The changes of elastic wave propagation in the process of rainfall induced slope surface failure

概要

Rainfall-induced landslides commonly occur in mountainous areas and cause severe human and infrastructural damage around the world. Most of the previous landslides have occurred at shallow depths, generally less than 3 m, and the average thickness of the failed surface layer was 1.2 m. To mitigate damage caused by rainfall-induced landslides, physical countermeasures such as retaining walls, ground anchors and dewater systems are common, however, they are not economically feasible for the amount of potentially unstable slope. Therefore, landslide early warning systems are an alternative soft countermeasure that can provide an efficient and economical way to reduce the damage of landslides. A typical landslide early warning system is based on monitoring of soil moisture and pore pressure, or on measuring mass movement events by linear displacement transducers, inclinometers or extensometers, or measuring both the soil moisture and the displacement by soil moisture sensors and tilt sensors. These methods have recently been used because they are simple and easy to install in the slope surface layer. However, they can only sense the local area surrounding the position of the sensor. To cover a wide area of unstable slope, many sensors are required.
Elastic wave propagation in soil as a non-destructive monitoring technique has received considerable attention in recent years. The application of elastic wave propagation in soil has been developed by many researchers, for example, shear waves were measured in laboratory specimens by means of piezoelectric transducers, and recently, both shear wave (S-wave) and compression wave (P-wave) velocities were designed to measure the unsaturated soil. It was found that both P-wave and S-wave velocities decreased by nearly half when soil saturation was increased from 20% to 80% in laboratory triaxial experiments. A series of model experiments found that elastic wave velocities continuously decreased in response to moisture content and deformation.
In this study, a method of evaluating slope shear deformation and soil moisture by elastic wave is presented. Elastic wave devices include an exciter and several receivers that are laid out within the slope surface layer to cover a relatively deep and wide area. To extend the former research, three main points have been improved in this study. Firstly, an exciter has been developed that can automatically generate clear and powerful elastic wave signals to propagate more than 1 m in soil. Secondly, an algorithm has been developed that can process the huge number of wave signals, and automatically detect the travel time and calculate the wave velocities. Thirdly, a fullscale multi-layer shear model was used to simulate the process of slope failure and observe the wave propagation. The detailed behavior of elastic wave propagation in soil over a wide range of soil moisture, shear stress, and shear deformation, can be explored. A series of tests were designed to reproduce the slope failure. The factors affect on elastic wave velocity have been confirmed.
In this study, not only the elastic wave velocities but also the wave attenuation has been investigated. A method using wave attenuation is presented to monitor slope deformations and soil moisture variations. It is an application of geometric spreading, which is as the wave moves away from the source, the area that the wave energy covers become larger and thus wave intensity decreases, and wave energy loss due to inelastic material behavior or internal friction during wave propagation. Laboratory experiments using a Multi-layer shear model were conducted, wave attenuation affected by shear forces corresponding with deformations on every layer, and the soil moistures in wet and dry processes have been analyzed.
To investigate the behavior of elastic wave propagation in the natural slope surface layer, elastic wave monitoring has been conducted at a slope located at Aso-shi, Kumamoto, Japan. This slope was suffered from the 2016 Kumamoto Earthquakes and some big cracks appeared on the slope surface. It is a typically unstable slope. The elastic wave monitoring devices include a fully automatic to generate elastic wave by the exciter, measure the wave signal by receivers. The layout of sensors and exciter underground and the monitoring parameters such as velocities, soil moisture, deformation from the tilt sensor are shown in detail. Elastic wave velocities and attenuation behaviors with soil moisture on-site is similar to laboratory experiments.
These findings show that monitoring the wave propagation in a slope surface layer can indicate the status of soil moisture content and shear deformation. Slope instabilities may be predicted based on the historical record. Monitoring the changes in elastic waves in the slope surface layer is valuable and can be applied to an early warning system.

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