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Shear stiffness of rock joints based on direct displacement measurements
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DOI：10.1016/j.jrmge.2026.05.040.png)
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Abstract
En 中文
Joint shear displacement is a fundamental factor governing the mechanical behaviour of rock joints under varying stress conditions. Shear stiffness, which relates shear displacement to shear stress, is therefore a key input parameter for predicting the mechanical stability and hydraulic permeability of rock masses. However, conventional laboratory measurements of relative displacements in shear boxes include deformations beyond those of the joint itself, arising from inherent deficiencies in the testing system. This paper presents novel insights into shear stiffness, enabled by optical displacement measurements obtained directly from the joints in a comprehensive experimental programme that significantly extends the range of previously investigated conditions. A total of 46 direct shear tests were conducted on natural and tensile-induced granite rock joints under two boundary conditions: constant normal stress and constant normal stiffness. An initial normal stress of 5 MPa was applied to joints of dimensions 35 mm 60 mm, 70 mm 100 mm, and 300 mm 500 mm. Shear stiffness values were determined using a novel, consistent approach during the elastic deformation interval, in which the displacement rate remained constant. Comparisons between indirect and direct displacement measurements clearly demonstrate that indirect measurements are unreliable, consistently underestimating shear stiffness by a factor of 5. Consequently, direct displacement measurements are essential for accurately determining the shear stiffness of rock joints. However, they cannot entirely replace conventional indirect measurements due to the risk of tracking loss during shearing at large displacements, which arises from crack formation along the joint traces. indicates that shear stiffness depends on joint type, with a pronounced negative correlation between small and large specimens, whereas the limited number of medium-sized specimens makes inferences for this group uncertain. Contact pressure measurements indicate that the initial contact pressure distribution primarily governs shear stiffness.
Keywords:
analysis of variance (ANOVA)
digital image correlation (DIC)
direct shear testing
rock joints
scale effects
shear stiffness
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