# Research on the Self-Drilling Anchor Pull-Out Test Model and the Stability of an Anchored Slope
## 2 Experimental Analysis of the Key Performance of Hollow Self-Drilling Anchor Bolts
### Analysis of the Experimental Results
This indicates that, within a specific range, increasing the anchorage length can effectively enhance the uplift performance of self-drilling anchor bolts. As for bond strength, theoretical studies on the elastic shear transfer model [12; 13; 14; 15] show that the bond strength distribution along the surface of the anchor bolt anchorage body is uneven. Simultaneously, by observing the blue curve in Figure 5b, we find that the coefficient of variation for longer anchorage sections is significantly lower than for shorter ones,suggesting that short anchorage sections are more affected by rock mass defects, whereas longer sections reduce randomness through spatial averaging effects. Moreover, as the anchorage length increases, the bond strength shows a decreasing trend, with the RS4 group exhibiting the highest bond strength and the RS10 group the lowest, indicating a significant negative correlation. This is consistent with the \"effective anchorage length\" theory, which suggests that simply increasing the anchorage length does not necessarily improve the load-bearing performance. Additionally, the non-uniformity of the spatial distribution of bond strength indicates that the average bond strength value obtained from traditional calculation formulas does not accurately reflect the actual stress field distribution. The shear stress at the interface of the anchorage section decays exponentially along the axis, with the maximum shear stress concentrated in the proximal region. As the anchorage length increases, the proportion of the effective load-bearing section decreases. Beyond the critical length, the distal section does not participate in load transfer, leading to an overall reduction in bond strength, particularly noticeable in soft rocks and fractured strata. We calculated the increment in ultimate uplift capacity and the decrease in average bond strength between adjacent anchor bolt groups, from which we can plot the ultimate uplift capacity increment curve (blue curve) and the decrease in average bond strength curve (red curve). From Figure 6, we can observe that as the anchorage length increases, particularly in the range of 6 to 10 m, the increment in the ultimate uplift capacity of the self-drilling anchor bolts shows a significant decrease, dropping from 46.5% to 14.7%. Correspondingly, the decrease in the average bond strength gradually increases from 5.9% to 9.3%. Although a comparison of the magnitude changes reveals that between anchorage lengths of 5.68 m and 9.65 m, the increase in the decrease in average bond strength is not significant, and it follows a nearly linear relationship with a relatively mild curve, the increment in the ultimate uplift capacity shows a diminishing trend with increasing anchorage length. For the 10-m group (RS10), the increment has rapidly decreased to 14.7%. This suggests that, from an efficiency perspective, the anchorage length of self-drilling anchor bolts in this type of soil layer should not exceed 8 m.  
Figure 6: Graph of the relationship between the increase in ultimate uplift capacity and the decrease in bonding strength.