Stratigraphic analysis of the field observations
Figure 2 shows the tephra layer field observations. At each area, the kuroboku colour was nearly black, which is darker than the scoria layers. Kuroboku layers were located on the scoria layers in each area. Takadake 1 has two kuroboku layers with two scoria layers, both of which are located on scoria layers (N1 kuroboku is located on the N2 scoria and N3-4 kuroboku is located on OJS scoria). Furthermore, Takadake 2 has three kuroboku layers with two scoria layers: N1 kuroboku and N2 kuroboku are located on the N2 scoria layer and N3-4 kuroboku is located on the OJS scoria layer. N3-4 kuroboku layer was divided into 2 sub-layers in each area, N3-4 kuroboku (U) and N3-4 kuroboku (L), which aimed at simplifying the slip surface identification in this study.
Dissimilarity in soil hardness was observed between the kuroboku and scoria layers. The soil hardness in Takadake 1 showed that N2 scoria had the highest soil hardness value (average = 18.55 mm) and topsoil had the lowest soil hardness value (average = 12.6 mm). However, the soil hardness value in Takadake 2 showed that OJS scoria had the highest value (average = 23.9 mm) and N3-4 kuroboku had the lowest value (average = 18.1 mm). The low average soil hardness indicates the location of the slip surface.
Miyabuchi and Daimaru (2004) reported that the landslide slip surfaces were formed near the boundary between the kuroboku and scoria layers. In this study, the low average soil hardness value in each area was located at the N3-4 kuroboku layer; therefore, according to the stratigraphic analysis results of the field observations, the N3-4 kuroboku layer was a slip surface in the studied area.
Tephra layer soil properties
The particle size accumulation curve (Figure 3) showed no dissimilarities in the tephra layers in the research area and all the tephra layers indicated well-graded soil material. However, the tephra layer fine fraction content (less than 0.075 mm) (Figure 4) shows a difference between the kuroboku and scoria layers, where the kuroboku layers have a higher fine fraction content than the scoria layers in each area. Moreover, in Takadake 1 and Takadake 2 the N3-4 kuroboku (L) layers have the highest fine fraction content (Figure 4).
Based on the results, the particle size accumulation curve did not show any dissimilarity between the kuroboku and scoria layers; however, the fine fraction content showed a dissimilarity between them. For this reason, in this study the particle size accumulation curve is difficult to use for estimating the slip surface, but the fine fraction content can be used as a factor for estimating the slip surface.
Figure 5 shows the tephra layer soil property results, which generally showed that the scoria and kuroboku layers are different. Scoria layers have a low fine fraction content, plasticity index, ignition loss, and organic matter content and a high density of soil particles. Meanwhile, kuroboku layers have high fine fraction content, plasticity index, and ignition loss and low density of soil particles and organic matter content.
Ignition loss and organic matter content were performed to observe the tephra layer carbon content. Previous research performed by Kato (1964) described kuroboku as having humic acids, black in colour and high carbon content. Unfortunately, the density of soil particles, ignition loss, and organic matter content values presented in Figure 5 were not differentiated between the kuroboku and scoria layers in this study; therefore, these values could not be utilized as slip surface indication factors.
Figure 5 shows that the plasticity index was different between the kuroboku and scoria layers. The liquid limit and plastic limit test results were plotted on a Casagrande plasticity chart (Figure 6) to examine the soil property characteristics of each layer, which were separated between the sampling location (Takadake 1 and Takadake 2) and the tephra layer type (kuroboku and scoria). The Takadake 1 and Takadake 2 tephra layer data are denoted by filled and un-filled symbols, respectively.
According to the Casagrande plasticity chart, the plotted data showed similar soil type results for Takadake 1 and Takadake 2. Figure 6 shows that all of the kuroboku layers were inorganic silts of high compressibility and organic clays and the OJS scoria layers plotted at the same location as the kuroboku layers. The N2 scoria layers, however, were inorganic silts of medium compressibility and organic silts.
Furthermore, the plotted data showed that the tephra layer could be separated into kuroboku and scoria groups according to the plasticity index and liquid limit values, and is denoted by the ellipse symbol in Figure 6. The plasticity index and liquid limit values of the kuroboku layers were different, but higher than the scoria layers. The Takadake 1 and Takadake 2 data both showed that the N3-4 kuroboku (L) layer had the highest plasticity index and liquid limit values, while the N2 scoria layer had the lowest plasticity index and liquid limit values.
The Casagrande plasticity chart (Figure 6) shows the differences between the kuroboku and scoria layers. Furthermore, Figure 6 shows that the N3-4 kuroboku (L) layers (slip surface layers) had the highest values and were located in the kuroboku group. Therefore, the plasticity index and liquid limit can be used as factors for estimating the slip surface.
According to the factors for estimating the slip surface, a correlation between the plasticity index and fine fraction content was observed (Figure 7), showing nearly the same result as the Casagrande plasticity chart. The correlation showed that the plotted data were distinguished between the sampling location (Takadake 1 and Takadake 2) and the tephra layer type (kuroboku and scoria). The Takadake 1 and Takadake 2 tephra layer data are denoted by filled symbols and un-filled symbols, respectively.
Figure 7 shows the Takadake 1 data fitted to the Takadake 1 trend line, and the Takadake 2 data were also fitted to the Takadake 2 trend line. The trend lines show that the plasticity index is directly proportional to the fine fraction content. Furthermore, the correlation shows dissimilarity between the kuroboku and scoria layers. Scoria layers showed low fine fraction content and plasticity index values and kuroboku layers showed high fine fraction content and plasticity index values. The kuroboku and scoria layers were denoted using the ellipse symbol in this correlation. The Takadake 1 and Takadake 2 data both showed that the N3-4 kuroboku (L) layer had the highest plasticity index and fine fraction content values, while the N2 scoria layer had the lowest plasticity index and fine fraction content values.
The correlation (Figure 7) showed that the slip surface layers (N3-4 kuroboku (L)) were plotted in the kuroboku group. With respect to the sampling location, the slip surface layers have the highest plasticity index and fine fraction content values. However, the plots of this correlation have a wide scattering, which could be caused by the difference of period on volcanic activities in soil materials and the historical landslides in the Aso volcanic mountains.