###### Abstract  
Compared with slope surface measurement, subsurface monitoring has the potential to detect abnormal changes at an earlier stage. Due to the large-scale deformation of landslides and the complex geological environment, the existing subsurface devices generally have limited measuring range or high economic cost. Thus, it is urgent to develop an advanced method to improve the sustainable monitoring of large subsurface deformation. In this study, a novel flexible device is designed for slope subsurface monitoring, which combines an active waveguide of acoustic emission and the distinctive structure of a large-deformation bolt. An experimental system is built and a loading process of three-stage deformation is carried out to test the performance of the flexible device. The kinematic, mechanical and acoustic emission parameters are synchronously obtained in the whole landslide model test. The results indicate that different evolution stages of a landslide can be distinguished by sliding acceleration. The axial tension within the anchor cable changes consistently with sliding thrust. In addition, using acoustic emission to quantify slope deformation has achieved strong correlation. An extra experiment with constant velocity is conducted to explore the precision of deformation quantification by acoustic emission. A schematic diagram is presented for how this novel device would be used for landslide monitoring in the field, with the potential practicability of stabilizing the slope. l 2021 2 August 2021 2 August 2021 2  
the surface monitoring cannot perceive the geological activity inside the slope, nor can it monitor the development and failure of the sliding surface. In fact, landslide is caused by the progressive damage of the internal structure of a slope, and the sliding surface plays a significant part in landslide evolution. Therefore, the evolution information of a landslide is sent out from the inner area, and the initial information can only be perceived inside the slope mass. Only when there is enough change in the slope mass can the slope surface deform macroscopically. Then the macroscopic deformation can be captured by the surface monitoring equipment if it exceeds a certain degree. In order to realize the sensitive perception and early warning of landslide hazards, it is necessary to carry out subsurface deformation monitoring. The subsurface device is directly attached to the borehole in the slope mass to obtain the direct information more efficiently and early, which can provide powerful support for the stability evaluation and disaster mitigation. Subsurface deformation monitoring devices of slope movement usually have high measurement accuracy and time resolution, although they can only collect the information of the slope mass around the borehole and thus the spatial coverage is limited [15; 16]. As shown in Table 1, three high-precision subsurface deformation monitoring methods are the inclinometer, ShapeAccelArray (SAA) and acoustic emission (AE) technology [9; 17; 18; 19]. However, due to the rigidity and britteness of inclinometer casing, the casing can be bent excessively when the local shear displacement reaches about 50 mm, resulting in the failure of the device [15; 20; 21]. As for SAA, the measured displacement of shear plane is more than hundreds of millimetres [9; 22; 23]. However, the expensive cost limits the wide application of SAA. It is noteworthy that AE is a sensitive and efficient monitoring technology, which appears in the initiation period of a landslide, along with the whole process of slope deformation. AE technology has the characteristics of being reliable, of low price, with high accuracy and can be conducted in real time [24; 25]. AE is sensitive to minor changes in displacement and velocity, which can detect extremely slow-moving landslide and has the potential to release early warning [26; 27; 28; 29]. AE monitoring of landslide deformation has been carried out for decades.