The report discusses the methodology of the landslide risk evaluation of the earthquake triggered slope movement at the artificial land reclamation area. The two case studies are used to evaluate the adaptability of this method. One is the case of Takamachi New Residential area damaged by Mid Niigata Earthquake 2004 and the other is Tsurugaya New town damaged by the Miyagi-oki Earthquake 1978. Recently, artificial land reclamation by civil earthwork in slope areas has been widely developed in and around urbanized area. Some area such as the valley filled with reclaimed soil is relatively weak compared with cut portion. In spite of the weakness of the ground, there were poor techniques of the evaluation of slope instability. The authors have developed the risk evaluation technique titled “Three dimensional slope stability analysis based on the simplified RBSM” to apply to the artificial land reclamation area. This has the advantage of not only being simple but also including the earthquake force and direction of seismic wave for calculation. Thus, the technique will be used to get the spatial distribution of the degree of Safety Factor (Fs). The Fs value distributions in the two case studies are evaluated by the analyses and well explain the real distributions of the slope damages.
A formulation for passive type cotter pile against the landslide with multiple slip surfaces has been proposed in this paper. The prime focus was the coupling of two different approaches. One is Chang's pile equation, which was extended to deal with displacement discontinuity at slip surface and the other is the slope stability assessment theory similar to Janbu's. By using the proposed formulation, an engineer can obtain almost all the state parameters with regard to both the pile and landslide mass, necessary to design the pile for landslide prevention. Example calculations show the significant differences between one layer of landslide mass and two or more layers with state quantities, such as stress, displacement etc.
Slope stability analysis for landslide along reservoir is performed in consideration of influence of residual pore water pressure caused by the drawdown of reservoir water level. The authors carried out seepage flow analysis of simple slope models and field dataset analysis of the residual ratio of pore water pressure, in order to examine the factors that have an influence on the residual ratio. As a result of analysis, it has high correlation between the residual ratio and two factors which are drawdown time of reservoir water level and horizontal distance from water's edge of reservoir to slip surface. We also showed representation method of the residual ratio by these two factors.
Changes in pore-pressure and normal flow height in the granular mass flow had been investigated on sand and pumiceous gravel samples by quasi-real scale model flume tests. The excessive pressure head was observed under the sand flow, which was about 1.5 times as much as normal flow height. In the case of pumiceous sample, no excessive pressure head was observed, however, it traveled longer than the sand sample. This was because pumiceous sample had very small unit weight and it must have almost floated in the water during the experiments. The series of this experiments revealed that the both views of maximum travel distance and the extent of congregation of soil deposits were of great importance. For the sake of expressing the granular flow phenomena from those viewpoints, the new parameter “run-out characteristics index (ƒ)” was proposed as the ratio of the equivalent coefficient of friction (tan φa) to the equivalent coefficient of friction between gravity centres before and after failure (tan φag).
This study clarifies geomorphological and geological background of landslides caused by 2004 Niigata Chuetsu earthquake, using GIS analysis. And it also discusses soil mechanical features of the landslides before the earthquake. It is summarized as follows; 1. Most of newly formed deep seated landslides are reactivated ones distributed along Imokawa River that incises Uonuma Hills, forming steep valley side slope more than 30 degrees. 2. GIS processing reveals detail ground deformation of each large scale landslide, using DEM detected before and after the earthquake. 3. More than 60% of surface failure type landslides occurred on steep slope more than 45 degrees. 4. Most of deep seated landslides occurred on dip slope. Gradient of original slopes range from 13 to 26 degrees and the mode is from 21 to 26 degrees. That means original slope before the earthquake were quite stable, considering that the gradient mode of original slope consisted of the same types of geology in Northeastern Japan is from 15 to 20 degrees. And it implies the earthquake just triggered the landslides. 5. Soil tests and inversion analysis on slope stability confirm that earthquake tremor induced numerous landslide in the study area.
Observation boreholes for measurement of groundwater potential and movement, water sampling at arbitrary sections were designed and constructed for field test. Casing pipes were attached with sealing belts between the casing pipe and the borehole wall at one meter intervals. Another part of the casing pipe was perforated up to 5% of the surface area. A sonde equipped with five rubber packers was used to seal inside the casing pipe for water pressure measurement and sampling. Groundwater pressure lower than hydrostatic pressure was measured below the observable water level in the borehole and positive water pressure was measured above the observable water level at a landslide area in Shizuoka Prefecture. Groundwater potential distribution varied after rainfall. Vertical water movement in the borehole was measured accurately using a flow meter sonde. Water flow was concentrated into the flow meter by a packer which stopped a gap between the flow meter and the casing pipe. Changes in the oxidation-reduction potential and electric conductivity of water sampled at different depths with the packers was larger than those sampled without packers. Electrical and temperature logging methods can also be conducted for whole the entire depth. Observation methods that enable measurements or sampling at specified depth are necessary to evaluate groundwater potential and water characteristics in complicated layers of a landslide slope.