## 3 Test Scheme and Result Analysis  
The measured prestress of bolt support is visually displayed using the drawing software Surfer 8.0, which facilitates better analysis of the distribution law of prestress field in composite rock layer anchor rod support. The prestress field of anchoring bolt support in hard rock is shown in Figure 3; the prestress field of anchoring bolt support in soft rock is shown in Figure 4. By comparing and analyzing the compressive stress area (Y = 1000-2100 mm) near the anchorage section of the two bolts, it can be seen from Figures 3 and 4 that this range includes the structural plane at the junction of soft and hard rocks. It can be seen that when the anchorage section is in hard rock, the compressive stress concentration degree and diffusion range are higher than that of anchorage section in soft rock, and its stress distribution along the direction of stress diffusion is wider than that of soft rock. The comparison of compressive stress area and tensile stress area at the upper end of the free section is shown in Table 3. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Scheme No.**} & **Crossing Direction of** & \multirow{2}{*}{**Anchoring Length**} & \multirow{2}{*}{**Test Variables**} \\  & **Anchor Bolt** & & \\ \hline
1 & Soft-hard & 445.1 mm & / \\
2 & Hard-soft & 445.1 mm & Properties of anchoring surrounding rock \\
3 & Soft-hard & 1271.7 mm & Anchoring length \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Test scheme. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multicolumn{1}{c}{**Properties of Anchoring**} & \multirow{2}{*}{**Hard**} & \multirow{2}{*}{**Soft**} & \multicolumn{2}{c}{**Properties of Anchoring**} \\
**Surrounding Rock** & & & \\ \hline Peak value of compressive & & & \\ stress/MPa & \(-\)0.35 & \(-\)0.2 &
\begin{tabular}{c} Peak value of compressive \\ stress/MPa \\ \(-\)0.2 MPa range area/m\({}^{2}\) \\ \(-\)0.15 MPa range area/m\({}^{2}\) \\ \end{tabular} & 0.26 \(\times\) 2 & 0.021 \(\times\) 2 & \(-\)0.2 MPa range area/m\({}^{2}\) \\
0.42 \(\times\) 2 & 0.13 \(\times\) 2 & \(-\)0.15 MPa range area/m\({}^{2}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Comparison of compressive stress area at the upper end of free section. The stress distribution of surrounding rock near the structural plane is shown in Figure 5. When the anchorage section is in hard rock, the stress values at the soft and hard sides of the structural plane are quite different, and the stress value of the hard rock side is significantly higher than that of the soft rock side. The compressive stress near the structural plane decreases rapidly along the anchor bolt diffusion direction. Taking the compressive stress at the X = 125 mm location at the hard rock side of the structural plane as the benchmark, the compressive stress at 250 mm, 375 mm, and 500 mm locations from the bolt body decreases by 27.1%, 50.4% and 73.9%, respectively, showing a law of linear attenuation. Taking the stress at the x = 125 mm location at the soft rock side as the benchmark, the compressive stress at 250 mm, 375 mm and 500 mm locations from the  
Figure 4: Prestress field of end anchoring bolt support in soft rock. Figure 3: Prestress field of end anchoring bolt support in hard rock. bolt body decreases by 26.5%, 39.4% and 46.9%, respectively, and the reduction amplitude decreases rapidly near the bolt and slowly far from the bolt, showing a law of exponential attenuation. Considering the diffusion range of compressive stress, the anchoring effect of the anchoring section in hard rock is better than that in soft rock. This phenomenon can be analyzed by using the [PERSON]. [PERSON] [18] stress solution of a concentrated force acting in the interior of the foundation in elastic space problems. Assuming that the distance from a location in the anchorage section to the surrounding rock surface is \(\omega\), the micro element with the length of \(d\omega\) is taken at this position, and the corresponding concentration force is \(dp=2\pi br(\omega)d\omega\).