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C. Ng, Abraham Chiu (2001)BEHAVIOR OF A LOOSELY COMPACTED UNSATURATED VOLCANIC SOIL
Journal of Geotechnical and Geoenvironmental Engineering, 127
(2014)Debris flow in Izu
(2014)Groundwater. In: Investigation report of Izu Oshima landslide caused by Typhoon No. 26 in October
(2014)Mechanism of landslides occurrence. In: Investigation report of Izu Oshima landslide caused by Typhoon No
(2006)Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System)
H Ikeya (2014)Debris flow in Izu Oshima Island on October 16, 2013
A. Skempton (1954)THE PORE-PRESSURE COEFFICIENTS A AND B
(2014)Investigation report of Izu Oshima landslide triggered by Typhoon No. 26 in 2013
Geological map of Izu Oshima volcano
(2014)The past landslides
(1998)Geological map of Izu Oshima volcano. Geological Map of Volcanoes 10, scale 1:25000
(2014)Landslide disaster of Izu-Oshima Island by typhoon No.26 in 2013
(2014)Geotechnical characters of soil
(2013)Disaster Research Team of Kanto Branch (2014) Landslide disaster of Izu-Oshima Island by typhoon No
Background: Typhoon Wipha struck Izu Oshima Island on 16 October 2013, bringing heavy rainfall. It triggered widespread landslides on the western slopes of Izu Oshima Island, and caused significant loss of life and serious property damage. Authors have conducted both field investigations and laboratory experiments in an effort to understand the initiation mechanism of the shallow landslides triggered by heavy rainfall. Results: Pyroclastic-fall deposits on the slopes are well-graded fine sand with silt, and with high specific gravity and void ratio. These soil properties will affect the mechanical and hydraulic characteristics of soil. The results of consolidated-undrained triaxial tests show that the effective internal friction angle of soil is 38.7 degrees. The results of triaxial tests using pore-water pressure control show that static liquefaction can occur in porous pyroclastic-fall deposit layers due to rainfall infiltration. Conclusions: The effective strength of pyroclastic-fall deposits on the upper slope is quite high. Even though the slope is very steep (over 30 degrees), it can remain stable while in an unsaturated condition. Due to heavy rainfall and the porosity of the pyroclastic-fall deposits, rainfall can quickly infiltrate into soil layer. Moreover, the interface above the underlying basalt will stop groundwater infiltration, acting as an impervious boundary. With increase of groundwater level, the effective strength of the porous soil will decrease. Finally, static liquefaction can be triggered, leading to the generation of shallow landslides on the upper slopes. Keywords: Shallow landslide; Rainfall; Triaxial test; Initiation mechanism; Izu Oshima Island Background were killed, four people remain missing, 73 houses were Typhoon Wipha passed the Izu Islands on 16 October completely destroyed, and 129 houses were damaged 2013. These islands are located at the southeast of the (Ministry of Land Infrastructure and Transport and Izu Peninsula, Japan (Fig. 1). Most regions on the Japan 2013). typhoon track, especially the Izu Oshima Island, suffered Several researchers and organizations carried out field heavy rainfall. It triggered widespread shallow landslides investigations after this geo-disaster event (Sakurai and on the western slopes of Izu Oshima Island (Fig. 2). Disaster Research Team of Kanto Branch 2014; Ikeya Pyroclastic-fall deposits mixed with branches and trunks 2014; Disaster Prevention Division of Tokyo Metropolitan flowed downslope along gullies and drainage channels. Government (TMG) (2014)). Their investigation reports High speeddebrisflows floodedthe settlement of describe the occurrences of the landslides, and the damage Motomachi, which is situated at the toe of the slopes. caused by the debris flows. These studies provide key This disaster caused serious loss of life and property background information for further research. Authors had damage. According to the damage report, 35 people conducted both field investigations and laboratory experi- ments in an effort to understand the initiation mechanism of shallow landslides triggered by heavy rainfall during Ty- phoon Wipha. Consolidated-undrained triaxial tests were * Correspondence: email@example.com Department of Geoscience, Shimane University, Matsue 690-8504, Japan Full list of author information is available at the end of the article © 2015 Yang et al. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited. Yang et al. Geoenvironmental Disasters (2015) 2:15 Page 2 of 10 Study site Geology Izu Oshima Island is a volcanic island. This active volcano consists of pre-caldera, syn-caldera and post-caldera volcanoes (Kawanabe 1998). Many hazardous eruptions have occurred in the past. The ejecta in each stage were mostly coarse-grained pyroclastic materials, with basaltic lava flows and pyroclastic-fall deposits. Younger deposits and lava flows covered the deposits and lava flows from the older volcano stage. This type of volcanic activity led to the formation of resistant lava flows interbedded with poorly consolidated pyroclastic-fall deposits. Fig. 3 shows the geological map of the main landslide area, which is outlined in Fig. 2 (right) with a yellow frame. Most of the shallow landslides which were the source of the debris flows occurred within the distribution of basalt (Y5L). Pyroclastic-fall deposits overlie the basalt (Y5L) in the sec- tion which is exposed in the upper slope along the road (Fig. 4). This type of structure with interbedded resistant Fig. 1 Location of Izu Oshima Island. Inset is the track of basalt and weak pyroclastic deposits can easily cause land- Typhoon Wipha slides. Moreover, the interface between these two layers tends to be a confining boundary for groundwater passing conducted to determine the effective soil strength. This through the upper soft and porous pyroclastic-fall deposits. paper also presents the results of triaxial tests with pore- water pressure control, which were conducted to simulate Topography the initiation mechanism of the shallow landslides on The topography of Izu Oshima Island is typical of a vol- the upper slopes due to rainfall infiltration. canic island. The outline of the island is oval, elongated Fig. 2 Landslides in the west of Izu Oshima Island (red areas indicate the landslide distribution, modified from Geospatial Information Authority of Japan, 2013) Yang et al. Geoenvironmental Disasters (2015) 2:15 Page 3 of 10 Fig. 3 Geological map of main landslide area (modified from Kawanabe 1998; photograph from the Geospatial Information Authority of Japan). Y5L: Basalt (Syn- and post-caldera volcano, 1338?); Y5C: Basalt scoria and spatter (Syn- and post-caldera volcano, 1338?); N4L: Basalt (Syn- and post-caldera volcano, 8th Century); N4C: Basalt scoria (Syn- and post-caldera volcano, 8th Century); C: Basalt scoria (Pre-caldera volcano); YE: Pyroclastic-fall deposits (Pre-caldera volcano) in the NNW-SSE direction, with a length of about island was planned as a residential area. A caldera with a 15 km, and width of about 9 km (Kawanabe 1998). The diameter of about 4 km is present at the summit of the central part of the island has the highest elevation due volcano, but in the east its wall is buried by younger to volcano eruption. The highest point is Mt. Mihara lava. On the west side, the caldera wall is clearly defined. (764 m a.s.l.), and the terrain becomes gentler towards In addition, the slopes of the outer rim of the western the coast. Sea cliffs up to 350 m high are developed from caldera wall are quite steep (Fig. 5). Most of the shallow the north to the east, and in the southwest of the island, landsides occurred on the steep slopes. due to wave erosion. The highest cliff is found on the east coast. Relatively flat land in the northwest of the Heavy rainfall The main triggering factor of these shallow landslides was the short-period downpour of heavy rainfall. On the early morning of 16 October 2013 Typhoon Wipha was close to the Izu Islands (Fig. 1). As Typhoon Wipha approached, the northern Izu Islands suffered heavy rainfall, especially Izu Oshima Island. Records from the meteorological sta- tion at Motomachi in the Izu Oshima Island show the maximum one-hour precipitation was about 118.5 mm. The maximum 24-hour accumulated precipitation was more than 824 mm (Fig. 6). This was 2.5 times the average monthly precipitation for October in Izu Oshima Island (Tokyo District Meteorological Observatory (TDMO) 2013). Field investigation Fig. 7 shows a plan view of the areas that were impacted Fig. 4 Basalt (Y5L) overlain by pyroclastic-fall deposits (at red spot by debris flows. According to the information from local in Figure 3) residents and seismic data, shallow landslides (light blue Yang et al. Geoenvironmental Disasters (2015) 2:15 Page 4 of 10 drained out through the drainage channel (brown arrow) (P2 in Fig. 8). Finally, this fine material flowed into the ocean and was deposited at the coast (P3 in Fig. 8). Sabo dam 2, sit- uated just behind the Volcano Museum, stopped the debris from damaging the museum and nearby residential houses. Shallow landslides on the upper slopes which were the source of debris flows were examined in the main inves- tigation area (Fig. 9). Slope angle is one of the most im- portant factors on slope instability. In the field, Laser Range Finder (LRF) was used to survey the longitudinal profile (A-A’ in Fig. 9) of the shallow landslide selected for study. The measuring result shows that slope is with steep terrain, and slope angle is over 30 degrees. Methods Soil samples were collected from the pyroclastic-fall deposit layer above the basalt at location S1 to study soil properties (Fig. 9). Conventional laboratory experiments on the soil sample were conducted to obtain the basic parameters (grain size distribution, specific gravity, in- situ dry density and void ratio) which will be used to control the parameters of specimens for triaxial tests. Triaxial text using strain control Fig. 5 Slope terrains on Izu Oshima Island. Red frame shows the Consolidated-undrained triaxial tests were conducted to de- main landslide area termine the effective soil strength. According to the in-situ soil dry density, dry soil passed 2 mm sieving was used to area) occurred on the upper slopes in the early morning make a cylindrical specimen. In order to get a homogenous (Ikeya 2014). Pyroclastic-fall deposits mixed with branches specimen, the dry soil sample was divided into several parts and trunks flowed downslope along the gullies (beige ar- to fill the cylindrical specimen tube with rubber membrane. rows). Debris flow flushed away the surface soil and vegeta- After filled the all soil into the specimen tube, the cylin- tion along the gullies (gullies A and B in Fig. 7). Because no drical surface of the sample is covered by the rubber mem- drainage channels were present, the debris flow flooded the brane sealed by rubber O-rings on the top and base of load houses situated on the down slopes area (light red area), system. After that, the specimen was fully saturated. Carbon and caused serious loss of lives and properties. The sabo dioxide (CO ) was slowly supplied from the base of the spe- dam 1, which is located at the mouth of gully A, played an cimen to gradually replace the air within it. Then de-aired important role in protecting this area. It effectively reduced water was slowly supplied to replace and absorb the CO to the speed of the debris flow, and captured branches and achieve a saturated state. The specimen was confirmed to trunks. Furthermore, fine-grained particles were easily be fully saturated when the Skempton’sBvalue, whichis Fig. 6 Hourly and accumulated precipitation in Izu Oshima Island from 15 October (8:00 a.m.) to 16 October (8:00 a.m.) 2013 Yang et al. Geoenvironmental Disasters (2015) 2:15 Page 5 of 10 Fig. 7 Plan view of the main landslide area (photograph from the Geospatial Information Authority of Japan) called pore-pressure coefficients (Skempton 1954), was principal stress can be assumed to be 40 kPa, based on the higher than 0.95. With the fully saturated specimens, four unit weight of the soil. The minor principal stress can be consolidated-undrained compression tests were conducted obtained as 15 kPa, based on the lateral earth pressure co- under four different confining stresses (25, 50, 75 and 100 efficient (K ), which can be calculated using Equation (1). kPa). After normal consolidation, the specimens were com- pressed at 1.0 % of axial strain per minute under the un- K ¼ 1− sinϕ ð1Þ drained condition. The data of deviatoric stress, pore-water pressure and axial strain was obtained by data logging sys- where ϕ′ is effective friction angle of the soil. tem. Through these tests, the shear strength parameters The process of specimen preparation and saturation was can be obtained. the same as that of the triaxial tests using strain control. The confining pressures were σ = 40 kPa and σ =15 kPa. 1 3 Triaxial text using pore-water pressure control After consolidation, de-aired water is supplied through a Triaxial tests with pore-water pressure control were con- pore-water pressure controller to increase the pore-water ducted to simulate the initiation mechanism of the shallow pressure. The rate of increase in pore-water pressure landslide on the upper slopes due to rainfall infiltration. was 0.2 kPa per minute. Through this step, the situ- The thickness of the upper layer soil varies from 1 to 3 ation of pore-water pressure accumulation on the po- meters, and hence for the initial stress condition the major tential sliding surface is simulated. Fig. 8 Photograph locations are marked on Figure 7 Yang et al. Geoenvironmental Disasters (2015) 2:15 Page 6 of 10 saturated specimens is shown in Figure 10b. Positive ex- cess pore-water pressure was recorded in all four tests. In all tests, the positive excess pore-water pressure increases to a peak at low strain (about 5 %), reduces from its max- imum value as the strain increases continuously, and fi- nally reaches a steady state at the end of the tests. The initial buildup of the positive excess pore-water pressure suggests that the specimens exhibit contractive behavior. The reduction in the excess pore-water pressure after the peak indicates that the specimens change from contractive to dilative behavior during shear, as illustrated by the ef- fective stress paths shown in Fig. 11. Ng and Chiu (2001) described the similar shear behaver of loosely compacted saturated volcanic soil under undrained conditions in their paper. All the effective stress paths show a similar trend. Each path moves toward the right-hand side initially, and then moves toward the left-hand until reaching a turning point, after which they turn right, before reaching the crit- ical state line CSL at the end of the test. The critical states of saturated specimens can be represented by the CSL in Fig. 9 Main investigation area of the shallow landslides on the the stress plane, as shown in Fig. 11. The gradient of the upper slopes (photographs from the Geospatial Information Authority of Japan) critical state line is 0.625. This corresponds to a critical state angle of internal friction angle of 38.7 degrees. Results The relationships between deviatoric stress, axial strain In the Table 1, it can be observed that the specific gravity and pore-water pressure under loose and dense conditions and void ratio of the pyroclastic-fall deposits are 2.903 and are shown in Fig. 12a and b, respectively. For the soil in 1.183, respectively. That shows the typical property of loose condition (initial void ratio of 1.079), effective stress pyroclastic-fall deposits. These soil properties will affect starts to decrease when the pore-water pressure increases the mechanical and hydraulic characteristics of the soil. to about 8 kPa. Obvious decrease of effective stress is gen- Grain size distribution of sample shows that the soil is erated when the pore-water pressure reaches about 12 kPa. well-graded fine sand with silt (ASTM D2487-06 2006). Excess pore-water pressure builds up with rapid increase After the occurrence of shallow landslides, these fine ma- of axial strain. For the dense condition (initial void ratio of terials on steep slopes can easily transform into debris 0.808), effective stress starts to decrease when pore water flow during heavy rainfall. pressure increases to about 13 kPa. Obvious decrease of The stress–strain relationship of the consolidated- effective stress is generated when the pore-water pressure undrained tests on the saturated specimens (Fig. 10a) reaches about 17 kPa. In contrast to the loose condition, shows that the deviatoric stress increases with the axial the rate of increase of pore-water pressure slows with the strain. The stress–strain curves indicate the evidence of rapid increase in axial strain. From these results, it is evi- strain hardening. When the shear strain is over about dent that obvious decrease of effective stress occurs in 15 %, the deviatoric stress begins to flatten. Critical state a shorter time in the soil in loose condition than it is in was reached in all four tests with a high axial strain. The dense condition. relationship between the excess pore-water pressure and the axial strain of the consolidated-undrained tests on the Discussion Two types of failure mode Two types of failure mode can be proposed, based on Table 1 Soil parameters the pattern of shallow landslides on the steep slopes Parameters Value along the road (Fig. 13). The first type of failure mode Specific gravity, G 2.903 shows that pyroclastic-fall deposits partially mantling Coefficient of uniformity, C 12.188 u the upper most slopes slid first, during heavy rainfall. Coefficient of curvature, C 1.154 Thesoil massthencrossed theroad, anddrove the pyroclastic-fall deposits lying below the subgrade, to Mean grain size, D (mm) 0.143 slide downslope together. For this type of failure mode Dry density, ρ (kg/m ) 1,330 slope cutting during road construction affects the sta- Void ratio, e 1.183 bility of the pyroclastic-fall deposits on the upper Yang et al. Geoenvironmental Disasters (2015) 2:15 Page 7 of 10 Fig. 10 (a) stress–strain relationship; (b) relationship between pore-water pressure and axial strain for the consolidated-undrained tests slopes. Slope cutting without any preventive construc- Slope angle tion will result in a free boundary, and remove the re- According to historical literature, landslide disasters sistance at the toe of slope. Therefore, the slope can caused by heavy rainfall have occurred several times on easily become unstable under trigger factors such as the slopes near Motomachi (National Research Institute rainfall and dynamic load. for Earth Science and Disaster Prevention (NIED) 2013; The second type of failure mode shows that the entire Sakurai 2014). From the slope angle distribution of the pyroclastic-fall deposits below the road started to slide Izu Oshima Island (Fig. 5), it is evident that the slopes during heavy rainfall. The main scarp of this shallow where landslides have often occurred are quite steep. To landslide was located at the subgrade of the upper part discuss the effect of slope angle on the slope stability, a of the road. The topographic map shows that the section simplified slope model for limit equilibrium analysis can of subgrade of the upper road near the main scarp was be assumed (Fig. 15) based on the slope structure. of relatively low elevation (Fig. 14). The lower-lying and For the unit width, the factor of safety (F ) of slope winding road surface will collect surface water during can be obtained using Equation (2). heavy rainfall. Subsequently, runoff water will continu- c′ þ γH−γ h cos α tan ϕ′ ously infiltrate the pyroclastic-fall deposit layer on the F ¼ ð2Þ γH sin α cos α slope below the road subgrade. Finally, groundwater increase in the slope will lead to the pyroclastic-fall where, deposits sliding. H is the thickness of the pyroclastic-fall deposit layer; h is the height of groundwater table; α is the slope angle; γ is the average unit weight of soil, 17 kN/ m ; γ is the unit weight of water, 9.8 kN/m ; c′ is the effective cohesion of soil; ϕ′ is the effective friction angle of soil, 38.7 degrees. Due to that, the pyroclastic-fall deposits can be considered as a cohesionless soil Equation (2) can be simplified as follows: γ h tan ϕ′ F ¼ 1− ∙ ð3Þ γ H tan α For the slope without rainfall infiltration, and in which there is no obvious groundwater table, h in Equation (3) Fig. 11 Effective stress paths under different confining pressures can be assumed to be 0. Therefore, the relationship be- (25, 50, 75 and 100 kPa); CSL – Critical State Line tween factor of safety and slope angle can be plotted as Yang et al. Geoenvironmental Disasters (2015) 2:15 Page 8 of 10 Fig. 12 Relationship between deviatoric stress, axial strain and pore-water pressure under (a) loose condition and (b) dense condition in Fig. 16. This result shows that slope will remain 20 degrees) can remain stable even when the height of stable, even if the slope angle is steep as 38 degrees. groundwater table nearly reaches the ground surface. Due to rainfall infiltration during heavy rainfall, the However, relatively steep slopes (such as 30 degrees) groundwater table in the slope will increase gradually, will reach a critical situation when the groundwater and this will affect the slope stability. Taking a slope table reaches half the thickness of the pyroclastic-fall with a two-meter thick pyroclastic-fall deposit layer as deposit layer. an example, we can plot the relationship of factor of safety (F ) with the height of the groundwater table and Static liquefaction slope angle, using Equation (3) (Fig. 17). From this fig- The profile of the pyroclastic-fall deposits on the slope ure, it is apparent that relatively gentle slopes (such as (Fig. 18) shows that several obvious layers are present. Layer 1 is surface soil, mixed with plant roots. Layer 2 and Layer 4 are both yellowish-brown deposits with a tough and cemented structure. Their yellow color originates from abundance of fine clay particles. Therefore, the permeability of these layers is quite Fig. 14 Topographic map of the upper slope (modified from Fig. 13 Two types of failure mode of shallow landslides Geospatial Information Authority of Japan) Yang et al. Geoenvironmental Disasters (2015) 2:15 Page 9 of 10 Fig. 17 The relationship of factor of safety (F ) with the height of the groundwater table and slope angle Fig. 15 Simplified slope model for limit equilibrium analysis low. In contrast to Layers 2 and 4, Layer 3 and Layer 5 are fresh, dark gray pyroclastic-fall deposits with porous structure. Consequently, their permeability and water re- tention capability are quite high. With this sandwich panel structure, confined groundwater will be generated dur- ing continuous groundwater infiltration from the upper slopes. The results of triaxial tests using pore-water pressure control show that static liquefaction can occur in these porous pyroclastic-fall deposit layers. Piping holes were observed on the main scarp in the field, confirming the phenomenon of static liquefaction in porous pyroclastic-fall deposit layers (Wakai et al. 2014; Inagaki 2014; Ueno 2014). Fig. 16 The relationship between factor of safety (F ) and slope angle Fig. 18 Profile of pyroclastic-fall deposits (S2 in Figure 9) s Yang et al. Geoenvironmental Disasters (2015) 2:15 Page 10 of 10 Conclusions Disaster Prevention Division of Tokyo Metropolitan Government (TMG) (2014) Investigation report of Izu Oshima landslide triggered by Typhoon No. 26 in 2013. Sabo 115:7–11, In Japanese 1. The effective strength of pyroclastic-fall deposits on Ikeya H (2014) Debris flow in Izu Oshima Island on October 16, 2013. Sabo the upper slope is quite high, and the effective internal 115:2–6 (In Japanese) Inagaki H (2014) Mechanism of landslides occurrence. In: Investigation report of friction angle is 38.7 degrees. Consequently, even Izu Oshima landslide caused by Typhoon No. 26 in October 2013. P 74-77 though the slope is very steep (over 30 degrees), it can (In Japanese) remain stable while in an unsaturated condition. Due Kawanabe Y (1998) Geological map of Izu Oshima volcano. Geological Map of Volcanoes 10, scale 1:25000. Geological Survey of Japan to heavy rainfall and the porosity of the pyroclastic- Ministry of Land Infrastructure and Transport, Japan (2013) Report of the damage fall deposits, rainfall can quickly infiltrate into soil caused by heavy rainfall during the typhoon No. 26 (No. 16). layer. Moreover, the interface above the underlying http://www.bousai.go.jp/updates/h25typhoon26/pdf/h25typhoon26_16.pdf. Accessed 4 February 2014 basalt will stop groundwater infiltration, acting as an National Research Institute for Earth Science and Disaster Prevention (NIED) impervious boundary. With increase of groundwater (2013) Disaster in the history of Izu Oshima. http://dil.bosai.go.jp/disaster/ level, the effective strength of the porous soil will 2013H25T26/pdf/izuoshima_history.pdf. Accessed 22 January 2014 Ng C, Chiu A (2001) Behavior of a loosely compacted unsaturated volcanic soil. decrease. Finally, static liquefaction can be triggered, J Geotech Geoenviron 127(12):1027–1036 leading to the generation of shallow landslides on the Sakurai M (2014) The past landslides. In: Investigation report of Izu Oshima upper slopes. landslide caused by Typhoon No. 26 in October 2013. P 9-10 (In Japanese) Sakurai M, Disaster Research Team of Kanto Branch (2014) Landslide disaster of 2. Slope cutting for road construction on the soft-hard Izu-Oshima Island by typhoon No. 26 in 2013. Journal of the Japan Landslide slope structure (porous and shallow soil layer mant- Society 51(1):25–28, In Japanese ling the hard basalt layer) should be considered very Skempton AW (1954) The Pore-Pressure Coefficients A and B. Geotechnique 4(4):143–147 carefully. Without any preventive measures, the Tokyo District Meteorological Observatory (TDMO) (2013) Quick report about upper layer soil can easily slide along the interface Typhoon No. 26 in 2013. http://www.jma-net.go.jp/tokyo/sub_index/bosai/ between the soft and hard layers. disaster/ty1326/ty1326_tokyo.pdf. Accessed 4 February 2014 Ueno S (2014) Groundwater. In: Investigation report of Izu Oshima landslide 3. The drainage system along the road, especially in caused by Typhoon No. 26 in October 2013. P 19 (In Japanese) areas of low terrain, provides an environment which Wakai A, Uchimura T, Araki K, Inagaki H, Goto S (2014) Geotechnical characters of easily gathered runoff water. An amount of water soil. In: Investigation report of Izu Oshima landslide caused by Typhoon No. 26 in October 2013. P 35-39 (In Japanese) will continuously infiltrate into the slope below the road, leading to shallow landslides. From the above, it is very important to evaluate the threshold rainfall for shallow landslide occurrences on Izu Oshima Island. This will be useful to build early warning systems to protect the local people during heavy rainfall events. Competing interests The authors declare that they have no competing interests. Authors' contributions All authors participated the field investigations. YH conducted the triaxial tests and drafted the manuscript. All authors read and approved the final manuscript. Acknowledgements The authors would like to sincerely thank Dr. Barry Roser for his helpful and constructive comments to improve the manuscript. Valuable and constructive comments from anonymous reviewers are also appreciated. This work was financially supported by JSPS KAKENHI Grant Number A-2424106 for landslide dam failure prediction. Submit your manuscript to a journal and beneﬁ t from: Author details Department of Geoscience, Shimane University, Matsue 690-8504, Japan. 2 7 Convenient online submission School of Environmental Design, Kanazawa University, Kanazawa 920-1192, 7 Rigorous peer review Japan. 7 Immediate publication on acceptance Received: 23 October 2014 Accepted: 5 May 2015 7 Open access: articles freely available online 7 High visibility within the ﬁ eld 7 Retaining the copyright to your article References ASTM D2487-06 (2006) Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System). ASTM International, West Submit your next manuscript at 7 springeropen.com Conshohocken, PA, doi:10.1520/D2487-06
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