###### Abstract
Most studies on AE monitoring are qualitative research, that is, qualitatively judging the state of a slope according to the level of AE [24]. AE was used to quantify the magnitude change of landslide displacement rate for the first time in 2007 [32]. Furthermore, the proportional relationship between AE rate and displacement rate was first discovered and mathematically described in 2014 [33]. In order to avoid rapid attenuation of the AE signal which is difficult to detect, the waveguide is usually used in slope monitoring [28]. Passive waveguide (grouted waveguide) is often used in rock slope monitoring [34; 35]. Steel pipe and granular backfills are used to form an active waveguide applied in soil slope monitoring [25; 28; 33; 36]. With the movement of surrounding soil, the active waveguide itself deforms and generates AE. The feature parameters of AE can be used to quantify the deformation behaviour of the soil slope [17; 23; 29; 33; 35; 37]. As a common parameter of AE, ring down count (RDC) refers to the number of times that the AE signal amplitude exceeds the pre-designed voltage threshold within a period of time [33]. In order to filter out environmental noise, AE signals are collected only when they are in a specific frequency range of 20-30 kHz, which is the dominant frequency range generated from an active waveguide [28; 29; 33]. However, the current waveguides are mostly metal tubes, which may undergo shear or bending failure during a landslide; thus, metal tubes are difficult to apply under the circumstance of large deformation (i.e., \(>\)500 mm). A kind of large \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Range** & **Precision** & **Cost** \\ \hline ShapeAccelArray (SAA) & hundreds of millimetres [23] & \(\pm\)1.5 mm per 30 m [30] & very high \\ \hline Inclinometer & \(<\)50 mm [23] & \(\pm\)0.01 mm per 500 mm & high \\ \hline Acoustic emission (AE) & \(>\)400 mm [23] & 0.0001 mm/h to more than 400 mm/h [31] & low \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison of three subsurface devices for slope monitoring. deformation bolt was invented and tested extensively [38; 39]. The structure of the AE waveguide can be improved with reference to this large-deformation bolt. There seem to be few studies on the design of AE waveguide aiming at large-deformation monitoring of slope. Furthermore, the advantages have been underexploited for a novel flexible monitoring device combined with active waveguide and large-deformation bolt. It is worthwhile to develop an AE monitoring method using a flexible structure to sustainably measure the subsurface deformation of a slope. The objectives of this study are fourfold: (1) to design a novel flexible device that is suitable to be applied in slope subsurface monitoring under large deformation; (2) to realize the synchronous monitor of AE and mechanical parameters using one flexible device; (3) to explore the correlation between the measured force of a flexible device and the thrust to the sliding body; (4) to examine the performance of deformation quantification by AE and the influence of a flexible device on the slope. In this study, a novel flexible monitoring device is designed and manufactured, and a landslide experiment system is built to test the feasibility and application effect of the proposed device. Of note, the proposed monitoring device is mainly used for slow moving landslides, and monitoring tools for fast-moving landslides are different from those of the other typologies of phenomena. In the following sections, an iron box is used to simulate the local structure of a soil slope, and a three-stage loading program is designed to simulate a landslide process. Then, the mechanical and AE parameters in the whole process of landslide experiment are obtained synchronously using the proposed flexible device. Furthermore, the change characteristic and relationship of multiple parameters are explored. Finally, an extra landslide experiment with constant velocity is conducted to investigate the precision of quantified deformation by AE.