ABSTRACT
While laboratory direct shear testing of rock fractures in constant normal stress (CNL*) conditions are well established in practice, testing in Constant Normal Stiffness (CNS) boundary conditions is less common and few recommendations on CNS testing are available regarding appropriate laboratory testing conditions. This paper provides practical guidelines needed to design an effective suite of CNS test conditions on rock joints and other fractures that can be applied to a variety of rock types based on intact elastic properties. The results of 42 direct shear tests on NQ and NQ3 size granitic drill core under both CNS and CNL* boundary conditions are analyzed and discussed in this study. Boundary conditions were found to have minimal effect on pre-yield sample deformation properties including normal stiffness and shear stiffness, but have significant influence on peak shear strength and post-peak residual shear strength and dilation. Finally, sources of error that may arise in direct shear testing that are relevant to boundary conditions are discussed and potential solutions are presented.
