A kinematic method for calculating shear displacements of landslides using distributed fiber optic strain measurements

Deformation monitoring of landslides is of great significance to characterize and understand their evolutions. Recently the distributed fiber optic strain sensing (DFOSS) technique has emerged as a powerful tool for landslide monitoring by enabling distributed and real-time measurement along a sensing optical fiber (SOF) over dozens of kilometers with high strain accuracy. However, the correlation between landslide displacements and axial strains exerted on an SOF remains elusive. Here we present a preliminary attempt to calculate shear displacements of landslides based on distributed strain measurements via a kinematic method. Parametric studies on the sliding direction, width of shear zone and magnitude of shear displacement indicate that, under certain circumstances, the shear displacements of a landslide can be well estimated without going much deep into the pattern of shear zone or the slope mass-SOF coupling condition. The proposed calculation method is validated through field shear tests of geologic granular materials, and has immediate application to the analysis of the Majiagou landslide, Three Gorges Reservoir region, China. The distributed strain measurements captured by a borehole-embedded SOF allow two sliding surfaces of this landslide to be located; one occurred at the contact between surface deposits and bedrock, whereas the other was the main sliding surface occurring within bedrock. The shear displacements along the main sliding surface are calculated using the proposed method. It is revealed that the landslide responds more aggressively to the fluctuation of reservoir water level than to the rainfall over a 1-year period.