## 4 Discussion  
Over the past few decades, the use of rock bolts for the stability of underground excavations has increased. For rock tunnel stability empirical rock-mass classification systems like, RMR, Q, and RMi, are used for preliminary support, which include rock bolts during both the design and construction. In the empirical-support design, the excavation span and rock-mass quality predict the rock-bolt pattern of 20 mm diameter fully grouted rock bolts, along with other support and reinforcement materials. Figure 12: Groundwater (slight leakage) effect on the performance of grouted rock bolts. Figure 13: Groundwater effect on the performance of (**a**) grouted rock bolts and (**b**) inflated steel-tube rock bolt. The criteria for the rock-bolt pattern has been modified with time in the empirical system [59], and in its current version, the rock-bolt pattern is used in rock-mass quality calculation for the extension of the empirical system to such in situ environments that were not previously covered by these systems [13; 14]. Although the applications of the empirical classification system in the tunnel-support design are widespread, in the support design, it should be noted that the application of fully grouted rock-bolt patterns are based on average values. The actual circumstances while using rock bolts in the tunnel support are not so simple that they can be predicted from the rock-mass characterization, followed by classification of supporting tunnels, using empirical systems. To design rock-bolt patterns in a safe and economical way, using empirical systems, all the factors affecting the performance of rock bolts should be evaluated. In this study, major factors are evaluated from the rock-bolt pull-out test results, as per ASTM standards for this purpose [18]. For accomplishing this task, several field tests were conducted, and the results of the previous studies are also included. Based on the field tests, laboratory test, and results from numerical modeling, the factors affecting the fully grouted rock-bolt performance are broadly divided into five types: (1) rock-bolt-related factors, (2) grout-related factors, (3) stresses, (4) rock-bolt installation, and (5) rock-mass condition. The performance of the bolts depends on the load-transfer mechanism; the shape and type of bolt have a direct influence on this load-transfer mechanism [24]. RMR, Q, and RMi systems are based on the tunneling experience, where rock bolts are frequently used when compared to the use of cable bolts in the mining industry. Owing to ribs on the rock-bolt longitudinal profile, mechanical interlocking enhances the load-transfer mechanism. The host rock plays a significant role in the performance of fully grouted rock bolts [31], and field pull-out test results (Figure 4) indicate that, as the rock-mass quality improves, the load-carrying capacity of the rock bolt increases. Average pull-out resistance results with variation in rock-bolt length shows that the pull-out resistance increases with the length of the rock bolt for moderate rock. In soft and weathered rocks, the weak bond between the grout and bore-hole surface results in a slight change in the anchorage force with the length of rock bolt. In weathered rocks, the rock-bolt length has a negligible influence on the pull-out strength and displacement. The pull-out displacement decreases more with the increase in rock-bolt length; however, the influence of rock mass on this decrement is high. Although the load-carrying capacity of the bolt increases, this capacity should not be increased up to the ultimate tensile strength of the rock bolt [50]. In the field, different diameters of rock bolts are used, and the increase in their diameter has a linear positive impact. For the same grade of steel, the load-carrying capacity of the rock bolt increases with diameter.