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Characteristics of tuff breccia-andesite in diverse mechanisms of landslides in Oita Prefecture, Kyushu, Japan

Characteristics of tuff breccia-andesite in diverse mechanisms of landslides in Oita Prefecture,... Heavy rainfall frequently occurred in Kyushu and triggers the landslides every year. This study observes the landslides which occurred in Oita Prefecture, Kyushu, Japan. The landslides in this study, consisting of the same soil materials, tuff breccia and andesite materials; however, the landslide mechanisms were different. Two landslides occurred caused by heavy rainfall in the different timing of the landslide occurrence, and another landslide occurred without the heavy rainfall or an earthquake occurs. Therefore, this study aims to analyse the physical and mechanical properties of tuff breccia and andesite materials with diverse landslide mechanisms. This study performed soil stratigraphic analysis and soil hardness measurements in the field, and performed physical properties, saturated permeability, mechanical properties, and XRD tests in the laboratory. This study found that characteristics of tuff breccia and andesite in diverse mechanisms of landslides were not very different, especially on the landslides caused by heavy rainfall. Furthermore, the landslide in the andesite and tuff breccia areas could be divided into three types based on the timing of the landslide occurrence, scale of the landslide, and landslide mechanisms. Keywords: Andesite, Landslide mechanisms, Mechanical properties, Physical properties, Saturated permeability, Tuff breccia Introduction landslide also consists of river deposits from the Holocene, Heavy rainfall frequently occurred in Kyushu and triggers Quaternary period (Geology Survey of Japan 2020). the landslides every year. The rainfall lead to the landslide However, the landslide mechanisms were different. The because a lot of factors such as groundwater and moisture landslides in Ohtsuru and Ono, Hita City, were influenced content could reduce the strength (Li et al. 2015). This by heavy rainfall whereas the landslide in Yabakei, study observes the landslides which occurred in Oita Nakatsu City, was not influenced by earthquakes or rain- Prefecture, Kyushu, Japan. The landslides in this study fall. The discontinuous layer of permeability in tuff breccia consisted of the same materials. Figure 1 shows that the layer was influenced the landslide in Yabakei (Kubota landslides in this study consisting of the same geological et al. 2018). Noviyanto et al. (2020)also reported that formation, igneous rock formation: andesite, basaltic variations in volcanic rock and soil characteristics with andesite lava, and pyroclastic rocks from the Neogene clay-rich content can potentially form the sliding layer in period (Geology Survey of Japan 2020). The Yabakei the discontinuity layer. The characteristics of tuff breccia and andesite at diverse landslide mechanisms in Oita Prefecture has not been studied in detail. Istiyanti et al. (2020) studied the physical * Correspondence: goto@yamanashi.ac.jp properties of tephra layers and reported that the kuroboku Faculty of Engineering, Graduate Faculty of Interdisciplinary Research, and scoria layers have different physical properties of soil. University of Yamanashi, Kofu, Japan Full list of author information is available at the end of the article © The Author(s). 2021 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. ISTIYANTI et al. Geoenvironmental Disasters (2021) 8:4 Page 2 of 14 Fig. 1 Location of the landslides and geological map in this study (Geology Survey of Japan 2020) The kuroboku layers have higher fine fraction content and exposed in the lower collapse and weathered tuff breccia plasticity index than the scoria layers, furthermore, sliding was exposed in the upper collapse (Fig. 5). The layer was located at N3–4 kuroboku layer. Therefore, this observation was performed in the upper collapse, and study aims to analyse the physical and mechanical the soil sample was taken from this location. Ono land- properties of tuff breccia and andesite materials with slide was occurred probably because the permeability be- diverse landslide mechanisms. haviour in thick collapsed of soil layer which has a depth around 20 to 30 m (Ochiai et al. 2017). Ochiai et al. Research area (2017) also reported that many cracks have occurred in Three landslides that occurred in Oita Prefecture, two the upper part of the landslide and some of them are landslides in Hita City, and another landslide in Nakatsu considered to be old cracks and cliffs. Furthermore, City were selected as the research area. The landslides in Chigira et al. (2018) reported that the sliding layer was Hita City are named Ohtsuru and Ono landslides, and formed in the exposed layer, the reddish-brown layer. the distance between these landslides was around 3300 This reddish-brown layer is clayey tuff breccia and the m. Ohtsuru landslide occurred near Yanase, Hita City, clay probably forms on this layer and make the imper- and the Ono landslide occurred near the Ono River, meable zone in this layer; the upper layer (andesite lava) Hita City in Joguyama Mountain (645 m). Another is saturated and the water pressure increases near the landslide, called the Yabakei landslide, occurred in the boundary (Chigira et al. 2018). Figure 6 shows the Yabakei area, Nakatsu City. topographic map and cross-section of the Ono landslide. On 5th July 2017, a wide area in Hita City was The evidence of landslide activity in the past was also affected by heavy rainfall (Fig. 2 (a)). The Ohtsuru observed in the Ono landslide at the previous studies landslide occurred during the peak rainfall on 5th (Ochiai et al. (2017), Chigira et al. (2018). April 2017 with the total rainfall on that day was 336 The Yabakei landslide was a large-scale landslide (Fig. 7) mm (Japan Meteorological Agency 2020). The that occurred on 11th April 2018 without the occurrence Ohtsuru landslide was a shallow landslide and has a of heavy rainfall or an earthquake. Figure 2 (b) showed narrow slope (Fig. 3). Other landslides occurred that heavy rainfall was not occurred from the March, one surrounding the Ohtsuru landslide, on the opposite month before the landslide, in Yabakei area. The Yabakei side of Ohtsuru landslide, and beside the Ohtsuru area repeatedly suffered from landslides, and the landslide (Fig. 4). These landslides were also shallow landslides always occurred in the same location at Yabakei and have the narrow slope. area (Yabakei landslide investigation committee 2018). Large-scale landslides in Ono occurred the day after The smectification process was observed on the sliding the peak rainfall on 6th April 2017 with the total rainfall layer at highly weathered tuff breccia layer in the Yabakei of 66.5 mm (Japan Meteorological Agency 2020). Two landslide (Fig. 8) (Yabakei landslide investigation collapses occurred in the Ono landslide; andesite was committee 2018), and this study observed that sliding ISTIYANTI et al. Geoenvironmental Disasters (2021) 8:4 Page 3 of 14 Fig. 2 Precipitation data on research area; (a) Hita City and (b) Nakatsu City (Japan Meteorological Agency 2020) layer. In addition, Kubota et al. (2018)reportedthatthe Ohtsuru landslide, the Ono landslide was divided into 3 major cause of the Yabakei landslide was the weathered sub-layers: weathered andesite, weathered tuff breccia, and clay layer formed from the tuff breccia by a mineral- tuff breccia. Unfortunately, because of the condition of the groundwater reaction, and groundwater was gushed out field which difficult to exposed the soil layers, the depth of from approximately 20 m above sea level in the landslide the soil layers in the Ono landslide could not be identified. with the surrounding bedrock was significantly altered The soil layers in the Yabakei landslide also could not be and clayey. exposed due to unfavourable conditions. Moreover, a depth of 1 m on the highly weathered tuff breccia was Research methods observed (Istiyanti and Goto 2020). Field measurements Soil hardness was measured using a Yamanaka-type Soil stratigraphic analysis and soil hardness measurements soil hardness meter, which measures the soil hardness were performed in the field. The stratigraphy analyses of by inserting the device into the exposed soil layer at the each landslide were performed by scraping the surface to site. Sasahara et al. (1995) reported that there is a strong expose the soil layers, as shown in Fig. 9. The main soil positive correlation between the soil hardness and the layer in the Ohtsuru landslide was divided into 3 sub- shear strength parameters of soil in the Mohr–Coulomb layers from the surface to a depth of 6 m. Ohtsuru failure criterion. Furthermore, Tokunaga and Goto landslides consist of topsoil, highly weathered andesite, (2017) found a discontinuity in the strength of pyroclastic weathered andesite, and tuff breccia. Similar to the materials in Aso volcanic mountains using a soil hardness ISTIYANTI et al. Geoenvironmental Disasters (2021) 8:4 Page 4 of 14 Fig. 3 Condition of Ohtsuru landslide (Photos taken on July 2018) tester. In this study, the soil hardness values of each sub- characteristics of tuff breccia and andesite. The tests for layer at Ohtsuru and Ono, and highly weathered tuff physical and mechanical properties were performed accord- breccia at the Yabakei landslide were measured. The ing to the laboratory testing standards of Geomaterials Vol. undisturbed samples were collected from the centre of 1 (The Japanese Geotechnical Society 2015). Moreover, the each soil layer to observe the characteristics of the tuff test for saturated permeability properties was performed breccia and andesite in laboratory tests. according to the methods for the permeability of saturated soils by Daiki (Daiki n.d.) using undisturbed samples. Laboratory tests Physical property tests of the soil performed in this Tests for physical properties, saturated permeability, and study include the particle size distribution, liquid limit, mechanical properties were performed to observe the and plastic limit tests. A particle size distribution test Fig. 4 Topographic map (Geospatial Information Authority of Japan 2020) with cross-section and photos of Ohtsuru landslide (taken on July 2017) ISTIYANTI et al. Geoenvironmental Disasters (2021) 8:4 Page 5 of 14 Fig. 5 Condition of Ono landslide (Photos taken on July 2017) was performed to observe the relation between particle Geotechnical Society 2015). Furthermore, the mechan- size (mm) and mass percentage passing (%) (JIS A1204 ical properties were determined by the consolidated 2009 cited in The Japanese Geotechnical Society 2015). constant-pressure direct box shear test on soils (JGS The water content of the soil under different conditions, 0561 2009 cited in The Japanese Geotechnical Society transition from plastic to the liquid state (liquid limit), 2015) using undisturbed samples. This study also and transition from plastic to semi-solid state (plastic performed X-ray diffraction (XRD) tests to observe the limit) can be determined by the liquid limit and plastic clay minerals on tuff breccia and andesite; it was per- limit tests (JIS A1205 2009 cited in The Japanese formed according to the randomly oriented powder Fig. 6 Topographic map (Geospatial Information Authority of Japan 2020) with cross-section (modified Chigira et al. 2018) (photos taken on July 2017) ISTIYANTI et al. Geoenvironmental Disasters (2021) 8:4 Page 6 of 14 Fig. 7 Condition of Yabakei landslide (Photos taken on July 2018) mounts, ethylene glycol treatment, and heat treatment the soil hardness was observed between the tuff methods from U. S. Geological Survey (US Geological Survey breccia and the upper layer in the Ohtsuru and Ono 2020). landslides. The soil hardness in the Ohtsuru landslide shows that the tuff breccia has the highest soil hardness value, and the weathered andesite has the Results lowest soil hardness value. Furthermore, the soil Soil hardness measurements hardness in the Ono landslide also shows that tuff Figure 9 shows the soil hardness measurements breccia has the highest soil hardness value, and the during the field observation. An obvious difference in weathered tuff breccia has the lowest soil hardness Fig. 8 Topographic map (Geospatial Information Authority of Japan 2020) with cross-section (modified Yabakei landslide investigation committee 2018), photo of slope from Asia Air Survey (2018), and photo of soil taken on July 2018 ISTIYANTI et al. Geoenvironmental Disasters (2021) 8:4 Page 7 of 14 Fig. 9 Soil stratigraphy on the field, soil hardness, and saturated permeability properties values in (a) Ohtsuru landslide, (b) Ono landslide, (c) Yabakei landslide Fig. 10 Particle size distribution curves of soil materials ISTIYANTI et al. Geoenvironmental Disasters (2021) 8:4 Page 8 of 14 value (Istiyanti and Goto 2020). The soil hardness Table 1 Activity values in soil materials (Skempton 1953) value in the Yabakei landslide was measured 1 m from Classes of activity Activity value the surface on the highly weathered tuff breccia layer. Inactive < 0.5 We could not observe other soil layers in the Yabakei Inactive 0.5–0.75 landslide. Moreover, the highly weathered tuff breccia Normal 0.75–1.25 from the Yabakei landslide has a lower soil hardness Active 1.25–2 value than the tuff breccia from Ohtsuru and Ono Active > 2 landslides. Physical properties of tuff breccia and andesite landslide; on the weathered andesite and weathered tuff The density of soil particles (Fig. 9) showed that the soil breccia from the Ono landslide; and on the highly materials from the Ohtsuru and Ono landslides have no weathered tuff breccia from the Yabakei landslide. The dissimilarities; however, highly weathered tuff breccia coefficient of saturated permeability, void ratio, and dry from the Yabakei landslide has a low density of soil density from the test for the permeability of saturated particles. The particle size distribution curve (Fig. 10) soils showed no dissimilarities among soil materials. also shows no dissimilarities among the soil materials, Figure 12 (a) and (b) show the dissimilarities between and all the soil materials indicated a well-graded soil the tuff breccia and andesite materials. Figure 12 (a) material. The particle size distribution curve for tuff shows tuff breccia from the three landslides has the breccia in the Yabakei landslide has the highest fine same range of coefficient of saturated permeability. The fraction content. Figure 11 shows the plasticity chart for void ratio differed according to the weathering process, classifying the soil materials using the liquid limit and and highly weathered tuff breccia has a lower void ratio plastic limit test results, which are separated among the than tuff breccia. Furthermore, Fig. 12 (b) shows sampling locations denoted by different colours, and slid- andesite materials from the Ohtsuru and Ono landslides ing layers on each area are denoted by white circles with have different coefficients of saturated permeability and differently coloured outlines. The plotted data on plasticity void ratios. Andesite materials from the Ohtsuru land- chart showed that soil materials in the Ohtsuru landslide slide have a lower coefficient of saturated permeability are inorganic silts of medium compressibility and organic and a higher void ratio than those from the Ono land- silts. Furthermore, the plotted data of the Ono and slide. Different void ratios based on the weathering Yabakei landslides are inorganic silts of high process are also observed in andesite materials from the compressibility and organic clays. Ohtsuru landslide; highly weathered andesite has a lower The ratio of the plasticity index to the clay size fraction void ratio than weathered andesite. (percentage by weight of particles finer than 2 μm), termed the activity (Skempton 1953). Skempton (1953)also Mechanical properties of tuff breccia and andesite suggested three classes of activity: active, normal, and Tests for mechanical properties were performed on the inactive, and subdivided them into five groups (Table 1). highly weathered andesite from the Ohtsuru landslide, The greater the activity, the more important the influence weathered andesite from the Ono landslide, and highly of the clay fraction on the properties, and the more weathered tuff breccia from the Yabakei landslide. Two susceptible their values are to changes in factors such as different colours were found on the highly weathered thetypeof exchangeablecations and pore fluid composition tuff breccia from the Yabakei landslide, and the direct (Mitchell and Soga 2005). The activity values are presented shear test was performed on the red and brown samples in Table 2 and the activity in soil materials from the Ohtsuru of this layer. Direct shear test for the highly weathered and Ono landslides have values of approximately 1–2. andesite from the Ohtsuru landslide was performed at Moreover, the activity value in the highly weathered tuff effective stress of 10–30 kPa. Direct shear test for the breccia from the Yabakei landslide is greater than 3. The soil weathered andesite from the Ono landslide was materials analysed in this study were active soil; however, the performed at effective stress of 10–40 kPa, and that for weathered andesite from the Ohtsurulandslideand thetuff the highly weathered tuff breccia from the Yabakei breccia from the Ono landslide were normal active soil. landslide at effective stress of 100–300 kPa. The relation- ship between the vertical displacement and shear Saturated permeability properties of tuff breccia and displacement in Fig. 13 (b) shows that the weathered andesite andesite in the Ono landslide has a dilative behaviour. Figure 9 shows the saturated permeability properties of Furthermore, the relationship between the maximum the soil materials. This study measured the coefficient of shear stress and vertical stress in soil materials is shown saturated permeability on the highly weathered andesite, in Fig. 14. Unfortunately, the relationship between the weathered andesite, and tuff breccia from the Ohtsuru maximum shear stress and vertical stress in highly ISTIYANTI et al. Geoenvironmental Disasters (2021) 8:4 Page 9 of 14 Fig. 11 Plotted data of soil materials on plasticity chart weathered andesite from the Ohtsuru landslide could consists of halloysite, sepiolite, montmorillonite, and not be used in this study because of the dispersed test dickite. The layer bed in the Ono landslide, i.e. tuff results. breccia, consists of halloysite, sepiolite, and montmorillonite. Moreover, highly weathered tuff breccia from the Yabakei Clay minerals in tuff breccia and andesite landslide consists of illite, chlorite, and montmorillonite A laboratory manual for XRD by the US Geological (Fig. 15 (c)). Survey (2020) divided the clay minerals into seven Generally, the soil materials in this study consist of groups: chlorite, illite, kaolinite, mixed-layer clays, the smectite group minerals in each layer. Mitchell smectite, sepiolite, palygorskite, and vermiculite. and Soga (2005) reported that the minerals of the Figure 15 (a) shows that the clay minerals in highly smectite group have a prototype structure similar to weathered andesite and weathered andesite from the that of pyrophyllite, consisting of an octahedral sheet Ohtsuru landslide consists of halloysite, illite, vermiculite, sandwiched between two silica sheets. Water adsorbed and montmorillonite. Furthermore, tuff breccia consists of onto the smectite between the unit layers and swell, halloysite, vermiculite, and montmorillonite. The content and the smectite minerals are the dominant source of of clay minerals on soil materials from Ohtsuru landslide swelling in the expansive soils (Mitchell and Soga does not shows the dissimilarity. 2005). In addition, sliding layer in the landslides In the Ono landslide (Fig. 15 (b)), the weathered consist of the montmorillonite which is the expansive andesite consists of sepiolite. Weathered tuff breccia character of the group of smectite. Table 2 Activity values in soil materials Location of landslide Soil materials Activity Type of soil Ohtsuru landslide Highly weathered andesite 1.46 Active Weathered andesite 1.19 Normal Tuff breccia 1.46 Active Ono landslide Weathered andesite 1.62 Active Weathered tuff breccia 1.39 Active Tuff breccia 1.11 Normal Yabakei landslide Highly weathered tuff breccia 3.82 Active ISTIYANTI et al. Geoenvironmental Disasters (2021) 8:4 Page 10 of 14 Fig. 12 Correlation between coefficient of permeability (m/s) and void ratio on (a) tuff breccia materials and (b) andesite materials Discussion behaviour. Unfortunately, the strength behaviour by the Ohtsuru landslide direct box shear test on weathered andesite and tuff breccia The sliding layer was located at the boundary between the were not observed. Further study is needed to observe the weathered andesite and tuff breccia, approximately 5 m from strength behaviour by the direct box shear test in the the surface in the Ohtsuru landslide. An obvious difference Ohtsuru landslide. in the soil hardness was observed between the weathered Many shallow landslides occurred in the Ohtsuru area. andesite and tuff breccia and showed the different strength The landslides occurred due to heavy rainfall and on that layers. However, no dissimilarity was observed in the probably occurred during the peak rainfall. The mechanism physical and saturated permeability properties. The upper of the Ohtsuru landslide may be influenced by the strength layer, highly weathered andesite, exhibits a contractive behaviour. The different strengths may be caused by the ISTIYANTI et al. Geoenvironmental Disasters (2021) 8:4 Page 11 of 14 Fig. 13 Shear stress-shear displacement and vertical displacement-shear displacement behaviour in soil materials in (a) Ohtsuru landslide, (b) Ono landslide, (c) Yabakei landslide Fig. 14 Relation between maximum shear stress and vertical stress in soil materials in (b) Ono landslide and (c) Yabakei landslide ISTIYANTI et al. Geoenvironmental Disasters (2021) 8:4 Page 12 of 14 Fig. 15 X-ray diffraction on soil materials in (a) Ohtsuru landslide, (b) Ono landslide, and (c) Yabakei landslide weathering process. The mechanism of other landslides weathered andesite, exhibits a dilative behaviour. near the Ohtsuru landslide could also be influenced by the Unfortunately, the permeability behaviour of tuff same reason. Moreover, even though this study did not find breccia and the strength behaviour by the direct box any dissimilarities in permeability behaviour, this landslide shear test on the weathered tuff breccia and tuff occurred during the peak rainfall and is probably related to breccia were not observed. Further studies are permeability behaviour. required to observe the permeability and strength behaviour in the Ono landslide. Ono landslide The timing of occurrence of the Ono landslide, which The sliding layer was located at a depth of around 30 m occurred the day after the peak rainfall, could be from the surface (Chigira et al. 2018) at the boundary influenced by permeability behaviour and the scale of between the weathered tuff breccia and tuff breccia, in the landslide. Furthermore, the landslide activity in the the Ono landslide. An obvious difference in the soil past could also influence this occurrence. hardness was also observed between the weathered tuff breccia and tuff breccia, and showed the different Yabakei landslide strength on that layers. However, no dissimilarity Compared with the tuff breccia from the Ohtsuru and was found in physical properties. The upper layer, Ono landslides, the highly weathered tuff breccia from ISTIYANTI et al. Geoenvironmental Disasters (2021) 8:4 Page 13 of 14 the Yabakei landslide has low strength and could be Conclusions caused by the weathering process in this layer. The fine Sediment disasters have frequently occurred in Oita fraction content, plasticity behaviour, and activity value Prefecture in 2017 and 2018. The characteristics of tuff in this layer were also higher than those of tuff breccia breccia and andesite at diverse landslide mechanisms in from the Ohtsuru and Ono landslides. Oita Prefecture has not been studied in detail, in For the soil permeability in the highly weathered tuff addition, the landslides which observed on this study breccia from the Yabakei landslide, the permeability consisted of the same soil materials. Therefore, observations values of this study were higher than expected from field of the characteristics of tuff breccia and andesite in diverse observations. The soil materials from the field observa- landslide mechanisms in Oita Prefecture are of interest. This tion were more clayey and should have had a low per- study performed soil stratigraphic analysis and soil hardness meability. However, our results show that the highly measurements in the field,and performed physical weathered tuff breccia from the Yabakei landslide has a properties, saturated permeability, mechanical properties, medium permeability, which could be caused by the and XRD tests in the laboratory. landslide activity in the past, forming the complex layers. Three landslides that occurred in Oita Prefecture were The strength behaviour of this layer exhibits a contract- observedinthisstudy.Ohtsuru andOno landslides ive behaviour. occurred caused by heavy rainfall on 5th July 2017 in a wide The weathering process and landslide activity in the area in Hita City. Ohtsuru landslide occurred during the past could be the reason for this landslide. Landslide peak rainfall on 5th July 2017, and Ono landslide occurred activity in the past caused higher weathering compared in the next day, on 6th July 2017. Furthermore, different to the Ohtsuru and Ono landslides, due to which the landslide mechanisms triggered the large-scale landslides in fine fraction content and activity value increased and the Yabakei, which occurred on 11th April 2018. strength of the soil decreased. Characteristics of tuff breccia and andesite; physical properties, saturated permeability properties, mechanical properties, and the content of clay minerals in diverse Types of landslide in the andesite and tuff breccia areas mechanisms of landslides on this study were not very Fan et al. (2017) classified the landslide dams associated with different, especially for the soil materials from Ohtsuru the earthquake in three types based on the composition and Ono landslides. However, weathering process in material and sedimentological. Furthermore, this study also Yabakei landslides formed the highly weathered tuff found that landslides in the andesite and tuff breccia areas breccia which has high fine fraction content, plasticity could be divided into three types based on the timing of the index, activity value and low soil strength. landslide occurrence, scale of the landslide, and landslide According to the results, this study found that mechanisms. landslides in the andesite and tuff breccia areas are of three types. The first type was a landslide with a narrow 1) The landslide which occurred during the peak slope that occurred during the peak rainfall, such as the rainfall and it has the narrow slope. Generally, this Ohtsuru landslide. This type of landslide was found type of landslide was found in the one area. This frequently in the one area. The second type was the study also found the other three landslides in large-scale landslide that occurred the day after the peak Ohtsuru area which have the same type with of heavy rainfall, such as the Ono landslide. The scale of Ohtsuru landslide. the landslide and landslide activity in the past delayed 2) The large-scale landslide that occurred the day after the timing of the landslide occurrence. Furthermore, the the peak of heavy rainfall, such as the Ono third type was a large-scale landslide that occurred landslide. The landslide activity in the past formed without the occurrence of heavy rainfall or an the low permeability zone and the infiltration of earthquake, such as the Yabakei landslide. rainfall to the large-scale of slope was taking a several time. Therefore, the scale of the landslide Acknowledgements and landslide activity in the past delayed the timing The field observations in this study were made possible with the of the landslide occurrence. participation and assistance from the forestry department of Oita Prefecture and of our laboratory member, Tomohiro Hanaki. 3) The large-scale landslide that occurred without the occurrence of heavy rainfall or an earthquake, such as Yabakei landslide. The landslide activity in the Authors’ contributions MLI, SG, and HO visited the landslide sites and selected a location to expose past, which repeatedly occurred in Yabakei, made the soil materials, collected a sample, and conducted a field investigation to the groundwater gushed out. Furthermore, the measure soil hardness. MLI and SG analysed and interpreted the soil weathering process in the soil high and the strength behaviour regarding the materials at the landslide sites. The authors read and approved the final manuscript. of the soil decreased. ISTIYANTI et al. Geoenvironmental Disasters (2021) 8:4 Page 14 of 14 Funding Tokunaga S, Goto S (2017) Study on measurement of strength discontinuity by Part of this study was carried out with the help of the Japan Society for the digitized Yamanaka soil hardness tester for shallow landslide site at Aso Promotion of Science Grants-in-Aid for Scientific Research Grant-in-Aid for volcano. Paper presented at the 52nd. Jpn Soc Geotech Nagoya. July 2017, Scientific Research (B) 17H03303. pp. 1935–1936. (in Japanese) US Geological Survey. A laboratory manual for X-ray powder diffraction. https:// pubs.usgs.gov/of/2001/of01-041/index.htm. Accessed July 2020 Availability of data and materials Yabakei landslide investigation committee (2018) Final summary report about The datasets used and/or analysed during the study are available from the landslide in Yabakei-cho, Nakatsu city that occurred in April 2018. http:// corresponding author upon reasonable request. www.pref.oita.jp/uploaded/life/2045589_2349104_misc.pdf. Accessed July 2020 (in Japanese) Competing interests The authors declare that they have no competing interests. Publisher’sNote Author details Springer Nature remains neutral with regard to jurisdictional claims in Civil Management and Engineering Major, Environmental and Social System published maps and institutional affiliations. Science Course, Integrated Graduate School of Medicine, Engineering, and Agricultural Sciences, University of Yamanashi, Kofu, Japan. Faculty of Engineering, Graduate Faculty of Interdisciplinary Research, University of Yamanashi, Kofu, Japan. Japan Forest Technology Association, Tokyo, Japan. Received: 15 October 2020 Accepted: 6 January 2021 References Asia Air Survey, Co., Ltd (2018) https://www.ajiko.co.jp/news_detail/?id=12850. Accessed July 2020 Chigira M, Ling S, Matsushi Y (2018) Landslide disaster induced by the 2017 northern Kyushu rainstorm. Disast Prevent Res Institut Ann 61:28–35. (in Japanese with English abstract) Daiki (DIK-4012) (n.d) Manual for soil permeability analysis using an instrument with 4 points Daiki Rika Kogyo Co., Ltd. (in Japanese) Fan X, Xu Q, van Westen CJ, Huang R, Tan R (2017) Characteristics and classification of landslide dams associated with the 2008 Wenchuan earthquake. Geoenviron Disast 4:12 https://doi.org/10.1186/s40677-017-0079-8 Geology Survey of Japan https://gbank.gsj.jp/geonavi/geonavi.php. Accessed July Geospatial Information Authority of Japan. https://maps.gsi.go.jp/. Accessed September 2020 Istiyanti ML, Goto S (2020) differences of physical properties between tephra and tuff breccia-andesite on sliding layers and surrounding layers of landslides proceedings of the 10th symposium on sediment-related disasters, Japan, Istiyanti ML, Goto S, Kimura T, Sato G, Hayashi S, Wakai A, Higaki D (2020) Sliding layer estimation of shallow landslides on Aso volcanic mountains in Japan based on tephra layer-physical properties of soil. Geoenviron Disast 7:28 https://doi.org/10.1186/s40677-020-00163-x Japan Meteorological Agency. www.data.jma.go.jp/obd.stats.etrn/. Accessed August 2020 Kubota T, Jitousono T, Nagai Y, Shimizu O, Mizuno H, Nomura Y, Suzuki Y, Yamakoshi T, Koi T, Ohishi H, Hirakawa Y (2018) Overview of the Yabakei landslide occurred on April 11th 2018 in Nakatsu city, Ōita prefecture, Japan. J Jpn Soc Erosion Control Eng 71:34–41. (in Japanese with English abstract) Li P, Zhang X, Shi H (2015) Investigation for the initiation of a loess landslide based on the unsaturated permeability and strength theory. Geoenviron Disast 2:24 https://doi.org/10.1186/s40677-015-0032-7 Mitchell JK, Soga K (2005) Fundamentals of soil behavior, John, 3rd ed. Wiley. United States and America, pp 56–59 and 97 Noviyanto A, Sartohadi J, Purwanto BH (2020) The distribution of soil morphological characteristics for landslide-impacted Sumbing volcano, Central Java – Indonesia. Geoenviron Disast 7:25 https://doi.org/10.1186/ s40677-020-00158-8 Ochiai H, Sakurai M, Higaki D, Goto S, Wakai A, Fukuoka H, Nakamura S, Kimura S (2017) Landslide disasters induced by the northern Kyushu heavy rainfall in July 2017. J Jpn Landslide Soc 54:225–228. (in Japanese) Sasahara K, Ebihara K, Tsunaki R (1995) Evaluation of the Shirasu slope stability using soil hardness values. J Jpn Soc Erosion Control Eng 48:20–24. (in Japanese with English abstract) Skempton AW (1953) The colloidal activity of clay proceedings of the third international conference on soil mechanics and foundation engineering. Zurich 1:57–61 The Japanese Geotechnical Society (2015) Japanese Geotechnical Society Standards-Laboratory testing standards of geomaterials, 1 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Geoenvironmental Disasters Springer Journals

Characteristics of tuff breccia-andesite in diverse mechanisms of landslides in Oita Prefecture, Kyushu, Japan

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Abstract

Heavy rainfall frequently occurred in Kyushu and triggers the landslides every year. This study observes the landslides which occurred in Oita Prefecture, Kyushu, Japan. The landslides in this study, consisting of the same soil materials, tuff breccia and andesite materials; however, the landslide mechanisms were different. Two landslides occurred caused by heavy rainfall in the different timing of the landslide occurrence, and another landslide occurred without the heavy rainfall or an earthquake occurs. Therefore, this study aims to analyse the physical and mechanical properties of tuff breccia and andesite materials with diverse landslide mechanisms. This study performed soil stratigraphic analysis and soil hardness measurements in the field, and performed physical properties, saturated permeability, mechanical properties, and XRD tests in the laboratory. This study found that characteristics of tuff breccia and andesite in diverse mechanisms of landslides were not very different, especially on the landslides caused by heavy rainfall. Furthermore, the landslide in the andesite and tuff breccia areas could be divided into three types based on the timing of the landslide occurrence, scale of the landslide, and landslide mechanisms. Keywords: Andesite, Landslide mechanisms, Mechanical properties, Physical properties, Saturated permeability, Tuff breccia Introduction landslide also consists of river deposits from the Holocene, Heavy rainfall frequently occurred in Kyushu and triggers Quaternary period (Geology Survey of Japan 2020). the landslides every year. The rainfall lead to the landslide However, the landslide mechanisms were different. The because a lot of factors such as groundwater and moisture landslides in Ohtsuru and Ono, Hita City, were influenced content could reduce the strength (Li et al. 2015). This by heavy rainfall whereas the landslide in Yabakei, study observes the landslides which occurred in Oita Nakatsu City, was not influenced by earthquakes or rain- Prefecture, Kyushu, Japan. The landslides in this study fall. The discontinuous layer of permeability in tuff breccia consisted of the same materials. Figure 1 shows that the layer was influenced the landslide in Yabakei (Kubota landslides in this study consisting of the same geological et al. 2018). Noviyanto et al. (2020)also reported that formation, igneous rock formation: andesite, basaltic variations in volcanic rock and soil characteristics with andesite lava, and pyroclastic rocks from the Neogene clay-rich content can potentially form the sliding layer in period (Geology Survey of Japan 2020). The Yabakei the discontinuity layer. The characteristics of tuff breccia and andesite at diverse landslide mechanisms in Oita Prefecture has not been studied in detail. Istiyanti et al. (2020) studied the physical * Correspondence: goto@yamanashi.ac.jp properties of tephra layers and reported that the kuroboku Faculty of Engineering, Graduate Faculty of Interdisciplinary Research, and scoria layers have different physical properties of soil. University of Yamanashi, Kofu, Japan Full list of author information is available at the end of the article © The Author(s). 2021 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. ISTIYANTI et al. Geoenvironmental Disasters (2021) 8:4 Page 2 of 14 Fig. 1 Location of the landslides and geological map in this study (Geology Survey of Japan 2020) The kuroboku layers have higher fine fraction content and exposed in the lower collapse and weathered tuff breccia plasticity index than the scoria layers, furthermore, sliding was exposed in the upper collapse (Fig. 5). The layer was located at N3–4 kuroboku layer. Therefore, this observation was performed in the upper collapse, and study aims to analyse the physical and mechanical the soil sample was taken from this location. Ono land- properties of tuff breccia and andesite materials with slide was occurred probably because the permeability be- diverse landslide mechanisms. haviour in thick collapsed of soil layer which has a depth around 20 to 30 m (Ochiai et al. 2017). Ochiai et al. Research area (2017) also reported that many cracks have occurred in Three landslides that occurred in Oita Prefecture, two the upper part of the landslide and some of them are landslides in Hita City, and another landslide in Nakatsu considered to be old cracks and cliffs. Furthermore, City were selected as the research area. The landslides in Chigira et al. (2018) reported that the sliding layer was Hita City are named Ohtsuru and Ono landslides, and formed in the exposed layer, the reddish-brown layer. the distance between these landslides was around 3300 This reddish-brown layer is clayey tuff breccia and the m. Ohtsuru landslide occurred near Yanase, Hita City, clay probably forms on this layer and make the imper- and the Ono landslide occurred near the Ono River, meable zone in this layer; the upper layer (andesite lava) Hita City in Joguyama Mountain (645 m). Another is saturated and the water pressure increases near the landslide, called the Yabakei landslide, occurred in the boundary (Chigira et al. 2018). Figure 6 shows the Yabakei area, Nakatsu City. topographic map and cross-section of the Ono landslide. On 5th July 2017, a wide area in Hita City was The evidence of landslide activity in the past was also affected by heavy rainfall (Fig. 2 (a)). The Ohtsuru observed in the Ono landslide at the previous studies landslide occurred during the peak rainfall on 5th (Ochiai et al. (2017), Chigira et al. (2018). April 2017 with the total rainfall on that day was 336 The Yabakei landslide was a large-scale landslide (Fig. 7) mm (Japan Meteorological Agency 2020). The that occurred on 11th April 2018 without the occurrence Ohtsuru landslide was a shallow landslide and has a of heavy rainfall or an earthquake. Figure 2 (b) showed narrow slope (Fig. 3). Other landslides occurred that heavy rainfall was not occurred from the March, one surrounding the Ohtsuru landslide, on the opposite month before the landslide, in Yabakei area. The Yabakei side of Ohtsuru landslide, and beside the Ohtsuru area repeatedly suffered from landslides, and the landslide (Fig. 4). These landslides were also shallow landslides always occurred in the same location at Yabakei and have the narrow slope. area (Yabakei landslide investigation committee 2018). Large-scale landslides in Ono occurred the day after The smectification process was observed on the sliding the peak rainfall on 6th April 2017 with the total rainfall layer at highly weathered tuff breccia layer in the Yabakei of 66.5 mm (Japan Meteorological Agency 2020). Two landslide (Fig. 8) (Yabakei landslide investigation collapses occurred in the Ono landslide; andesite was committee 2018), and this study observed that sliding ISTIYANTI et al. Geoenvironmental Disasters (2021) 8:4 Page 3 of 14 Fig. 2 Precipitation data on research area; (a) Hita City and (b) Nakatsu City (Japan Meteorological Agency 2020) layer. In addition, Kubota et al. (2018)reportedthatthe Ohtsuru landslide, the Ono landslide was divided into 3 major cause of the Yabakei landslide was the weathered sub-layers: weathered andesite, weathered tuff breccia, and clay layer formed from the tuff breccia by a mineral- tuff breccia. Unfortunately, because of the condition of the groundwater reaction, and groundwater was gushed out field which difficult to exposed the soil layers, the depth of from approximately 20 m above sea level in the landslide the soil layers in the Ono landslide could not be identified. with the surrounding bedrock was significantly altered The soil layers in the Yabakei landslide also could not be and clayey. exposed due to unfavourable conditions. Moreover, a depth of 1 m on the highly weathered tuff breccia was Research methods observed (Istiyanti and Goto 2020). Field measurements Soil hardness was measured using a Yamanaka-type Soil stratigraphic analysis and soil hardness measurements soil hardness meter, which measures the soil hardness were performed in the field. The stratigraphy analyses of by inserting the device into the exposed soil layer at the each landslide were performed by scraping the surface to site. Sasahara et al. (1995) reported that there is a strong expose the soil layers, as shown in Fig. 9. The main soil positive correlation between the soil hardness and the layer in the Ohtsuru landslide was divided into 3 sub- shear strength parameters of soil in the Mohr–Coulomb layers from the surface to a depth of 6 m. Ohtsuru failure criterion. Furthermore, Tokunaga and Goto landslides consist of topsoil, highly weathered andesite, (2017) found a discontinuity in the strength of pyroclastic weathered andesite, and tuff breccia. Similar to the materials in Aso volcanic mountains using a soil hardness ISTIYANTI et al. Geoenvironmental Disasters (2021) 8:4 Page 4 of 14 Fig. 3 Condition of Ohtsuru landslide (Photos taken on July 2018) tester. In this study, the soil hardness values of each sub- characteristics of tuff breccia and andesite. The tests for layer at Ohtsuru and Ono, and highly weathered tuff physical and mechanical properties were performed accord- breccia at the Yabakei landslide were measured. The ing to the laboratory testing standards of Geomaterials Vol. undisturbed samples were collected from the centre of 1 (The Japanese Geotechnical Society 2015). Moreover, the each soil layer to observe the characteristics of the tuff test for saturated permeability properties was performed breccia and andesite in laboratory tests. according to the methods for the permeability of saturated soils by Daiki (Daiki n.d.) using undisturbed samples. Laboratory tests Physical property tests of the soil performed in this Tests for physical properties, saturated permeability, and study include the particle size distribution, liquid limit, mechanical properties were performed to observe the and plastic limit tests. A particle size distribution test Fig. 4 Topographic map (Geospatial Information Authority of Japan 2020) with cross-section and photos of Ohtsuru landslide (taken on July 2017) ISTIYANTI et al. Geoenvironmental Disasters (2021) 8:4 Page 5 of 14 Fig. 5 Condition of Ono landslide (Photos taken on July 2017) was performed to observe the relation between particle Geotechnical Society 2015). Furthermore, the mechan- size (mm) and mass percentage passing (%) (JIS A1204 ical properties were determined by the consolidated 2009 cited in The Japanese Geotechnical Society 2015). constant-pressure direct box shear test on soils (JGS The water content of the soil under different conditions, 0561 2009 cited in The Japanese Geotechnical Society transition from plastic to the liquid state (liquid limit), 2015) using undisturbed samples. This study also and transition from plastic to semi-solid state (plastic performed X-ray diffraction (XRD) tests to observe the limit) can be determined by the liquid limit and plastic clay minerals on tuff breccia and andesite; it was per- limit tests (JIS A1205 2009 cited in The Japanese formed according to the randomly oriented powder Fig. 6 Topographic map (Geospatial Information Authority of Japan 2020) with cross-section (modified Chigira et al. 2018) (photos taken on July 2017) ISTIYANTI et al. Geoenvironmental Disasters (2021) 8:4 Page 6 of 14 Fig. 7 Condition of Yabakei landslide (Photos taken on July 2018) mounts, ethylene glycol treatment, and heat treatment the soil hardness was observed between the tuff methods from U. S. Geological Survey (US Geological Survey breccia and the upper layer in the Ohtsuru and Ono 2020). landslides. The soil hardness in the Ohtsuru landslide shows that the tuff breccia has the highest soil hardness value, and the weathered andesite has the Results lowest soil hardness value. Furthermore, the soil Soil hardness measurements hardness in the Ono landslide also shows that tuff Figure 9 shows the soil hardness measurements breccia has the highest soil hardness value, and the during the field observation. An obvious difference in weathered tuff breccia has the lowest soil hardness Fig. 8 Topographic map (Geospatial Information Authority of Japan 2020) with cross-section (modified Yabakei landslide investigation committee 2018), photo of slope from Asia Air Survey (2018), and photo of soil taken on July 2018 ISTIYANTI et al. Geoenvironmental Disasters (2021) 8:4 Page 7 of 14 Fig. 9 Soil stratigraphy on the field, soil hardness, and saturated permeability properties values in (a) Ohtsuru landslide, (b) Ono landslide, (c) Yabakei landslide Fig. 10 Particle size distribution curves of soil materials ISTIYANTI et al. Geoenvironmental Disasters (2021) 8:4 Page 8 of 14 value (Istiyanti and Goto 2020). The soil hardness Table 1 Activity values in soil materials (Skempton 1953) value in the Yabakei landslide was measured 1 m from Classes of activity Activity value the surface on the highly weathered tuff breccia layer. Inactive < 0.5 We could not observe other soil layers in the Yabakei Inactive 0.5–0.75 landslide. Moreover, the highly weathered tuff breccia Normal 0.75–1.25 from the Yabakei landslide has a lower soil hardness Active 1.25–2 value than the tuff breccia from Ohtsuru and Ono Active > 2 landslides. Physical properties of tuff breccia and andesite landslide; on the weathered andesite and weathered tuff The density of soil particles (Fig. 9) showed that the soil breccia from the Ono landslide; and on the highly materials from the Ohtsuru and Ono landslides have no weathered tuff breccia from the Yabakei landslide. The dissimilarities; however, highly weathered tuff breccia coefficient of saturated permeability, void ratio, and dry from the Yabakei landslide has a low density of soil density from the test for the permeability of saturated particles. The particle size distribution curve (Fig. 10) soils showed no dissimilarities among soil materials. also shows no dissimilarities among the soil materials, Figure 12 (a) and (b) show the dissimilarities between and all the soil materials indicated a well-graded soil the tuff breccia and andesite materials. Figure 12 (a) material. The particle size distribution curve for tuff shows tuff breccia from the three landslides has the breccia in the Yabakei landslide has the highest fine same range of coefficient of saturated permeability. The fraction content. Figure 11 shows the plasticity chart for void ratio differed according to the weathering process, classifying the soil materials using the liquid limit and and highly weathered tuff breccia has a lower void ratio plastic limit test results, which are separated among the than tuff breccia. Furthermore, Fig. 12 (b) shows sampling locations denoted by different colours, and slid- andesite materials from the Ohtsuru and Ono landslides ing layers on each area are denoted by white circles with have different coefficients of saturated permeability and differently coloured outlines. The plotted data on plasticity void ratios. Andesite materials from the Ohtsuru land- chart showed that soil materials in the Ohtsuru landslide slide have a lower coefficient of saturated permeability are inorganic silts of medium compressibility and organic and a higher void ratio than those from the Ono land- silts. Furthermore, the plotted data of the Ono and slide. Different void ratios based on the weathering Yabakei landslides are inorganic silts of high process are also observed in andesite materials from the compressibility and organic clays. Ohtsuru landslide; highly weathered andesite has a lower The ratio of the plasticity index to the clay size fraction void ratio than weathered andesite. (percentage by weight of particles finer than 2 μm), termed the activity (Skempton 1953). Skempton (1953)also Mechanical properties of tuff breccia and andesite suggested three classes of activity: active, normal, and Tests for mechanical properties were performed on the inactive, and subdivided them into five groups (Table 1). highly weathered andesite from the Ohtsuru landslide, The greater the activity, the more important the influence weathered andesite from the Ono landslide, and highly of the clay fraction on the properties, and the more weathered tuff breccia from the Yabakei landslide. Two susceptible their values are to changes in factors such as different colours were found on the highly weathered thetypeof exchangeablecations and pore fluid composition tuff breccia from the Yabakei landslide, and the direct (Mitchell and Soga 2005). The activity values are presented shear test was performed on the red and brown samples in Table 2 and the activity in soil materials from the Ohtsuru of this layer. Direct shear test for the highly weathered and Ono landslides have values of approximately 1–2. andesite from the Ohtsuru landslide was performed at Moreover, the activity value in the highly weathered tuff effective stress of 10–30 kPa. Direct shear test for the breccia from the Yabakei landslide is greater than 3. The soil weathered andesite from the Ono landslide was materials analysed in this study were active soil; however, the performed at effective stress of 10–40 kPa, and that for weathered andesite from the Ohtsurulandslideand thetuff the highly weathered tuff breccia from the Yabakei breccia from the Ono landslide were normal active soil. landslide at effective stress of 100–300 kPa. The relation- ship between the vertical displacement and shear Saturated permeability properties of tuff breccia and displacement in Fig. 13 (b) shows that the weathered andesite andesite in the Ono landslide has a dilative behaviour. Figure 9 shows the saturated permeability properties of Furthermore, the relationship between the maximum the soil materials. This study measured the coefficient of shear stress and vertical stress in soil materials is shown saturated permeability on the highly weathered andesite, in Fig. 14. Unfortunately, the relationship between the weathered andesite, and tuff breccia from the Ohtsuru maximum shear stress and vertical stress in highly ISTIYANTI et al. Geoenvironmental Disasters (2021) 8:4 Page 9 of 14 Fig. 11 Plotted data of soil materials on plasticity chart weathered andesite from the Ohtsuru landslide could consists of halloysite, sepiolite, montmorillonite, and not be used in this study because of the dispersed test dickite. The layer bed in the Ono landslide, i.e. tuff results. breccia, consists of halloysite, sepiolite, and montmorillonite. Moreover, highly weathered tuff breccia from the Yabakei Clay minerals in tuff breccia and andesite landslide consists of illite, chlorite, and montmorillonite A laboratory manual for XRD by the US Geological (Fig. 15 (c)). Survey (2020) divided the clay minerals into seven Generally, the soil materials in this study consist of groups: chlorite, illite, kaolinite, mixed-layer clays, the smectite group minerals in each layer. Mitchell smectite, sepiolite, palygorskite, and vermiculite. and Soga (2005) reported that the minerals of the Figure 15 (a) shows that the clay minerals in highly smectite group have a prototype structure similar to weathered andesite and weathered andesite from the that of pyrophyllite, consisting of an octahedral sheet Ohtsuru landslide consists of halloysite, illite, vermiculite, sandwiched between two silica sheets. Water adsorbed and montmorillonite. Furthermore, tuff breccia consists of onto the smectite between the unit layers and swell, halloysite, vermiculite, and montmorillonite. The content and the smectite minerals are the dominant source of of clay minerals on soil materials from Ohtsuru landslide swelling in the expansive soils (Mitchell and Soga does not shows the dissimilarity. 2005). In addition, sliding layer in the landslides In the Ono landslide (Fig. 15 (b)), the weathered consist of the montmorillonite which is the expansive andesite consists of sepiolite. Weathered tuff breccia character of the group of smectite. Table 2 Activity values in soil materials Location of landslide Soil materials Activity Type of soil Ohtsuru landslide Highly weathered andesite 1.46 Active Weathered andesite 1.19 Normal Tuff breccia 1.46 Active Ono landslide Weathered andesite 1.62 Active Weathered tuff breccia 1.39 Active Tuff breccia 1.11 Normal Yabakei landslide Highly weathered tuff breccia 3.82 Active ISTIYANTI et al. Geoenvironmental Disasters (2021) 8:4 Page 10 of 14 Fig. 12 Correlation between coefficient of permeability (m/s) and void ratio on (a) tuff breccia materials and (b) andesite materials Discussion behaviour. Unfortunately, the strength behaviour by the Ohtsuru landslide direct box shear test on weathered andesite and tuff breccia The sliding layer was located at the boundary between the were not observed. Further study is needed to observe the weathered andesite and tuff breccia, approximately 5 m from strength behaviour by the direct box shear test in the the surface in the Ohtsuru landslide. An obvious difference Ohtsuru landslide. in the soil hardness was observed between the weathered Many shallow landslides occurred in the Ohtsuru area. andesite and tuff breccia and showed the different strength The landslides occurred due to heavy rainfall and on that layers. However, no dissimilarity was observed in the probably occurred during the peak rainfall. The mechanism physical and saturated permeability properties. The upper of the Ohtsuru landslide may be influenced by the strength layer, highly weathered andesite, exhibits a contractive behaviour. The different strengths may be caused by the ISTIYANTI et al. Geoenvironmental Disasters (2021) 8:4 Page 11 of 14 Fig. 13 Shear stress-shear displacement and vertical displacement-shear displacement behaviour in soil materials in (a) Ohtsuru landslide, (b) Ono landslide, (c) Yabakei landslide Fig. 14 Relation between maximum shear stress and vertical stress in soil materials in (b) Ono landslide and (c) Yabakei landslide ISTIYANTI et al. Geoenvironmental Disasters (2021) 8:4 Page 12 of 14 Fig. 15 X-ray diffraction on soil materials in (a) Ohtsuru landslide, (b) Ono landslide, and (c) Yabakei landslide weathering process. The mechanism of other landslides weathered andesite, exhibits a dilative behaviour. near the Ohtsuru landslide could also be influenced by the Unfortunately, the permeability behaviour of tuff same reason. Moreover, even though this study did not find breccia and the strength behaviour by the direct box any dissimilarities in permeability behaviour, this landslide shear test on the weathered tuff breccia and tuff occurred during the peak rainfall and is probably related to breccia were not observed. Further studies are permeability behaviour. required to observe the permeability and strength behaviour in the Ono landslide. Ono landslide The timing of occurrence of the Ono landslide, which The sliding layer was located at a depth of around 30 m occurred the day after the peak rainfall, could be from the surface (Chigira et al. 2018) at the boundary influenced by permeability behaviour and the scale of between the weathered tuff breccia and tuff breccia, in the landslide. Furthermore, the landslide activity in the the Ono landslide. An obvious difference in the soil past could also influence this occurrence. hardness was also observed between the weathered tuff breccia and tuff breccia, and showed the different Yabakei landslide strength on that layers. However, no dissimilarity Compared with the tuff breccia from the Ohtsuru and was found in physical properties. The upper layer, Ono landslides, the highly weathered tuff breccia from ISTIYANTI et al. Geoenvironmental Disasters (2021) 8:4 Page 13 of 14 the Yabakei landslide has low strength and could be Conclusions caused by the weathering process in this layer. The fine Sediment disasters have frequently occurred in Oita fraction content, plasticity behaviour, and activity value Prefecture in 2017 and 2018. The characteristics of tuff in this layer were also higher than those of tuff breccia breccia and andesite at diverse landslide mechanisms in from the Ohtsuru and Ono landslides. Oita Prefecture has not been studied in detail, in For the soil permeability in the highly weathered tuff addition, the landslides which observed on this study breccia from the Yabakei landslide, the permeability consisted of the same soil materials. Therefore, observations values of this study were higher than expected from field of the characteristics of tuff breccia and andesite in diverse observations. The soil materials from the field observa- landslide mechanisms in Oita Prefecture are of interest. This tion were more clayey and should have had a low per- study performed soil stratigraphic analysis and soil hardness meability. However, our results show that the highly measurements in the field,and performed physical weathered tuff breccia from the Yabakei landslide has a properties, saturated permeability, mechanical properties, medium permeability, which could be caused by the and XRD tests in the laboratory. landslide activity in the past, forming the complex layers. Three landslides that occurred in Oita Prefecture were The strength behaviour of this layer exhibits a contract- observedinthisstudy.Ohtsuru andOno landslides ive behaviour. occurred caused by heavy rainfall on 5th July 2017 in a wide The weathering process and landslide activity in the area in Hita City. Ohtsuru landslide occurred during the past could be the reason for this landslide. Landslide peak rainfall on 5th July 2017, and Ono landslide occurred activity in the past caused higher weathering compared in the next day, on 6th July 2017. Furthermore, different to the Ohtsuru and Ono landslides, due to which the landslide mechanisms triggered the large-scale landslides in fine fraction content and activity value increased and the Yabakei, which occurred on 11th April 2018. strength of the soil decreased. Characteristics of tuff breccia and andesite; physical properties, saturated permeability properties, mechanical properties, and the content of clay minerals in diverse Types of landslide in the andesite and tuff breccia areas mechanisms of landslides on this study were not very Fan et al. (2017) classified the landslide dams associated with different, especially for the soil materials from Ohtsuru the earthquake in three types based on the composition and Ono landslides. However, weathering process in material and sedimentological. Furthermore, this study also Yabakei landslides formed the highly weathered tuff found that landslides in the andesite and tuff breccia areas breccia which has high fine fraction content, plasticity could be divided into three types based on the timing of the index, activity value and low soil strength. landslide occurrence, scale of the landslide, and landslide According to the results, this study found that mechanisms. landslides in the andesite and tuff breccia areas are of three types. The first type was a landslide with a narrow 1) The landslide which occurred during the peak slope that occurred during the peak rainfall, such as the rainfall and it has the narrow slope. Generally, this Ohtsuru landslide. This type of landslide was found type of landslide was found in the one area. This frequently in the one area. The second type was the study also found the other three landslides in large-scale landslide that occurred the day after the peak Ohtsuru area which have the same type with of heavy rainfall, such as the Ono landslide. The scale of Ohtsuru landslide. the landslide and landslide activity in the past delayed 2) The large-scale landslide that occurred the day after the timing of the landslide occurrence. Furthermore, the the peak of heavy rainfall, such as the Ono third type was a large-scale landslide that occurred landslide. The landslide activity in the past formed without the occurrence of heavy rainfall or an the low permeability zone and the infiltration of earthquake, such as the Yabakei landslide. rainfall to the large-scale of slope was taking a several time. Therefore, the scale of the landslide Acknowledgements and landslide activity in the past delayed the timing The field observations in this study were made possible with the of the landslide occurrence. participation and assistance from the forestry department of Oita Prefecture and of our laboratory member, Tomohiro Hanaki. 3) The large-scale landslide that occurred without the occurrence of heavy rainfall or an earthquake, such as Yabakei landslide. The landslide activity in the Authors’ contributions MLI, SG, and HO visited the landslide sites and selected a location to expose past, which repeatedly occurred in Yabakei, made the soil materials, collected a sample, and conducted a field investigation to the groundwater gushed out. Furthermore, the measure soil hardness. MLI and SG analysed and interpreted the soil weathering process in the soil high and the strength behaviour regarding the materials at the landslide sites. The authors read and approved the final manuscript. of the soil decreased. ISTIYANTI et al. Geoenvironmental Disasters (2021) 8:4 Page 14 of 14 Funding Tokunaga S, Goto S (2017) Study on measurement of strength discontinuity by Part of this study was carried out with the help of the Japan Society for the digitized Yamanaka soil hardness tester for shallow landslide site at Aso Promotion of Science Grants-in-Aid for Scientific Research Grant-in-Aid for volcano. Paper presented at the 52nd. Jpn Soc Geotech Nagoya. July 2017, Scientific Research (B) 17H03303. pp. 1935–1936. (in Japanese) US Geological Survey. A laboratory manual for X-ray powder diffraction. https:// pubs.usgs.gov/of/2001/of01-041/index.htm. Accessed July 2020 Availability of data and materials Yabakei landslide investigation committee (2018) Final summary report about The datasets used and/or analysed during the study are available from the landslide in Yabakei-cho, Nakatsu city that occurred in April 2018. http:// corresponding author upon reasonable request. www.pref.oita.jp/uploaded/life/2045589_2349104_misc.pdf. Accessed July 2020 (in Japanese) Competing interests The authors declare that they have no competing interests. Publisher’sNote Author details Springer Nature remains neutral with regard to jurisdictional claims in Civil Management and Engineering Major, Environmental and Social System published maps and institutional affiliations. Science Course, Integrated Graduate School of Medicine, Engineering, and Agricultural Sciences, University of Yamanashi, Kofu, Japan. Faculty of Engineering, Graduate Faculty of Interdisciplinary Research, University of Yamanashi, Kofu, Japan. 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