Yasuhiko Okada

Landslide Experiments on Artificial and Natural Slopes

An almost real-size slope model was used to study the initiation process of landslide fluidization during torrential rain. Experiments were conducted by filling an inclined flume with loose sand under the rainfall simulator to induce the sand to collapse. Both the movement, volumetric strain and the pore water pressure of the sand were monitored throughout the experiments, from the start of spraying to the cessation of the landslide. These experiments showed: (1) Landslide fluidization caused by undrained rapid loading undergoes three stages: compaction of the sand layer by the sliding mass from upper slope, generation of excess pore water pressure in saturated zone, and induction of fast shearing; (2) Fluidization at the collapse source area undergoes also three stages: destruction and compaction of sand layer skeleton by outbreak of shearing, increase of pore water pressure in saturated zone, and shift to hispeed shearing, these three stages take place almost simultaneously.

Observations on Earthquake Acceleration and Pore Water Pressure in a Hilly Region

Earthquake accelerations and pore water pressures were measured on a hill slope to constrain slope stability reduction caused by earthquakes. Acceleration responses were observed at various locations on and under the ground surface on a ridge slope and in neighboring valley fill units. Pore water pressure in the valley fill was also measured during earthquakes. The amplification of the acceleration response that was transmitted from underground to the ground surface was compared between sites. The acceleration responses became greater when an earthquake was transmitted from underground to the ground surface. Large earthquake accelerations on the ridge of the hill were amplified primarily in a direction perpendicular to the ridge crest. They were also amplified at specific frequencies by the topological effect. In contrast, earthquake accelerations in valley fill sediments were equally amplified in all directions and over a wide frequency band. Increases in the dynamic pore water pressure tended to be synchronized with increases in vertical acceleration.

Fluctuations in the Pore-Water Pressure of a Reactivated Landslide in a Snowy District

Pore-water pressure fluctuations and water that reaches the ground surface (MR) were monitored in a reactivated landslide which is located in a heavy snow district of Japan. Observations of pore-water pressure inside the moving landslide mass revealed that changes in pore-water pressure during snow cover periods were quite different from those in other periods. These results suggest that the hydrological properties of a moving landslide mass are strongly affected by snow load.

Examining Fluidisation Mechanisms of Hikagemori Landslide Triggered by Iwate-Miyagi Nairiku Earthquake in 2008 by Laboratory Soil Tests

The Iwate-Miyagi Nairiku Earthquake in 2008 triggered a complex debris slide—earth flow on the east slope of Mt Hikagemori, Kurihara city, Miyagi prefecture. In situ surveys 6 days after the occurrence revealed that the earth flow body was mostly unsaturated, but near the slip surface pore fluid pressure was built up with suspension of fine soil particles. From undrained cyclic loading triaxial compression tests on undisturbed samples taken from the head scarp of the landslide, a measured cohesion was at least 50 kPa, and a structural collapse could take place in a case. In mass flow experiments by means of a large-scale model flume, an unsaturated mass moved and slid smoothly over the muddy water travelling a long distance for a specimen with sufficient water inside. This may not reproduce the exact fluidisation behaviour of the complex Hikagemori landslide, though it may also explain a part of the long travelling mechanisms of the Hikagemori landslide.

Comparison of Shear Behaviour of sandy soils by Ring-Shear Test with Conventional Shear Tests

The relation of shear behaviour of dry Toyoura standard sand between the ring-shear test, triaxial compression test and two types of shear-box tests was compared. Test results shows that the peak internal friction angles from the ring-shear test fitted those from shear-box tests, while there was a certain difference from triaxial compression test. Then, the liquefaction potential of saturated Osaka-group coarse sandy soil by ring-shear test and triaxial compression test was examined. The liquefaction was observed only in the loose state by the triaxial compression test, while it appeared in all three tests by the ring-shear test. The stress paths of liquefaction in the medium and dense state by the ring-shear test is different from liquefaction in the loose state by both tests.