## 2 Experimental Test
### Experimental Results
#### 2.5.3 Sand Deformation  
To ensure the accuracy of the sand surface deformation measurements, the sand surface was leveled before applying displacement. Under push \(u_{\rm t}\), a void can be found at the sand surface near the proximal end of the FAS specimen, while a bump can be found at the sand surface near the distal end. The sand surface deformation of H63-Push under  
Figure 7: \(F\)-\(u\) curves obtained from tests and finite element models. a +50 mm \(u_{\text{t}}\) is illustrated in Figure 8. Under pull \(u_{\text{t}}\), a bump can be found at the sand surface near the proximal end of the FAS specimen, while a void can be found at the sand surface near the distal end. The sand surface deformation of H63-Push under a \(-50\) mm \(u_{\text{t}}\) is shown in Figure 9.  
The sand surface deformations of different test cases observed through observation windows \(O_{1}\) to \(O_{3}\) are compared in Figure 10.  
Figure 8: Sand surface deformation in H63-Push: (**a**) void near the proximal end; (**b**) bump near the distal end. Figure 10: \(Cont.\)  
Figure 9: Sand surface deformation in H63-Pull. From the \(F\)-\(u\) curves and the sand surface deformation diagrams under different \(u_{\rm t}\), it is found that the sand surface deformations under push and pull \(u_{\rm t}\) exhibit similar characteristics, which can be divided into three stages:  
* Elastic stage When \(u\) is less than \(u_{\rm a}\), the FAS specimens under push and pull \(u_{\rm t}\) demonstrate coordinated deformation characteristics with the sand. The elastic deformation of the sand can efficiently absorb \(u\). No obvious crack or deformation can be observed on the sand surface. * Elastoplastic stage When \(u\) is between \(u_{\rm a}\) and \(u_{\rm b}\), certain areas of the sand (as indicated by the dotted line in Figure 10) gradually transform from elastic deformation to plastic deformation. The sand in the area demarcated by the dotted lines moves as depicted by the arrows in Figure 10. Remarkable differences in the deformation modes of the sand under push and pull \(u_{\rm t}\) can be observed. Under push \(u_{\rm t}\), at the dashed line AB, the sand moves upwards to the point \(x_{\rm 2b}\) on the sand surface along the horizontal angle \(\theta_{\rm 2b}\) from A to B; at the dashed line CD, the sand moves upwards to the point \(x_{\rm 2a}\) on the sand surface along the horizontal angle \(\theta_{\rm 2a}\) from C to D; and at the dashed line EF, the sand moves downwards from the point \(x_{\rm 1b}\) on the sand surface along the horizontal angle \(\theta_{\rm 1b}\) from F to E. Consequently, a bump with a width of \(w_{\rm 2}\) and a height of \(v_{\rm 2}\) can be found on the sand surface from \(x_{\rm 2a}\) to \(x_{\rm 2b}\) near the distal end  
Figure 10: Measured sand surface deformation: (**a**) H21-Push; (**b**) H21-Pull; (**c**) H31.5-Push; (**d**) H31.5-Pull; (**e**) H63-Push; and (**f**) H63-Pull. (The dashed lines AB, CD, EF, and EG represent the slip surfaces; the red arrows indicate that the sand moves upward along the slip surfaces; the blue arrows indicate that the sand moves downward along the slip surfaces). of the FAS specimen and a void with a width of \(w_{1}\) and a depth of \(v_{\rm s1}\) can be found on the sand surface from \(x_{\rm 1a}\) to \(x_{\rm 1b}\) near the proximal end of the FAS specimen.