A widespread case of discontinuity shearing is found among local shallow rock blocks sliding along non-planer, rough, non-anchored discontinuities, which is characterized by an initial normal load in addition to free subsequent changes in local normal load/stiffness, provoked by shear dilatancy (INL). The existing direct shear apparatuses of rock discontinuities fitted for those cases are rare, and there is a lack of understanding of the shear behaviours of discontinuities under such INL boundary conditions. Here, we develop an INL direct shear tester of discontinuities for shear strength. This tester takes advantage of the direct shear test apparatus modified after the YZ-5 model without a servo-controlled system to permit the accomplishment of those boundary conditions. Shear behaviours of unfilled and infilled discontinuities under INL boundary conditions are investigated. Moreover, the possible influence of clay content on those shear behaviours of infilled discontinuities is clarified. The developed tester made it possible to test the shear performance of the local shallow rock blocks sliding along non-planer, rough, non-anchored discontinuities. This work contributes to a limited body of research on the shear behaviours of discontinuities under those INL boundary conditions. Limitations of this work are also revealed.

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