## 2 Materials and Methods
### Test Procedure  
In this test, the self-developed large-scale interface shear equipment was used to carry out interface shear tests under two kinds of variable normal stress, namely the repeated loading and unloading of variable normal stress and the continuous loading of variable normal stress, as shown in Figure 6. The upper and lower shear boxes of the equipment were 400 mm \(\times\) 300 mm \(\times\) 100 mm and 500 mm \(\times\) 300 mm \(\times\) 100 mm, respectively. The normal pressure was applied by the gas booster cylinder. A large amount of load was applied by adjusting the main air valve of the cylinder, and the load was slightly adjusted by the auxiliary air valve. The maximum load could reach 50 kN, and the force stroke tolerance was +0.2 mm. The horizontal thrust was driven by the servo electric cylinder. The maximum thrust of the electric cylinder was 40 kN, the maximum stroke was 200 mm and the repeated positioning accuracy was \(\pm\)0.02 mm. The control system was also self-developed, with one-way push, one-way pull and cyclic loading functions. The loading mode could be controlled by the strain mode and load mode, respectively, through displacement sensor and force sensor to meet different loading requirements. During the shearing process, the feedback normal pressure change could be monitored in real time,  
Figure 4: Preparation of the standard sand: (**a**) Standard sand unsealing and (**b**) Standard sand box.  
Figure 5: Preparation of the clay sample: (**a**) Moisture content measurement. and (**b**) Clay box.  
which was convenient for analyzing the change of the real interface friction coefficient with the shear displacement.  
Among them, the research background of \"variable normal stress repeated multi-stage loading and unloading\" was the gravity anchor foundation of offshore wind power and the mooring system of offshore floating photovoltaic platform. Due to the influence of the marine environment, the base pressure was in a state of repeated change, that is, the process of loading, unloading and reloading. In order to study the change of the interface friction coefficient between the base material and the structure under the vertical load repeated loading and unloading, this type of scheme design was carried out, as shown in Figure 7.  
The research background of \"variable normal stress continuous multi-stage loading\" was the anchor foundation design of the world's first-span (2300 m) Zhang Jinggao Yangtze River Bridge. After the foundation construction was completed, the vertical pressure on the base continued to increase due to the construction of the cable tower and the lifting of the suspension cable and the bridge deck. In order to study the change of the interface friction coefficient between the base material and the structure under the continuous increase of the vertical load, this type of design scheme was carried out, as shown in Figure 8.  
Figure 6: Ultra-large interface shearing equipment.  
Figure 7: Repeated multi-stage loading and unloading of variable normal stress.  
In order to simulate the shear mechanical properties of the interface between the base soil and the structure under the condition of variable normal stress, clay, sand and cement soil were selected as the main base materials. Concrete was selected as the main structural materials in this test, and the curing time of cement soil was classified. The test scheme is shown in Table 1. The shear rate was 0.05 mm/s. Considering the large size of the test piece, it was very difficult to make cement soil samples on the surface of the concrete test piece and maintain them. The test only considered the sliding shear characteristics of the interface, and the influence of the bond strength between the cement soil and the concrete interface was not within the scope of this study.