## 4 Discussion  
### Influence of Rock Type, Lateral Stress, and Invert  
Two types of rocks are discussed here: layered schist and homogenous sandstone. The elastic modulus of schist was higher than that of sandstone, and the rock particles were relatively dense, with weak deformation ability. The failure modes of schist and sandstone were quite different. Due to the weak bond strength between muscovite and bedrock, the main failure modes of schist are splitting and tensile failure. The sample was separated along the bedding plane to form a rock slab resembling a platy and beam structure. The comparison of failure modes of schist and sandstone under uniaxial compression showed that rock bedding weak planes had a substantial influence on the distribution and propagation of cracks. When the Y-direction load was parallel to the joint plane, the cracks preferentially propagated along the joint plane with the weakest strength. In contrast,  
Figure 12: Displacement curve of the measurement line \(L_{m}\) under different loading levels. Figure 13: Displacement-time curve of the measurement points: (**a**) sample SC-BI-10; (**b**) sample SC-BP-10. sandstone typically suffered from compressive shear failure and still had good integrity after failure. The inverted arch was used to improve the stress conditions at the bottom of the tunnel; thus, the intact sample under the same stress condition was tested for comparison. As shown in Figure 14a-f, distinct delamination fractures and ejection features were observed in the two shapes of schist samples, and the inverted notch changed from a circular arc to an inverted trapezoid after failure. The shape and location of the fracture surface indicated the areas of weakness in the surrounding rock, which must be reinforced and carefully considered when designing tunnel support. As shown in Figure 14g-i, the sandstone exhibited no apparent signs of interlayer splitting or surface bulging, and the main failure mode was characterized by shear along the diagonal of the sample. Compared with samples containing prefabricated inverted arches, the cracked slabs of intact schist samples were longer, with more layers of slabs, more severe damage, greater uplift displacement, and a wider range of damage. The inverted arch played a positive role in preventing the occurrence of floor heave. Presumably, a new free surface first formed once the upper layer of the rock sample from the inverted arch had been peeled off. Due to the weakened optimization effect of the new free surface compared to the older one, internal failures persisted, and some fragments peeled off from the surface. The floor heave of the intact rock sample without the inverted arch was more obvious. The layered rock plate and rock sheet were successively broken and uplifted from the near free surface outside to the deeper inside. One of the key elements affecting tunnel floor heave is high ground stress. As the lateral stress increased, the maximum value of \(F_{MB}\) gradually increased. In contrast to the traditional biaxial test, the order of the axial strain values was as follows after the same level of load was applied: \(\varepsilon_{5}>\varepsilon_{10}>\varepsilon_{15}\). Given that the lateral stress restricted the deformation of the rock sample under low-stress conditions, the restriction effect became more evident as the lateral stress increased. The ultimate axial strain value of failure followed the general law: \(\varepsilon_{15}>\varepsilon_{10}>\varepsilon_{5}\). At the same time, the larger the lateral stress was, the longer it took to reach the limitations of the bearing capacity of the samples. This demonstrated that deeper-buried tunnels would suffer more severe damage and generate greater displacement in actual tunnel engineering due to the greater surrounding stress accumulating more energy inside the rock.