## 2 Experimental System and Test Procedure  
### Design of Flexible Device for Slope Subsurface Monitoring  
The AE essence of an active waveguide is the relative motion between sliding mass and slope bed on the backfill materials. It is necessary to fix the pile in the deep stable bed by boreholes, so as to block the backfill materials and thus the generated AE can continue. Flexible piles are bent when subjected to transverse load while the pile bottom remains static. An anchor cable is a kind of flexible pile. On the one hand, an anchor cable possesses a certain stiffness which can block the movement of the sliding body and stimulate AE of backfill materials. On the other hand, an anchor cable possesses some resilience which allow a greater degree of transverse shear and bending. However, if both ends of the anchor cable are fixed, failure is likely to occur under tension. A new subsurface monitoring device was designed with reference to the structure of a large-deformation bolt [38; 39], changing the material deformation into structural deformation. The main structure is shown in Figure 1 and is detailed as follows. An inner threaded sleeve was added outside the upper part of an anchor cable. The sleeve was embedded in the slope and was fixed on the slope surface. A conical metal head was installed on the top of the anchor cable. When the sliding body moved, the sleeve deviated upward from the anchored end. From another perspective, the anchor cable and its metal head moved flexibly along the axial direction in the sleeve; thus, the anchor cable was not easy to break as a result of tension. The anchor cable may be able to deform axially a large distance determined by the length of the sleeve and this structural characteristic may bring about an increased measurement range for slope deformation monitoring. The diameter of the metal head was slightly larger than the diameter of the thread on the inner wall of the sleeve, and the internal thread would be gradually worn down from top to bottom. The anchor cable fixed on the slope bed hindered the movement of the sliding body, played the role of reinforcing the slope mass and limiting its deformation. Consequently, the device had a potential ability to stabilize the slope. The sleeve was short in length and did not pass through the sliding surface to avoid shear failure. The movable pedestal circled outside the sleeve was located on the ground surface and provided upward support for the sleeve. The ring dynamometer was installed on the pedestal to measure the axial tension within the anchor cable and was fastened with a nut. The AE portion of the proposed device was described as follows. A rubber tube was placed along the inner wall of the borehole and passed through the sliding surface. The rubber tube was located outside the anchor cable, and sufficient backfill materials were filled into the space between the rubber tube and the anchor cable. The rubber tube can not only isolate the backfill and external geotechnical conditions to reduce the impact of the surrounding environment to AE generation, but also can maintain its resilience when subjected to large deformation. An AE transducer was fixed on the metal head of the anchor cable through the coupling agent. As the slope moved, the deformation of rubber tube would be much larger than that of anchor cable. The difference of internal and external deformation would cause the extrusion, collision and friction of backfill materials, and hence AE was generated. The AE characteristics during this period depended on the interaction between backfill materials and anchor cable, and the dominant frequency was concentrated within 20-30 kHz based on frequency spectrum analysis. As a metal waveguide, the AE signal attenuation along the anchor cable was low, and the AE parameters could be useful to quantify the deformation of the sliding body relative to slope bed. The designed device integrates the structure of a large-deformation bolt into an active waveguide.