Discriminant analyses were done using 25-m grid data for slope angle, precipitation, peak ground acceleration, geology, curvature, geological structure, and slope aspect of affected areas by the 2004 heavy rainfall in July and the Mid Niigata Prefecture Earthquake in October to explain occurrence of landslides. Amount of contribution to landslides was estimated by discriminant for each parameter. The results are as follows. The slope angle is an important contribution factor, but in the case of the heavy-rainfall, the precipitation contributes to landslides more than the slope angle. Remarkable differences of the contribution were observed in mud-rich turbidites of the Nishiyama stage and the Uonuma formation between the rain-driven failures and the earthquake-driven failures. The curvature showed larger contribution to the rain-driven failures than the earthquake-driven failures. The geology and geological structure showed large contribution to large landslides by the earthquake, and the slope aspect displays regional differences.
A new reinforcement method in forest conservation using rope nets and rock bolts to improve the earthquake resistance of mountainous slopes under the risk of shallow landslides was examined. The seismic behavior of slopes and the mechanism of the reinforcement method were studied through shaking table tests using Masado from the Rokko mountain range and cohesive soil from close to the Yamazaki fault in western Hyogo prefecture. The shaking test results showed that a combination of rock bolts, rope nets and head plates was most effective for seismic slope stability. Earthquake resistance was best performed in the test case of rock bolts with a 20 cm pitch. In terms of the mechanism of the reinforcement method, it was found that the structure of the method limited slope deformation and transmitted vibration to the whole site under the coverage of the rope nets, dispersing earthquake forces such that stress concentration was moderated. The reinforcement method brought about stability in the slope, preventing shallow landslides during earthquakes.
The issue and its resolution method of the risk evaluation of landslides in the area of Shimajiri-mudstone, Okinawa is considered. The hazard ranking system of landslides which is examined through the existing“Primary investigation manual for landslides”brings out the possibility of miss in picking up the first activated mudstone slides. The impact on landslides which can be caused by the exogenous factors of landslide and human-induced land alteration varies depending on the types of landslide. Therefore, we created the schematic illustration for risk evaluation processes of landslide. Lastly, we pointed out an importance of establishment of the risk evaluation method in which characteristics of regional landslides are reflected.
Recently some of landslides due to heavy rain and earthquake flow down with long distance and form landslide dam. Overflow of landslide dam sometimes may generate large flooding and debris flow. The establishment of estimation method for the debris flooding prone area is expected. This paper discusses the numerical method by using stabilized finite element method restrained the numerical oscillation due to the discretization of convection term for debris flooding. The presented schemes use the governing equation expressing directly the water depth and introduce the shock-capturing term resulting from the second-order accuracy of riverbed change term. The analysis schemes verify the utility in comparison with model analysis and are applicable for debris flooding analysis without the complex numerical scheme.
Stratum water and crack water through cracks of bedding planes and of joint surfaces give a great influence on the amount of displacement of moving mass in a landslide area. In this paper, at first, we examined the three-dimensional modeling of seepage flow analysis reflecting aspect cracks and then calculated water pressure distribution specifically by melting snow water in order to grasp a change of water pressure distribution of before-and-after construction of drainage facilities of well and drainage pipe. On that occasion, we made accurate modeling structure of various cracks and drainage facilities of well and drainage pipe. Using the results derived by the seepage flow analysis, we carried out a three-dimensional stability analysis. Based on the result of the stability analysis, we evaluated the stability of the landslide slope and the effect of drainage facilities. In OODAIRA Landslide area where landslides were reactivated by melting snow, by applying the above mentioned analysis, we were able to obtain the acceptable results that matches field situation between fluctuation of water pressure and displacement of moving mass. We were then able to simulate effectiveness of drainage facilities.
On July 13, 2004, heavy rainfalls due to the strong activities of rain front occurred in the Mid Niigata Region, Japan. They are as much as 400 mm in 24 hours, bringing about serious flooding by breaking the river banks. The heavy rainfalls also triggered more than 3359 landslides which were inventoried by air-photographs by Asia Air Survey Co. Ltd. The air-photographs interpretation and field research revealed that two types of landslides were inventoried; one is shallow failure and the other is deep failure, the latter of which is associated with mudflows. We are focusing the shallow failures in the Izumozaki area in this paper. Because the area is one of the typical failure areas affected by 2004 July rainfalls, and was also affected by many failures by heavy rainfalls in1961 August, and 1976, 1978. Therefore, we have interpreted the air-photographs of Izumozaki area of 1962 (one year later of the heavy rainfalls in 1961) , 1980 (1976-1978 rainfalls) and 2004 (just after the July 13 rainfalls) and compared using GIS in distribution between these failures, and then investigated the sequence of failure distribution, and their relationships to topography and geology.
Many slope failures occurred in the large area by the Mid Niigata earthquake in 2004. Numerical calculation methods such as Newmark method and finite element method are used as an evaluation method of the slope stability under strong seismic motion. In this study, we carried out centrifuge model tests to examine the applicability of a FEM `FLIP', the usual type Newmark method, and an improved type Newmark method considering seismic response characteristics of the slope. Furthermore, we carried out a lot of case studies to discuss the effect of slope height and input earthquake motion on the seismic slope stability. As a result, when slope height becomes tall, it was shown that in the difference of computed residual displacement by the difference of analysis code and the input earthquake motion becomes large. Therefore, it was suggested that the selection of an input earthquake motion and a numerical calculation method is important for seismic stability of a large-scale slopes.