Discontinuity geomechanical properties are becoming increasingly more important to measure as the use of numerical models with explicit structure becomes the state of practice. Joint normal stiffness and joint shear stiffness are two parameters that characterize the deformation behaviour of discontinuities, and in many cases when determined from laboratory testing are too low when used as inputs in numerical models. This paper presents practical guidelines to correct direct shear testing data for machine influences with regards to joint normal and joint shear stiffness using a method originally employed by Goodman (1976). In cases where joint deformations cannot be measured directly on the joint, an intact specimen should be tested under direct shear in order to determine the deformation associated with the system.
Rockmasses are composed of two components; intact rock and discontinuities. In the majority of cases, the rockmass behaviour for surface and near surface engineering projects is governed by the discontinuity component. For engineering design associated with these projects, laboratory direct shear testing is a critical but expensive tool used to characterize and measure the stiffness, strength, and dilative behaviour of rock joints and other fractures. Furthermore, these geomechanical properties are then used as inputs to numerical models with explicit structures (Figure 1).
While laboratory testing is the logical way to determine numerical model inputs for fractures, it is common practice to modify the determined properties from laboratory testing to prevent the model from becoming unstable or return model results that better represent reality (Li et al., 2010). In the case of discontinuities, joint normal and joint shear stiffness inputs are one of the most commonly modified laboratory determined properties as it is commonly found to be to low. One of the potential reasons for this consistent return of low joint stiffness properties may be due to the nature of direct shear laboratory testing and the machines that are used.