Conclusion

This book addresses the technologies of slope monitoring and landslide early warning with emphasis on the authors’ past efforts, with due reference to other monitoring technologies. In addition, the historical and climatic background of the technologies was reviewed. The contents of this book are summarized as its conclusion as follows: There were substantial efforts in the past for mitigating rainfall-induced landslide disasters. Nevertheless, the climate is believed to be changing nowadays, increasing the likelihood of short-term heavy rainfalls and the induced landslides. It is, therefore, important to make more efforts to control and mitigate the risk of rainfall-induced landslides in addition to the past achievements. Slope monitoring combined with early warning is a promising “appropriate” technology to mitigate disasters caused either by gravity-induced (slow) or by rainfall-induced (rapid) landslides. Therein, due consideration has to be made of the affordability of the local community and, therefore, the appropriateness from a financial viewpoint. There are significant social needs for mitigation of landslide disasters. At the same time, the funding available for them is limited. Therefore, the slope stabilization by retaining walls, rock anchors and others is not the only choice. Early warning, together with quick evacuation, if necessary, is an important choice as well because of the lower cost. Land-use control is also a common practice. It is possible to foresee slope failure by carefully observing the surface displacement and/or distortion in its preliminary stage. This observation is possible not only instrumentally but also in terms of observing cracks and distortion of existing structures with the naked eye. Early warning is issued by using a variety of criteria. The most traditional and widely used is the rainfall criterion, in which a warning is issued when the intensity of rainfall and the cumulative rainfall exceed empirical thresholds. The other combination of rainfall data is the intensity of rainfall and its duration. The cumulative effect of rainfall therein can be replaced by the amount of water content stored under slopes as assessed by the tank model. The rainfall criterion is useful because it does not require a high cost for installation, if a long-term rainfall record and its empirical correlation with past landslide disasters in the target locality are available. Noteworthy is that the rainfall-based criterion suits a regional early warning and cannot foresee the failure of a particular slope. The second type of early warning is based on the monitoring of slope deformation/displacement. This technology is intended to foresee the failure of a particular slope, and the regional warning is out of its scope. Therefore, the aforementioned rainfall criterion and the displacement/deformation criterion are complementary to each other. The authors’ slope monitoring uses tilt sensors that are placed at the top of vertical rods driven into the bottom of surface unstable layers. MEMS (Micro Electro Mechanical Systems) technology successfully reduced the cost of this sensor. Therefore, many sensors can be deployed over a slope that is heterogeneous and may start to experience instability in any part unexpectedly. This tilt angle monitoring has an empirical criterion to issue a warning that was derived on the basis of field monitoring and the onset of slope failures during past cases. Application of remote-sensing technology to slope monitoring is one of the future promising directions. Its advantage is the wide-scope capturing of the slope conditions. It contributes to the decision on which part of the slope needs to be monitored more in detail. On the other hand, it does not suit the warning of rainfall-induced landslides that occur rapidly within a few hours during heavy rain. The final goal of slope monitoring and early warning, which is the safety of human lives, is achieved through collaboration with local communities. Towards this goal, many efforts are underway. There is no single model for this effort, but some are in progress in collaboration with local communities who are anxious to protect themselves from natural disasters. The next stage of slope monitoring is the use of microtremors for health monitoring by using inexpensive but accurate MEMS vibration sensors. The advantage of microtremor monitoring is that the characteristics of microtremor, which are the amplitude, the frequency components and the dominant direction of motion, are related to the change of the slope properties. It is expected that the change of microtremor characteristics is a signal of the progress of material weathering, hydration and many other kinds of deterioration. Then, in turn, the microtremors can suggest the impending landslides. Data from slope monitoring can be stored and interpreted in digital form. This means that slope monitoring is an important factor of digital transformation that is applied to the safety management of infrastructures. An example of this type of digital transformation conducted by an industry was introduced in Chapter 11 . Slope monitoring technology can be applied to many more fields. Several candidate fields of application were selected and described in Chapter 12 with a brief review of their current situations. Those fields are (i) the remote detection of the onset of landslide, (ii) the warning of glacier lake outburst floods, (iii) the health monitoring of deteriorating natural slopes, which are undergoing rock weathering, and man-made geotechnical infrastructures and (iv) the forecast of volcanic eruptions. Certainly, there are many more future applications, but the authors are not able to discuss all of them.