Rock/Concrete Direct Shear Constant Normal Stiffness

In underground excavations, concrete dam foundations, drilled piers socketed into rock and rock anchors, dilation during sliding is constrained by the rock/concrete mass and the normal stress is not constant along a discontinuity. This boundary condition is better represented by a constant normal stiffness (CNS) direct shear test than the typical constant normal load (CNL) direct shear test. This is likely also the case for characterizing the strengths of concrete in large mass concrete dams where the normal stress may not be constant along the discontinuity during sliding. This research developed the control software for a CNS boundary condition for an existing rock/concrete direct shear machine. After the programming was complete, a series of commissioning tests were performed, including a number of identical: a) 6-inch diameter concrete specimens, and b) sawtoothed hydrostone specimens to mimic rock discontinuities. Intact concrete specimens were tested for break-bond and sliding friction strengths while the hydrostone specimens were only tested for sliding friction strength. Both sets of tests were performed under both CNL and CNS conditions to investigate the effects on shear strength due to the different boundary conditions. This paper details the system upgrades and the results of the comparison testing under CNL and CNS boundary conditions, and provides examples of CNS data use in design.