## 3 Test Results and Analysis  
### Mechanical and Deformation Properties  
Table 4 lists the testing results of the maximum compressive loads for intact schist, schist with a prefabricated inverted arch, intact sandstone, and sandstone with a prefabricated inverted arch, denoted as \(F_{M\!I}\). Since the cross-section area of the rock sample with an arch was uneven, the actual stress varied in the specimen. Therefore, the maximum load instead of the stress was used to evaluate the bearing capacity of the sample. Compared with the intact sample, the maximum loads of the schist and sandstone samples with a prefabricated inverted arch were reduced by 26.43% and 43.24%, respectively. The testing results of the biaxial step-loading test are listed in Table 5, and the maximum loads under biaxial tests are denoted by \(F_{M\!B}\). The step-loading path of the biaxial step-loading test is shown in Figure 5. It can be seen in Figure 6 that during the stage when the load remains constant, the strain continues to increase with time, and the strain of the sample with a prefabricated inverted arch shows a stepwise relationship with time. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & \multicolumn{2}{c}{**Schist**} & \multicolumn{2}{c}{**Sandstone**} \\ \hline
**Samples No.** & \(F_{M\!I}\)**/**kN** & **Mean** & **Samples No.** & \(F_{M\!I}\)**/**kN** & **Mean** \\ \hline SC-UI-1 & 85.33 & & SA-UI-1 & 196.73 & \\ SC-UI-2 & 99.48 & 107.00 & SA-UI-2 & 195.25 & 191.90 \\ SC-UI-3 & 136.20 & & SA-UI-3 & 183.70 & \\ SC-UP-1 & 57.00 & & SA-UP-1 & 109.23 & \\ SC-UP-2 & 89.55 & 78.72 & SA-UP-2 & 111.93 & 108.93 \\ SC-UP-3 & 89.60 & & SA-UP-3 & 104.95 & \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The maximum compressive load of the prismatic samples. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Loading Method** & **Lithology** & **Sample Shape** & **Sample No.** & **Lateral Stress (MPa)** & \(\Delta F\) \\ \hline  & & & SA-BI-5 & 5 & 0.2\(F_{M\!I}\) \\  & & Intact sample & SA-BI-10 & 10 & 0.2\(F_{M\!I}\) \\  & Sandstone & & SA-BI-15 & 15 & 0.2\(F_{M\!I}\) \\  & Sandstone & Sample with & SA-BP-5 & 5 & 0.2\(F_{M\!I}\) \\  & & prefabricated & SA-BP-10 & 10 & 0.2\(F_{M\!I}\) \\  & & invert arch & SA-BP-15 & 15 & 0.2\(F_{M\!I}\) \\  & & & SC-BI-5 & 5 & 0.2\(F_{M\!I}\) \\  & & Intact sample & SC-BI-10 & 10 & 0.2\(F_{M\!I}\) \\  & & & SC-BI-15 & 15 & 0.2\(F_{M\!I}\) \\  & & Sample with & SC-BP-5 & 5 & 0.2\(F_{M\!I}\) \\  & & prefabricated & SC-BP-10 & 10 & 0.2\(F_{M\!I}\) \\  & & invert arch & SC-BP-15 & 15 & 0.2\(F_{M\!I}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 3: _Cont._During the initial stage of stepwise load application, there was a significant and immediate deformation that accounted for a relatively large proportion of the total strain at approximately 46.56% (SC-BP-5), 35.74% (SC-BP-10), and 29.33% (SC-BP-15). At this point, the mechanical properties of the samples near the bottom deteriorated rapidly. As the load was continuously applied, the instantaneous strain increments gradually decreased. The open structural surface and internal pore structures were compacted during the early compression process, resulting in relatively large deformation. When the load continued to  
Figure 5: Load path of biaxial step-loading test: (**a**) intact schist; (**b**) schist with a prefabricated inverted arch; (**c**) intact sandstone; (**d**) sandstone with a prefabricated inverted arch. (Notes: \(F_{UP}\) denotes the maximum uniaxial compressive load of prism).