Akira Suemine

On the Initiation and Movement of Hanokidaira Landslide from the 2011 Tohoku Earthquake, Japan

A flow type landslide was triggered in Hanokidaira area by The 2011 off the Pacific coast of Tohoku Earthquake, killing more than ten people. The slope consists of pyroclastic flow deposits that were formed at different times, with a layer of paleosol that was outcropped. Above the paleosol there is a layer of pumice and scoria, which is very rich in natural moisture content (~145 %). From field observation, we inferred that the sliding surface originated on the boundary between the paleosol layer and the layer of pumice and scoria. To examine the possible trigger and movement mechanism of these landslides, we monitored the aftershocks on landslide area, and inferred the possible seismic response of the landslide area during the main shock. We took samples from both layers, and performed undrained static and cyclic shear tests on them in fully saturated or in natural water content states. The results showed that high pore-water pressure could be generated after failure with increase of shear displacement, resulting in great loss in the shear strength. We also performed seismic simulation tests on these samples by using the inferred seismic waveform, and the test results showed that the amplified vertical motion may have played a key role in the initiation of this landslide.

Application of the Newly Frequency Domain Electromagnetic Method Survey in a Landslide Area

Geophysical survey in a Landslide area is mainly perfomred by measuring the elastic wave velocity or electric resistivity. The electrical resistivity is well known to reflect the information in a slope, which can be obtained by the seismic prospecting or electrical resistivity survey. The commonly used electrical resistivity survey has disadvantage sometimes due to local ground conditions during the installation of electrodes and layout of electric cables on the hard material. On the other hand, the frequency domain electromagnetic method (FDEM), one of the electromagnetic methods, has advantages due to its nondestructive measurement nature and also its simple operation. In this study, we used a newly developed FDEM survey apparatus to examine its applicability in the exploration of landslide. The results showed good consistency with the structure of the landslide mass, which had been indentified by many other apporaches during the passing decades. Therefore, we concluded that FDEM is an effective method in exploring the area and also the inner structure of landsliding mass.

Enlargement of a Failed Area along a Sliding Surface

Progressive failure in short-term slope stability is examined in this study. The relative displacement between a sliding mass and a basement in a landslide was monitored precisely. The onset time of relative displacement at each borehole was determined and the speed of propagation of local relative displacement was calculated. The lowest speed of propagation was 4.6 mhr−1. Stresses inside the specimen were measured in the direct shear apparatus with a 400 × 400 mm shear plane. Results show that the horizontal sliding surface determined by the apparatus does not coincide with the plane in which Mohr’s failure criteria are first fulfilled, but with the plane of the maximum shear stress. Shear stress reductions were observed and the drop position migrated with the shear.

Groundwater Flow Behavior at Landslide Area in Crystalline Schist Mountains

In this study, we performed the 1-m-depth ground temperature survey and continuous ground temperature monitoring at the Zentoku landslide area and the Nishi-ikawa landslide in the crystalline schist mountains to clarify the actual groundwater flow in landslide slopes. Our results showed that the ground temperatures at differing locations of the slope were different, probably due to the influence of veins of groundwater flow. We also found that the ranges of these differences in ground temperature were different for different landslides in the same mountain or different mountains. To clarify the reasons for these differences, we compared the data obtained from these two different landslide areas and also those data obtained from other case studies with groundwater dating. We inferred that during a rainfall, the groundwater from deep layer, which is relatively cold in summer and warm in winter than shallow groundwater, and wasn’t formed just after heavy rain, but infiltrated previously (approximately two to nine years ago), might have also been expelled out, resulting the greater difference in ground temperature. Namely, the groundwater flowing in the crystalline schist slope involves not only the newly infiltrated rainwater, but also those water concentrated flow from deep layer with some residence time. Therefore, it is suggested that this kind of groundwater flow should be considered in the instability analysis of landslides on the crystalline schist slopes.

Vein of Groundwater Flow Behavior in a Land-Slide by Continuous Monitoring of Ground Temperature

To examine the dynamics of groundwater flow within landslide mass, we installed 29 thermocouples at the depth of one meter along three survey lines crossing the main body of Nishi-ikawa landslide in Tokushima, Japan, and continuously monitored the ground temperature since 30 April 2010. Through comparing the variation of these 1-m deep ground temperatures, we estimated the location of groundwater flow vein and found that: (1) characteristic of annual change in the 1-m deep ground temperature on Nishi-ikawa landslide was lower than the value estimated by Takeuchi’s equation, (2) some measurement points on Nishi-ikawa landslide read decreasing values, showing approximately the same tendency as the constant-temperature zone during heavy rain in summer season. We than inferred that these decrease in 1-m deep ground temperature resulted from the flow of groundwater from deeper layer such as the constant-temperature zone to the shallow layer through vein-like passes in the landslide mass during infiltration of heavy rain.

Warning/Evacuation and Monitoring Methods for Earthquake-Induced Landslides

Phenomena related to earthquake-induced landslides are diverse and include debris flows, floods, and overflowing due to breaching of landslide dams. They are also characterized by diversity in timing of occurrence, in changes in various phenomena and in the scope or scale of impact. We focus on diverse time-series changes as a characteristic of earthquake-induced landslides. On the basis of these characteristics, the main warning and evacuation measures and monitoring approaches for earthquake-induced landslides are divided into three stages in terms of time elapsed. The warning/evacuation and monitoring methods applicable to each stage are described as below. Stage 1: For several hours starting immediately after the earthquake (short term). People may voluntarily evacuate the area to avoid danger; alternatively, the local mayor may issue an evacuation advisory or order. Because quantitative monitoring with instruments is hard to conduct owing to the unavailability of equipment, warning/evacuation measures at this stage are usually determined by individuals (mayors), depending largely on these individuals’ experiences of sediment disasters. Stage 2: For several hours to several days after the earthquake (medium term). Government assistance and recovery efforts begin with a search for missing persons and efforts to rescue survivors. Monitoring equipment and warning equipment can be installed. On the basis of the results of field investigations and data from equipment, major warning and evacuation measures are put into practice. Monitoring at this stage is directed at (i) warning and evacuation and safety management, and (ii) understanding landslide behavior. Stage 3: For several days to several months after the earthquake (long term). Full recovery efforts are underway. Several days after the earthquake, the evacuation advisory or order may be lifted, depending on the results of observations of landslide behavior and on whether sensors are installed. The purposes of monitoring at this stage are the same as before, namely to understand landslide behavior, secure work safety. Disbanding warning and evacuation systems is added as an additional purpose. We also describe advance preparation for warning/evacuation, and educational activities.

Comparison of Crustal Deformations Observed with GPS and Strainmeters/Tiltmeters

Daily solutions from GPS observations can give uniform tectonic deformation and relative motions of stations more than 100km apart to each other. More precise wet delay correction is necessary in order to monitor local or the order of 10km deformations. On the other hand observation with strainmeters and tiltmeters show irregular changes which are probably associated with local stress accumulation or irregular concentration, but it is not easy to distinguish the irregular change caused by tectonic stress from that caused by meteorological or ground water disturbances. Combined observations with GPS and high sensitive strainmeters/tiltmeters are important to make clear the nature of observed irregular changes, and further to get information relating to the occurrence of earthquakes of M7 class and accordingly to approach the long-term earthquake prediction.

An Example of Unexpected Landslide in Crystalline Schist

An example of unexpected landslide in crystalline schist is reported. After conventional investigation; such as a survey of ground surface, boring and ground water survey; a hill is cut partially at its toe to construct a new road. The degree of the cut slope was very low to maintain the slope stability. Then a landslide occurred after heavy rainfall. Additional surveys; thus, boring, standard penetration tests, and measuring by pipe strain meter and extensometer, were conducted. The shape of landslide was identified by means of these surveys. After considering several countermeasures to prevent landslide, earth removal work was selected, which stabilized the landslide. We propose an idea that new knowledge about unexpected landslide will be obtained by selecting a characteristic common to many landslides that occurred at various geological features and underground water level in Japan.

Soil-Temperature Survey and Ground-Water Prospecting in a Crystalline-Schist Landslide

An underground-temperature survey was carried out in summer time and a ground-water prospecting was carried out after a typhoon in a crystalline-schist landslide. Drainage drillings were executed towards lower-temperature zones inferred from the underground-temperature survey. During a typhoon in the next year, the drainage flow rate was 40-720 1/min, which rate was from 4 to 50 times as great as that of low-water flow. At the period of the rain, the ground-water level decreased from one meter to nine meters. The lower temperature zones seem to be caused by high ground-water velocity, while the higher temperature zones by low ground-water velocity. But there was a high temperature spot, where ground water flowed rapidly. The velocity ranged between several cm/sec and 100cm/sec. The intermittent ground-water flow is suggested at two points in the landslide area.

An Example of Observation in Shobu Landslide

徳島県三好郡井川町正夫地すべり地 (三波川帯) における地すべり活動の観測結果について述べている。伸縮計と地中内部歪計と挿入型地中内部歪計の観測結果について主に述べている。観測の結果, 本地すべり地においては, 地表面においても地中においても, ある時には伸びたり, 次の時には縮んだりするというまるで尺取り虫的な地すべり活動が起っているらしいということが判明した。また, 抑制工としての排水ボーリングの結果, 排水量は数l/min位であるが地すべり活動が, 抑制工以前と比べてはるかに低減したという観測結果が得られている。