Scale effect is a phenomenon of increasing importance in rock mechanics and particularly in the field of rock joints, and as large scale experiments are difficult to handle, numerical modeling can be considered as an alternative way to study the macro joints mechanical behavior. A meso joint constitutive behavior model including an original dilatancy angle evolution law and an original rupture mechanism has previously been presented. This paper presents some improvements made on the previously developed model for large shear displacement of rock joints, and presents some results obtained using an originally developed finite displacement concept, taking in account the local opening phenomenon and the physical interactions. The results indicate changes in the dilation curves, peak shear stress decrease, initial shear stiffness decrease and peak shear displacement increase with joint size for shearing at constant normal stress. The results show a stabilization of the scale effect for macro joints of more than two meters.
Scale effect is a phenomenon observed in all geomaterials. In rock mechanics, the notion of scale can be defined at three levels: the micro scale (millimeter order), the meso scale (decimeter order) and the macro scale (more than one meter), with a differences in the mechanical properties of joints at each scale. The mechanical properties of rock joints at the micro scale (asperities) are excluded from the domain studied. The mechanical parameters related to the meso scale are accessible by specific mechanical tests and morphologic measurements in the laboratory (Hans et al. [1], Esaki et al. [2]). However, it is very difficult to evaluate properties at the macro scale because of handling difficulties.
The first practical study concerning scale effect phenomena on rock joints was made by Pratt et al. [3], by making direct shear tests on natural joints. As a result, a linear decrease in peak shear strength (semi logarithmic scale) as a function of the joint area (linear scale) was observed.
A more complete study of scale effect was carried out by Bandis et al. [4]. By increasing joint size, the following phenomena were observed: a decrease in the peak friction angle caused a decrease of the peak shear stress, and of the dilatancy angle. Later, another study was carried out by Barton et al. [5]. They developed a model able to predict macro joints behavior, with the same order of results as found by Bandis et al. [4]. Moreover, Yoshinaka et al. [6] observed a reduction of the shear strength with increase in specimen size. Castelli et al. [7] confirmed the reduction of shear strength with increasing joint size. A decrease in contact area with an increasing scale was also observed.
Previous studies on meso and macro constitutive modeling of rock joints have been carried out by the same authors (Vallier et al. [8]). However, at the macro scale, only small relative displacement assumption was considered (i.e. local physical interactions between elements neglected). Lack of changes in dilation curves made it necessary to implement the model for finite displacement. New results are presented in the following sections
