The behavior of joints and intact rock is characterized by some parameters which have interactive effects on rock mass problems. Moreover, some of these parameters are not easily measured and they may be variable in the region of the problem. Joint normal and shear stiffness are the mechanical parameters which can be assessed in distinct element modeling. Knowing that the strength conditions of discontinuities are determined by the stress distribution through the model and along discontinuities, this paper is concerned on the effect of selected stiffness values on Distinct Element Method (DEM) computed stresses in the rock masses.
Development of distinct element method (DEM) has been a significant step to identify the behavior of fractured rock masses. The behavior of joints and intact rock is defined by some parameters which have interactive effects on problems. Some of these parameters are not easily measured and furthermore may be variable in the region of the problem. Thus it is momentous to recognize the influence of variation of these parameters and their interactive effects on reliability of modeling results. Joint normal and shear stiffness are the mechanical parameters which can be evaluated by distinct element modeling. It sounds to be essential to know the effects of selected stiffness values on the obtained results.
Joint stiffness is not an easily measured or well known parameter. Methods of estimating joint stiffness have been derived. Two possible methods are presented here. One is based on the deformation properties of the rock mass and the intact rock; the other is adopted from the properties of the joint infilling material.
Approximate stiffness values can be backcalculated from information on the deformability and joint structure in the jointed rock mass and the deformability of the intact rock. If the jointed rock mass is assumed to have the same deformational response as an equivalent elastic continuum, then relations can be derived between jointed rock properties and equivalent continuum properties. For uniaxial loaded rock containing a single set of uniformly spaced joints oriented normal to the direction of loading, the following relation applies.
Another approach for estimating joint stiffness assumes that a joint has an infill material with known elastic properties. The stiffness of a joint can be evaluated from the thickness and modulus of the infilling material by the following equations.
Values for normal and shear stiffness of rock joints typically can range from roughly 10 to 100 MPa/m (for joints with soft clay infilling), to over 100 GPa/m (for tight joints in granite and basalt). Published data on stiffness properties for rock joints are limited; summaries of data can be found in Kulhawy , Rosso  and Bandis et al. .