Experiments on the landslide under the seismic forces were carried out in order to evaluate the effects of bedrock shape, which should restrict the seepage flows, on the occurrence of a landslide. A numerical simulation of the seepage flow by using a finite element method was done for each experimental case to get the values of soil properties such as density, cohesion, and angle of internal friction. By using these values, the seismic response analysis also was carried out. The response acceleration at the surface layer of the slope is slightly amplified in the calculation, which tendency also was obtained in the experiments. In the calculation, as the joint elements are taken into consideration the slide surface can be specified. The calculated safety factors of the joint elements did not correspond well with the slide surfaces in the experiments. The calculated safety factors which were less than 2.0, however, express the positions of the slide surface in experiments fairly well.
For the purpose of preservation of stream environment, accelerating use of thinning wood, cutting down the ex-penses, etc., construction of small dams made of wood has been examined recently. However, the design of wooden dam has been conducted based on experience up to this day, and the methods of designing wooden dams have not been established yet. The purpose of this study is to examine the methods of designing of small wooden crib dams. A small wooden crib dam was constructed in the field. Then, horizontal loading tests were conducted using the dam. The methods of design which are suitable for small wooden crib dams were examined based on the data of horizontal loading tests. The results are summarized as follows. (1) It was confirmed in the horizontal loadig tests and in the stability analysis in which all of forces are taken into acount that the crib structure type wooden dam is completely safe for stability of dam. (2) It was confirmed in the horizontal loading tests that a method of design in which occurrence of tensile stress is allowed at the upstream end of dam is applicable to design the wooden crib dams for fulfilling their function. A method of design in which occurrence of tensile stress is allowed at the upstream end of dam was presented. (3) In the horizontal loading tests, the unit weight of boulder packed in the crib is smaller than the value used in the exisiting design criteria. In the subsequent studies, it is necessary to examine the actual condition of the unit weight. (4) The horizontal loading tests showed that the friction coefficient between dam base and foundation ground is at least 0.91. (5) The horizontal resistance moment of dam body measured in the horizontal loading tests was at least 2.2 times larger than the shearing resistance moment of cobblestones filled in steel hollow sabo structures calculated by the existing formula.
In active volcanic areas, debris flow generally has higher fluidity than in ordinary areas, resulting in a wider extent of inundation. In such areas, an estimation of the hazard zone is required to prevent and mitigate the disasters due to debris flow. Therefore, many study on the two-dimensional simulation of debris flow have been done to estimate the flooding area. Reliability of the simulation mainly depends on the accuracy of the boundary conditions and the for-mula for sediment discharge. In this study, the boundary conditions at the upstream end of the simulation domain are set up from the estimated hydrograph of debris flow by using the runoff analysis model. Many studies on discharge of hyper-concentrated flow have been made, but, no reliable equation has been established yet. Existing formulae for sediment discharge of hyperconcentrated flow are examined by using the field data collected at the Mizunashi River. Simulations of debris flow flooding are made by using the chosen formula and the boundary conditions mentioned above. Reliability of the simu-lation is checked by applying it to the Mizunashi River at Unzen Volcano and the Fume River at Aso Volcano.
In the past, as a part of the project for erosion and sediment control, we have encouraged afforestation in the de-nuded or devastated areas. But in recent years, the main body of the work method changed from hillside works to tor-rent works (check dams or channel works), because of their easiness to evaluate the effect and stability. However, with the increase in public awareness of the environment, there has been pressures to look for ways to avoid the load on the ecosystem. Therefore, the investigations on the stability of hillside works at a downpour and the development of the root sys-tem of natural seeding trees in the representative hillside work areas were carried out to clarify the management stan-dard with hillside slope for erosion and sediment control. Here it was indicated that the stability of the hillside works was no less than that of natural slopes. As a result, the hillside slope management as Sabo project should be divided into three periods, that is, (1) period for hillside work construction and for growth of the planted vegetation, (2) period for smooth regeneration of the natural seeding trees and the maintenance for sediment yield reduction, (3) period for continuous succession of the satisfactory forest.
A major earthquake (M 7.3, Taiwan Central Weather Bureau) occurred near the town of Chi-Chi in Nantou country, central Taiwan, on September 21, 1999, about 150 km south of Taipei. The earthquake caused heavy damages to houses, buildings, public facilities, roads and bridges in cities and towns, and moreover some large-scale landslides and many shallow slope failures in mountainous regions. In order to know general conditions of debris disasters on slopes the research team conducted by the Japan Sabo Association in corporation with the Japan Society of Erosion Control Engineering and the Japan Landslide Society carried out a preliminary research from October 11 to 17. This paper summarizes the investigations related to the property of the earthquake damages, focused on debris disaster.
Although the present warning and evacuation of debris flow is determined by the amount of rainfall, it is pointed out that the volume of melting snow should be taken into consideration. The purpose of this research is, therefore, to construct a new system for the warning and evacuation from debris flow by incorporating the volume of melting snow into the present system using the amount of rainfall. In order to develop the basis of the warning and evacuation criteria of debris flows by incorporating the volume of melting snow, the authors verified the simple but accurate prediction technique for determining the volume of melting snow and investigated by measuring the soil moisture tension during the snowfall and snowmelt seasons. The results may be summarized as follows : (1) The volume of melting snow evaluated by this simple model using net radiation, air temperature and precipitation, advocated by Koike, Takahashi and Yoshino (1985), shows the same precision as the one evaluated by the heat balance model. (2) One of the characteristics of soil moisture condition during snowmelt season is that the saturated stratum had formed and continued over 10 days, due to the water supply with 30mm per day, which was observed for 13 days in April, 1998.