## 6 Conclusions  
The K39 landslide is large in scale, wide in influence range, and complex in geological structure. Deformation characteristics of landslides can be zoned in plans with the data from field fracture surveys, and the analysis of the monitoring data allows for the stratification of landslides according to different slide depths. Subsequent landslide management work can be carried out more effectively with an accurate understanding of the deformation mechanisms and deformation characteristics of landslides.  
1. The geological conditions and regional structure of the landslide area are extremely complex. The interface between the Quaternary alluvial deposits and the sandy mudstones of the toe group is the shallow landslide slide zone in Zone I and Zone II, and the deep landslide slide belt in Zone III is between the deep strongly weathered shale and the moderately weathered shale.
2. The landslide can be divided into three areas according to the statistics of the surface crack deformation: Zone I, Zone II, and Zone III. We determined that Zone I and Zone II are shallow landslides and Zone III is a deep landslide, according to the monitoring of the deep deformation of the landslide. The surface cracks in Zone I are concentrated near the rear edge retaining structure, and the strike of the cracks is mainly vertical to the sliding direction. The deformation was obviously controlled after drainage treatment. There are historical sliding tension fractures and traction structures in Zone II, and the current deformation is stable without deformation. Zone III is the main sliding area with the longest average fracture length and the deepest sliding depth, and there sliding deformation is always occurring.
3. The physical parameters of the sliding body, sliding belt, and sliding bed in Zone III were measured through on-site sampling and indoor tests. The groundwater level in the landslide area was lowered by setting a drainage tunnel, and the results showed that the slope was in critical condition when there was no drainage tunnel or the drainage tunnel depth was 15 m. The slope was basically stable when the drainage tunnel depth was 20 m or 25 m. The slope was stable when the depth of the drainage tunnel reached more than 30 m. The groundwater level is the main factor affecting the landslide deformation. According to the design of on-site support facilities, attention should be paid to the control of the slope groundwater level in the prevention and control of slopes with the same characteristics as the K39 landslide, to prevent the revival of the landslide.  
Conceptualization, X.B. and D.C.; methodology, D.C.; software, X.B.; validation, X.B., G.Z. and J.P.; formal analysis, M.L.; investigation, X.B.; resources, D.C.; data curation, X.B.; writing--original draft preparation, X.B.; writing--review and editing, D.C.; supervision, X.B.; funding acquisition, M.L. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the National Natural Science Foundation of China (No. 41772304 and No. 42277171).  
Not applicable.  
The data presented in this study are available on request from the author.  
The authors declare no conflict of interest.