## 2 Experimental System and Test Procedure
### Experimental System and Method  
The landslide experiment system consisted of three parts. The first was an experimental platform with a steel structure, the second was a mechanical loading system, and the third was a flexible monitoring device.  
As shown in Figure 2a, the size of the container was 1.4 \(\times\) 0.6 \(\times\) 0.9 m, and the bottom of the horizontal container was 0.3 m away from the ground. The inclination angle of the  
Figure 1: Flexible device for slope subsurface monitoring. 1: sleeve with inner wall threaded, 2: anchor cable, 3: conical metal head, 4: rubber tube, 5: backfill materials, 6: acoustic emission transducer, 7: ring dynamometer, 8: pedestal, 9: nut, 10: anchoring end.  
experimental platform could be adjusted flexibly within 0-20\({}^{\circ}\). The platform was equipped with a thick steel plate to simulate the sliding surface, and a test box (0.5 \(\times\) 0.5 \(\times\) 0.6 m) placed on the steel plate could move forward to simulate the local structure of the soil sliding body. The test box was made of 5 pieces of 6 mm-thick steel plate welded while the top plate could be disassembled. The centre of the test box was a hollow steel cylinder with the diameter of 11 cm, which simulated the borehole used for installing the flexible monitoring device.  
Figure 2: (**a**,**b**) are lateral view and top view photographs of landslide experiment system. (**c**) is the force analysis diagram of flexible device; F\({}_{1}\) represents the thrust applied to the rubber tube, F\({}_{2}\) denotes the upward axial tension to the sleeve, F\({}_{3}\) represents the lateral resistance of the anchor cable from the anchorage, F\({}_{4}\) denotes the downward binding force from the anchorage to the anchor cable.  
The mechanical loading system was mainly composed of a jack and a programmable logic controller (PLC) system. The PLC program was written in advance, and the jack could be controlled to move according to the velocity formula to realize a displacement-controlled shear test [37]. PLC could be used to design corresponding velocity formulas to be loaded in several stages during a landslide model test. The deformation process of a landslide could be reproduced, and the accelerating stage regarding slope failure was focused on.  
The flexible monitoring device was described in detail in the section \"Design of flexible device for slope subsurface monitoring', and was mainly composed of an anchor cable, sleeve and AE monitoring system. And the force analysis of the flexible device was shown in Figure 2c. The backfill materials consisted of granite gravel with average size of 6 mm and their properties are displayed in Table 2. The specific installation and operation procedure of the flexible device was as follows. First, the anchor cable was put through the hole in the middle of the iron box, and the bottom of the anchor cable was fixed under the thick steel plate (sliding surface) using the customized fixture. Second, the rubber tube was placed along the inner wall of the borehole and passed through the sliding surface. The part of the rubber tube below the sliding surface was fixed by iron bars. The space between rubber tube and anchor cable was filled with granular materials. Third, the upper part of the anchor cable was surrounded with the sleeve, and the conical metal head was installed on the top of the anchor cable. Fourth, the movable pedestal was circled outside the sleeve, and was put on the surface of the iron box. Then, the ring dynamometer was installed on the pedestal and was fastened with a nut. Finally, the AE transducer was fixed on the conical metal head. The signal wire, coupled with preamplifier, cable and acquisition instrument of an AE system were connected in order. AE data were exported to a dedicated computer and analysed by professional software.