Relationship between shear deformation and soil moisture content due to rainfall in a surface soil of granite slope

Monitoring the Vertical Distribution of Rainfall‐Induced Strain Changes in a Landslide Measured by Distributed Fiber Optic Sensing With Rayleigh Backscattering

“…If the air temperature change had a stronger effect on the frequency shift relative to the strain change, the map would show gradual changes in color from the top to the bottom. Furthermore, our measurements show abrupt increases in strain accompanied by rainfalls, and the strain changes are of the same order as measured in a physical model experiment with artificial rainfall by Brillouin optical time domain analysis (Yan et al, ) and in shallow landslides with inclinometers and strain gauges (Nakai et al, ; Sonoda & Kurashige, ). Several hundred micro strain of strain changes have been detected in this study, although the strain changes in the near‐surface zone (1–4 m BGL) could be overestimated by 125–175 με due to possible temperature changes (Figures , S2, and S3).…”

“…The strain largely increased because of rainfall and then gradually decreased toward the previous level until the next events occurred in July and early August. A similar fluctuation in strain changes before and after rainfall has also been reported via a strain measurement recorded using inclinometers or strain gauges for a shallow landslide (e.g., Nakai et al, ; Sonoda & Kurashige, ). Nakai et al () explained that shearing occurred during rainfalls and that the decreasing strain was caused by the soil drying process.…”

“…A similar fluctuation in strain changes before and after rainfall has also been reported via a strain measurement recorded using inclinometers or strain gauges for a shallow landslide (e.g., Nakai et al, ; Sonoda & Kurashige, ). Nakai et al () explained that shearing occurred during rainfalls and that the decreasing strain was caused by the soil drying process. The consistency in the trend of strain changes in Nakai et al () and Sonoda and Kurashige () and in the present study also suggests that the long‐term temperature change does not disturb the measurements of rainfall‐induced strain change.…”

“…The larger thickness of the deformation zone and fluctuation in the strain changes suggest that the deformation process in these sections is not a slip but a creep (Selby, ; Sonoda & Kurashige, ). For the uppermost section, based on our results and the interpretation by Nakai et al (), the fluctuation in strain changes can be interpreted as follows: (1) the increased weight of the rock caused by an increase in water saturation by rainfall deforms the section toward the descending slope, which leads to an increase in strain; (2) the strain decreases as the water saturation decreases, meaning that the weight of the rock reduces via drying or drainage after the rainfall until the next rainfall; and (3) repetitive fluctuations cause the section to become more solid and thus less sensitive to rainfall. Alternatively, the deformation process may have been changed in the bottom section because fluctuations similar to that in the uppermost section do not occur until 17 September.…”

“…Nakai et al () explained that shearing occurred during rainfalls and that the decreasing strain was caused by the soil drying process. The consistency in the trend of strain changes in Nakai et al () and Sonoda and Kurashige () and in the present study also suggests that the long‐term temperature change does not disturb the measurements of rainfall‐induced strain change.…”