Laboratory Investigation on Shear Behaviors of Bolt–Grout Interface Subjected to Constant Normal Stiffness

Shear behavior of the bolt–grout interface basically depends on site-specific boundary conditions, i.e., initial normal stress and normal stiffness of the borehole wall. Few studies have been conducted to investigate the shear behaviors between the bolt and grout material under different boundary conditions. To better understand the effect of boundary conditions on the shear behaviors of the bolt–grout interface, simplified two-dimensional (2D) bolt–grout interface specimens are prepared and tested using direct shear tests at various initial normal stresses and normal stiffnesses in this context. The testing results showed that both initial normal stress and normal stiffness significantly influence the bolt–grout interface shearing behaviors. Increasing the normal stiffness or initial normal stress would increase the peak and residual shear strength. However, the degree of the brittleness after the peak, normal displacement, and peak friction coefficient was reduced. It is noted that the peak and residual shear strength points under the constant normal stiffness conditions located near the peak and residual strength envelopes for the constant normal load tests, respectively. Besides, the shear failure process of the bolt–grout interface was captured by PAC acoustic emission (AE) monitoring and digital camera technology, a good correlation between the evolution of AE parameters and the shear stress curves was obtained.