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.

Distributed strain monitoring of geotechnical structures has gained increasing attention in the past decade. The distributed fiber optic strain sensing (DFOSS) technology enables the measurement of strain distribution in soil slopes. This paper aims to investigate the feasibility of strain based slope stability evaluation for locally loaded slopes. The measurements of horizontal strains at different elevations in a two-dimensional (2D) model slope subjected to a vertical surcharge load were analyzed. Empirical relationships between different types of strain parameters and factors of safety calculated by the conventional method were established. To verify the above findings, a 2D finite element model of a homogeneous soil slope was built. By applying a gradually increasing local load on the numerical model, the strains in the soil mass and eventually slope failure were induced. The strain distributions of several virtual monitoring lines under different loading levels were captured and analyzed in detail. At the same time, the strength reduction method (SRM) was used to perform slope stability analysis. The results show that the strain distribution characteristics are closely related to the propagation of plastic zones and the formation of the critical slip surface. Taking into consideration the convenience of field instrumentation and monitoring sensitivity, the maximum strains at different elevations can be used as characteristic parameters for estimating the slope stability condition. Compared with conventional displacement based slope stability evaluation method, the proposed methodology is more efficient and sensitive, which makes full use of the benefits of the DFOSS technology.