# Shear Mechanical Properties of Bolt-Grout Interface under Different Bolt Surface Profiles
## 3 Test Results
### Shear Stress-Shear Displacement Curves  
The shear stress-shear displacement curves of interfaces for five rib spacings under various normal stresses are displayed in Figure 8a-e. From these figures, the curves for each of the five groups, with varying rib spacings, clearly show a typical trend of peak-shaped curves. These curves can be classified into six stages, as shown in Figure 8f. Figure 8: Bolt-Grout interface, shear stress-displacement curves with different rib spacing (Points o, a, b, c, d and e in subfigure (**f**) are the initial point, the deformation critical point, the first peak point, the minimum point, the second peak point and the stable point respectively). The first stage is the pre-peak elastic deformation stage (o-a). The shear stress increases linearly and quickly, reaching roughly 80-90% of the peak strength. This stage exhibits an approximately linear mechanical behavior, and the shear stiffness is large. The second stage is the pre-peak nonlinear deformation stage (a-b). During this stage, the shear stiffness gradually drops and the shear stress reaches its peak. The microcracks at the interface sprout and expand rapidly, penetrating along the entire shear surface. The interface begins to produce irreversible plastic deformations. Under the continuous action of the shear force, the interfacial gypsum bumps continuously wear out, resulting in weakened interfacial engagement and reduced interfacial shear stiffness. The third stage is the stress drop stage (b-c). The shear stress drops sharply, showing typical brittle damage characteristics, and the sharp drop in stress is sometimes accompanied by a low sound of specimen fracturing. In this stage, due to the shear stress exceeding its shear strength, resulting in crushing or shearing of the gypsum between the ribs, the interface failure will produce a slight slip behavior within a short period of time. The fourth stage is the shear stress rebound stage (c-d). The shear stress rebound is followed by up-and-down fluctuations in the phenomenon, and the curve is jagged. This is because the original Bolt-Grout interface failure, with the shear displacement, continues to advance, and the upper and lower two specimens produce a new interface. The new boundary at the re-contact once again features interactions, so the shear stress begins to increase, coupled with the generation of new cracks. Changes in crack distribution and the lubricating effect of the debris caused the shear stress curve to fluctuate irregularly in a saw-tooth shape. The fifth stage is the strain softening stage (d-e). This stage of the interface still has a certain bearing capacity. This capacity aids in the increase in shear displacement and gradual reduction, demonstrating the clear strain softening phenomenon. The sixth stage is the residual friction stage (e-). The curve tends to flatten out, and the shear stress tends to the residual strength. Figure 9a-e shows the shear stress-displacement curves of interfaces under different normal stresses with five different rib angles. Two types of curves were observed. The first type, which is the peak-type, shows that the overall trend of the curve for the Bolt-Grout interface is relatively unaffected by the rib angle when it exceeds 15\({}^{\circ}\). In this case, the rib angle has minimal bearing on the curve's general behavior, and the curve's overall change rule essentially remains constant under varying normal stresses, which is in line with the typical trend of the peak curve. The second type is one with no peak where, at a rib angle of 15\({}^{\circ}\), the shear stress-displacement curve and the other rib angle exhibit a different shear stress-displacement curve trend with no discernible peak. The first peak-type curve has a similar trend to the shear stress-displacement curve, with different rib spacing. As shown in Figure 9f, the non-peak-type curves can be analyzed in two distinct stages:  
The first stage is the elastic deformation stage (o-a), which is similar to the first stage of the peak-type curve.