## 3 Test Results and Analysis
### Deformation Process of Schist Sample under Biaxial Step Loading  
To explore the evolution process and displacement variation of cracks in schist samples, sample SC-BP-15 was taken as an example for analysis. Figures 9 and 10 highlight eight crucial nodes during the biaxial step loading of sample SA-BP-15. The maximum principal strain \(e_{1}\), the strain \(e_{xx}\) in the X-direction, the displacement \(U\) in the X-direction, and the displacement \(V\) in the Y-direction were among the data acquired by the 3D-DIC system. To better comprehend, the contour was obtained by rotating the photo taken after the test counterclockwise by 90\({}^{\circ}\). Overall, the process can be divided into three stages: the quiet stage (A-D), crack initiation stage (D-G), and floor heave deformation stage (G-H). Figure 8: Image of sandstone after failure. The axial load started at point A after the lateral stress was applied. The strain field was currently quite symmetrical and uniform. As shown in Figures 9 and 10, when the second level of load was applied (point B), a local compressive stress concentration point emerged in the rock near the bottom of the invert center, which may also be considered as stress concentration. Due to the axial asymmetry of the structure, the application of axial force resulted in two effects: compression and bending torque. When the axial force was small, the neutral axis of the sample was located at the lower middle position, and there was compressive strain concentration on the weak plane between layers. Before the application of the seventh level of loading (point C), the local compressive concentration zones propagated and interconnected to form a complete strain concentration zone coinciding with the location of the layered weak surface. In the quiet stage, obvious macroscopic cracks were not observed on the rock surface. The displacement in the X and Y directions was symmetrically distributed along the middle position of the inverted arch, but the directions of the left and right parts of the displacement in the Y-direction were different. Under the influence of vertical stress and bending moment, the middle part of the sample tended to bulge outward. Then, the left half of the sample showed a shear strain concentration zone subparallel to the bedding weak plane at point D, indicating that a macroscopic shear crack \(\beta\) was formed, beginning the crack initiation stage. At this time, the difference in displacement between the two sides of crack \(\beta\) was also significant. The peeling process was similar to that of spalling. Next, when the load rose at point E, crack \(\beta\) continued to extend and develop. The displacement field was altered at this point, causing the rocks on both sides of the crack to deform each other and move toward the free surface. When the load was increased to the 12 th level (point F), the strain began to concentrate on the right side of the center of the invert bottom, forming a small area of strain concentration. When the load reached 92.33 MPa (near the maximum load at point G), several tensile strain concentration zones emerged, leading to crack propagation in the X-direction. The strain concentration area increased and extended away from the facing surface, and the displacement field near the strain concentration area also formed a clear layered area. The displacement in the X-direction gradually increased along its negative coordinate, and the displacement contour plot exhibited a layered arch shape, indicating that the rock was about to undergo stratification and arching outward. During the process from point G to point H, high strain concentration zones aggregated and nucleated into extensional cracks that traversed the rock, causing it to fracture into a platy structure. The fractured layers were subsequently continuously broken and bulged outward, and near parallel tensile cracks overlapped at their distal ends, forming a trapezoidal failure surface composed of multiple cracks.