Kyoji Sassa

Pore-pressure generation and movement of rainfall-induced landslides: effects of grain size and fine-particle content

Abstract Using a small flume, a series of tests was conducted to trigger rainfall-induced landslides. By performing tests on silica sand no. 7 (D50=0.13 mm), no. 8 (D50=0.05 mm) at different initial dry densities, the effects of grain size on pore-pressure generation and failure behavior of a landslide mass were analyzed. Results from tests of different initial densities showed that the optimal density index, at which the pore-pressure build-up after failure reached its maximum value, differed for samples of different grain size. Moreover, observed failure phenomena showed that the failure mode also depended greatly on the grain size. In fact, flowslides were initiated in the tests on finer silica sand (no. 8), whereas retrogressive sliding occurred in the tests on silica sand no. 7. Results of tests on mixtures of silica sand no. 8 with different contents of loess by weight showed that the existence of fine-particle soil (loess) could significantly change the flow behavior of a landslide mass during motion. The flow behavior of soils with 20% and 30% loess was different from these two silica sands and the mixture with 10% loess, showing greater velocity without deceleration. This suggests the existence of a mechanism that maintains high pore pressures during motion for these soils. In addition, by rotating saturated samples in a double-cylinder apparatus, a mechanism was examined in which pore pressure in saturated soils during motion was maintained. The results showed that the pore pressure of the saturated mixture increased with velocity because of the “floating” of sand grains that accompanied the movement for each test. In addition, the sample with finer grain sizes or greater fine-particle (loess) contents floated more easily, and high pore pressure could be maintained during motion. The floating ratios of grains reached a high value (>0.85) at a very slow velocity for samples with 20% and 30% loess. Based on these test results, it is concluded that grain size and fine-particle contents can have a significant impact on the mobility of rainfall-induced landslides.

PERFORMING UNDRAINED SHEAR TESTS ON SATURATED SANDS IN A NEW INTELLIGENT TYPE OF RING SHEAR APPARATUS

The design and construction of an undrained ring shear apparatus, which was newly developed at Disaster Prevention Research Institute, Kyoto University, are presented in detail. This apparatus is suited for undrained shear tests under all types of loading, and enables observing the undrained shear behavior of soils in high-speed motion (maximum rotating speed 2.24 m/s) to an endless displacement level. Using rubber edges and a gap control system, leakage of pore water was effectively prevented, and the friction of rubber edges was controlled with high credibility during shearing. For undrained shearing tests on sand, the related experimental procedures are introduced and the undrained shear behavior of sand in ring shear test is presented. Test results showed that high pore water pressures could be built up in both loose and dense sands, given the shear displacement is great enough.

Landslide Science and Practice

A proposal to supplement existing landslide classifications with a more detailed description of the geomechanical characteristics of the materials to include the effects of “structure”, stress history and initial state has been developed as part of the EC sponsored SafeLand project. This approach provides a valuable insight into and a rational basis for the modelling of the physical mechanisms that govern landslide triggering and subsequent development. The classification, behaviour and modelling of saturated clays and sands is summarized here. The use of advanced strain hardening plasticity models which, where necessary, include progressive damage to the “structure” of the material helps to replicate mechanical weathering, delayed failure and the triggering of flows or slides, depending on the type of material. A numerical example is presented, illustrating the different response of sensitive clays and mechanically overconsolidated clays to rapid erosion or excavation. More details of the proposed geotechnical classification and other numerical examples may be found in Deliverable 1.1 of the SafeLand project.

Mechanism of a long-runout landslide triggered by the August 1998 Heavy rainfall in Fukushima Prefecture, Japan

Abstract Heavy rainfall from 26 to 31 August 1998 triggered many landslides in Nishigo Village of southern Fukushima Prefecture, Japan. The Hiegaesi landslide, a long-runout landslide with travel angle of 11°, which occurred in loamy volcanic-ash/pumice layer and was deposited in a nearby rice paddy, was investigated. In an observation pit dug in the middle part of the landslide deposit, the sliding zone just above the deflected rice plants was observed, and it was confirmed that grain crushing occurred in the sliding zone. The triggering and sliding mechanisms of this landslide then were investigated by ring-shear tests in laboratory. For the triggering mechanism, one saturated naturally drained test (test A: torque-controlled test) and one saturated undrained test (test B: speed-controlled test) were conducted on the samples taken from the source area of the landslide. Even in the naturally drained test opening the upper drain valve of the shear box, a temporary liquefaction occurred. In the undrained test, excess pore-pressure was generated along with shearing, and “sliding-surface liquefaction” phenomenon was observed. The effective stress and shear resistance finally decreased to near zero. These results can explain the observed phenomenon of small friction resistance like a flow of liquid when the sliding mass slid out of the source area. For the sliding mechanism of the landslide in the rice paddy, saturated undrained test (test C: speed-controlled test) was performed on soil sample above the deflected rice plants. The apparent friction angle obtained in this test was 8°. In addition, the residual friction angle measured after test B and test C was the same value of 41°. Combining with the observation on the shear zone in the ring-shear box after test C, it is concluded that, during the sliding in rice paddy, the undrained shear strength of the soil layer itself mainly influenced the high mobility of the landslide, probably because the friction between rice plants and soils is greater than the undrained shear strength inside the soil mass.

Downslope volume enlargement of a debris slide–debris flow in the 1999 Hiroshima, Japan, rainstorm

Abstract Following a heavy rainstorm on 29 June 1999, hundreds of slope failures occurred in granitic mountains in Hiroshima Prefecture, Japan. Among these events, a highly mobile landslide (termed the Kameyama landslide in this paper), which occurred in Kameyama area of Hiroshima city, was the most catastrophic, and was investigated in detail. The displaced soil mass from the source area of this landslide traveled about 300 m and deposited a volume more than 10 times as great as that in the source area. The landslide originated in and traversed a valley-shaped concave slope covered by pre-existing colluvial debris deposits. In addition, a spring was visible in the source area and very shallow ground water was observed in an observation pit dug in the source area. Thus, it is inferred that the ground-water table rose quickly during the rainfall, and that this rise triggered the slope failure in the source area. Based on a field survey along the landslide cross section, a possible explanation for the mechanism of the landslide was obtained: the displaced soil mass from the source area impacted the debris deposit in the path of the landslide, thus triggering liquefaction failure of the saturated part of debris. The original landslide and the liquefied debris then moved downslope as a single mass. To examine this assumption, ring-shear tests were performed on samples taken from the source area. Undrained ring-shear tests on the saturated samples showed that the sample is highly liquefiable, and liquefaction failure could have been triggered in the debris deposits by a very small impact from the displaced soil mass of the initial failure. In addition, laboratory tests simulating the impacts on the debris deposits at natural water content, i.e., unsaturated (at the survey time, 2 days after the failure) showed that although shear failure could be caused by the assumed impact force, the displaced soils stopped after a few centimeters displacement, indicating that existence of a saturated zone in debris deposits is prerequisite for this kind of failure.

Mechanisms of Landslide Triggered Debris Flows

Debris flows are often caused by heavy rainfalls. Warning of debris flows is issued by rainfall pattern, rainfall intensity and cumulative rainfall(or duration) in the countries such as Japan, USA et al. However, some debris flows are not regulated by rainfall. Two big debris flow disasters occurred in Japan. One (Gamahara debris flow) in Nagano Prefecture in 1996 killed 14 persons, one (Harihara debris flow) in Kagoshima Prefecture killed 21 persons in 1997. Both took place without any rainfall intensity at the time of occurrence, and especially the Gamahara debris flow occurred in the day of almost no precipitation in a dry season, and cumulative rainfall was not high. The field investigation and dynamic loading undrained ring shear tests for these two debris flows presented two mechanisms of landslide triggered debris flows which do not need full saturation of debris for the high speed motion of debris with a very low shear resistance.

Mechanism of creep movement caused by landslide activity and underground erosion in crystalline schist, Shikoku Island, southwestern Japan

Abstract The mechanism of creep movement of the Zentoku landslide in crystalline schist has not been studied in detail because of the steepness of the slope, very slow movement, low population density and complex topographic and geologic characteristics. Sassa et al. (1980: Proc. INTERPRAEVENT 1, 85–106) and Sassa (1984: Proc. 4th International Symp. on Landslides, Toronto, vol. 2, pp. 179–184; 1985. Geotechnical classification of landslides, Proc. 4th International Conference and Field Workshop on Landslides, Tokyo, pp. 31–40; 1989: Landslide News, Japan Landslide Society, No. 3, pp. 21–24) monitored landslide movement and groundwater level at the Zentoku landslide on Shikoku Island, southwestern Japan, and suggested that the mechanism may be caused by underground erosion. To study the influence of underground erosion at this site, continual monitoring of suspended sediment and water discharge from a groundwater outlet (i.e. a spring) was implemented. The locations of groundwater flow paths were determined, as were the amounts of discharged sediment. Slope deformation was monitored by means of a borehole inclinometer. The conclusions were as follows: (1) the flow paths were found to be on or above the shear zones in which underground erosion has occurred; (2) in addition to being a result of precipitation and groundwater discharge, sediment discharge is affected by landslide activity; and (3) the mechanism of creep movement is an interrelated chain process that combines underground erosion caused by landslide activity with landslide activity caused by underground erosion. Thus, landslide activity increases erosion susceptibility and transportation of soils within the mass, and underground erosion causes instability of the landslide mass, in turn. This mechanism can explain the observed phenomenon that the Zentoku landslide not only moves actively during heavy rain, but also continues to creep throughout the year.

Displacement Monitoring and Physical Exploration on the Shuping Landslide Reactivated by Impoundment of the Three Gorges Reservoir, China

The Three Gorges Dam construction on the Yangtze River in China is the largest hydro-electricity project in the world. After the first impoundment in June 2003, many landslides occurred or reactivated. Shuping landslide is one of the most active landslides among them. In this paper, the deformation of the Shuping landslide monitored by GPS, extensometers, and crack measurements are summarized. Also, for the investigation of the groundwater situation, 1 m depth ground temperature measurement was conducted, and the groundwater veins were estimated. Based on the monitoring data and exploration results, a deformation model of the landslide caused by impoundment of reservoir was proposed.

ISDR-ICL Sendai Partnerships 2015–2025 for global promotion of understanding and reducing landslide disaster risk

At the 2nd United Nations World Conference on Disaster Reduction, which was held in Kobe, Japan, on 18-22 January 2005, the International Consortium on Landslides (ICL) co-organized a session which resulted in a global partnership and platform taking a holistic approach to research and learning on ‘Integrated Earth system risk analysis and sustainable disaster management’. This partnership was forged through a “Letter of Intent”, that was signed by UNESCO, UNISDR, WMO, FAO, UNU, ICSU, and WFEO. It further led to the adoption and implementation of the 2006 Tokyo Action Plan, thus creating a global partnership on Landslides, i.e., the current International Programme on Landslides (IPL) of ICL.

Monitoring, Prediction and Early Warning

Landslide risk reduction is a societal pressing need in for counties and also areas along coasts, lakes, rivers in relatively flat countries. Engineering measures to stabilize dangerous slopes needs very high cost and not feasible for many cases. Monitoring, Prediction, Early Warning is the most economical landslide risk reduction measure which is applicable for both developed and developing countries. This chapter presents monitoring of triggering factors, slope deformation, other indicators in indoor experiments, field experiments as well as in natural condition. Methodology of prediction and early warning is examined based on these monitoring and topographical, geological and hydrological conditions.