Today, we visited the Disaster Prevention Research Institute, which is part of Kyoto University. On this location, a lot of research is done on the prevention of the many disasters Japan and the rest of world are dealing with, using not only theoretical background but also the laboratories of the university.
The first presentation was presented by Assistant Professor Kyohei UEDA. He mentions that it is allowed to sleep during this session. The topic is Disaster Prevention through Geotechnics. There are several geohazards present in Japan, such as earthquakes, floods, landslides and volcanoes. Some examples from history are: the 1964 Niigata earthquake, with liquefaction and landslides and the 1995 Kobe earthquake with for example lateral spreading on the artificial island Rokko. Liquefaction is the process in which water and sediment are separated in case of an earthquake which results in groundwater on the surface and an unstable ground. This can lead to the collapsing of buildings with a lot of damage and fatalities. More recent examples are from the Great East Japan Earthquake in 2011, which resulted in sand boiling and the uplifting of light infrastructure. The uplifting is a result of the difference in density between the soil and the lighter structures such as manholes, which are separated by the earthquake. Recently, the Hokkaido 2018 earthquake caused enormous ground settlements, sometimes more than 1 meter, which resulted in tilted houses and visible foundations on the surface. In addition, landslides occured in mountainous regions in Hokkaido, which could be caused by the degradation of the present volcanic clay.
How to predict and prevent such geohazards? A theoretical, analytical and experimental approach could be taken to model the reality. The experimental approach is important as it can be used for the validation and verification of the scientific theories. Kyoto University has a laboratory of centrifugal model test, which is visited after the presentation. This machine is able to produce up to 200G by centrifugal force. In this laboratory, the same stress-strain relationship of larger soil structure systems can be achieved using only a small model to study its static and dynamic behaviour. This can be used for research on liquefaction, landslides and other geohazards.
Before we went to the laboratory, the Director of the Disaster Prevention Research Institute Prof. Dr. Hajime Nakagawa was met as a courtesy. We had a small talk about our experiences up til now and his experiences with the Netherlands. After taking a group picture, we went to the laboratory to see the centrifugal model in reality. In the control room was visible how incredibly fast the rotation of the system with a diameter of 5 meter was, it was set at 120 rotations per minute producing 40G. After stopping the system, we went inside to see it up close. In the shaking table on the outer side of the axe, we could see the separation of water and sand, which is transferable to a large scale earthquake event.
The research performed with this model is relevant for the prevention of disasters and can be used in the design phase. Currently, safety factors for earthquakes are high in Japan, as the Japanese are an uncertainty avoiding society. When the knowledge of earthquakes is improved, safety factors could be lowered as the effects are predicted more accurately which could result in less use of material and eventually cost reduction.
The second lecture was about sediment in dams and reservoirs. Japan has more than 3000 dams with a height of 15 meters or more. Sediment of upstream rivers accumulates in the reservoirs in front of the dam and settles. As a consequence of this settlement the volume of the reservoir decreases. Professor Dr. Sameh Kantoush works on integrated sediment management in dams and reservoirs and investigates different measures to reduce the impact of sediment on reservoirs and dams. The focus is not only on dams and reservoirs but also on the river basin. Natural disasters like earthquakes and typhoons, cause land slide and debris flow. At the locations of the land slide, sediment is produced. To understand the production of sediment the university created a map. This map shows the production of sediment per location in Japan by using a colour scale to display the amount of sediment. Furthermore, different tests in laboratories are performed to see the impact of sediment settlement on dams and reservoirs. Besides, samples of soil are taken and tested to understand the structure of the sediment. This information can be used to recycle the sediment for the construction industry.
In order to prevent a decrease in volume of the reservoirs by sediment settlement, the University of Kyoto investigates different measures. A useful and common used measure is flushing. For this measurement there are a few openings on the bottom of the dam. Because of the density of sediment, the sediment is at the bottom of the reservoir. By opening the slides of the bottom, water and sediment flows through the dam. The goal of this measurement is to sustain the reservoir. Due to the dam, some sediment will remain. So, returning to the initial stage of the reservoir before the building of the dam will not be possible. To sustain the reservoirs several reservoirs in Japan are fully discharged every year. At the bottom of the reservoirs small channels appear through which sediment discharges to downstream rivers. The channels remain even when the reservoir is filled up by rainwater again. During next flushing the sediment will discharge through these channels. To increase the efficiency of flushing, flow guidance structures can be placed in the reservoir. These structures will make it easier to create channels for the sediment.
Flushing is not sufficient for all reservoirs. Depending on the inflow and capacity of reservoirs other measurements might be more useful and efficient. Another common used measurement is dredging. The sediment can be recycled and used for the construction industry or the sediment can be replaced to a location upstream to repair the balance of sediment in the river basin. The capacity of dredging is relatively low compared to flushing. By-passes for sediment are also used to transport sediment to downstream rivers. Although, this measurement is expensive and sensitive to destruction caused by the pressure.