## 4 Discussion
Although the rock bolt diameter has a positive impact on the pull-out strength and increases the ultimate tensile strength of the rock bolt, rock bolts with a large diameter are uneconomical and pose difficulties in installation. Along with the shape, length, and diameter of the rock bolt, the presence of face plate and optimum annulus space during rock-bolt installation enhances the load transfer in rock bolts from the rock mass. By increasing the diameter of the rock bolt and decreasing the annulus, evaluation from the economical point of view and installation is important. In fully grouted rock bolts, as the annulus is filled with grout, the mechanical properties of the grout have a direct effect on the performance of the rock bolt. Although resin as grouting material has better mechanical properties than cement, the improper mixing of the chemical during rock-bolt installation decreases its efficiency. In cement grouts, the water-cement ratio has an adverse effect, and the grout's settling time is directly related to the pull-out strength of rock bolts. The effect of these parameters is also dependent on the rock-mass quality and the pull-out strength increase for the same water-cement ratio and settling time in good-quality rock masses (Figure 7). The load-carrying capacity of rock bolt increases significantly with curing time for both moderate and weathered rock. In pull-out tests, the pull-out resistance for moderate rock increased considerably; however, load-displacement curve slope did not show any significant difference for the curing times. In the weathered rock case, curing time not only affects the pull-out resistance but also affects the gradient in the load-displacement curves. The average values for the inflection point are 57, 103, and 137 kN for 12, 24, and 36 h grout settling time, respectively, in weathered rock (Figure 7). In the case of moderate rock, these values are 73 and 150 kN for 12 and 24 h grout settling time, respectively. The literature shows that load-carrying capacity increases up to 35 days [36]. Traditional grouted rock bolts have limitations in high-stress environments, owing to their rigid nature. In high-stress in situ environments, they are unable to absorb energy. Generally, in such situations, energy-absorbing rock bolts like split set, expansion shell bolt, inflated steel-tube rock bolt, and swellex are used (Figure 2). The development of fully grouted D-bolt has replaced these traditional energy-absorbing rock bolts, owing to its high load-carrying capacity and absorbing energy [22; 23]. During tunnel excavation, the installed rock bolts at different locations along the tunnel periphery behave differently, owing to the confining stresses on each rock bolt due to the induced stresses. Laboratory studies indicate that confining stresses not only increase the pull-out strength but also affect the fracturing mechanism of the rock surrounding the rock bolt (Figure 8). The field test indicates that blast-induced vibration does not affect the performance of fully grouted rock bolts and empirical classification, as well, not considering the dynamic effect as a characterizing factor. However, the wave reflection at tunnel periphery produces a negative effect on the rock-bolt performance. The rock-support and ground-reaction curves indicate that the time of rock-bolt installation with other support and reinforcement materials has a direct effect on the support performance [60]. The installation time depends on the stand-up time, which is a function of the rock-mass quality and tunnel span [61]. The empirical system proposes a composite support, and, therefore, the stability of the composite support is essential from the economic and safety points of view. As shown in Figure 9, with the increase in radial displacement (u), the elastic limit that can be reached by one support of the composite system and its input, in terms of stiffness (\(k_{i}\)), becomes zero.