as Cervenka et al. [1], Carol et al. [2] and Puntel et al. [3] have proposed the joint models based on fracture mechanic concept and cohesive crack model (cf. Galvez et al. [4] and Bazant [5] for cohesive crack models). These models however do not take into account the stiffness reduction of joint and focus on the resistance deterioration due to fracture development in connected segments of the joints. Jefferson [6] proposes a plastic-damage model for cementitious interfaces. Jefferson [7] proposes also a "tripartite cohesive crack model" for concrete. These two models developed by Jefferson take into account many physical aspects of quasi-brittle joints behavior, they are however relatively complex on mathematical formulation. In the present paper a relatively simple model is proposed for quasi-brittle joints and interfaces. In this model damage may occur under normal, shear or mixed loading condition and results to lose of resistance parameters as well as stiffness of the joint. Deterioration of joint parameters in each direction (normal/shear) affects the parameters in the other direction. This model is applicable to a variety of rock and concrete joints and rock-concrete interfaces. The application of this model is particularly interesting in the case of cyclic loading; for example fluctuation of water pressure in concrete 1008 joints or bed joints of a dam so as in adjacent rock joints, and dynamic loading in all type of aforementioned joints. Another example is thermal and hydraulic oscillations affecting the stability of a fractured rock slope.
