• Physical Properties Of Soil Materials
Figure 3 shows the results of physical properties in soil layers. Generally, the results of Kuroboku were different from those of Scoria. Scoria layers contain a lower fine fraction and plasticity index, a higher density of soil particles, and a lower ignition loss with low organic matter content. Oppositely, Kuroboku layers contain a higher fine fraction, higher plasticity index, a lower density of soil particles, and a higher ignition loss, but also feature low organic matter (except for the lower part of N3-4 Kuroboku at Takadake 1—which has a higher organic matter content).
In the field, slip surfaces located in the lower part of N3-4 Kuroboku were observed at each landslide sites. Characteristics of these slip surfaces include a higher fine fraction, higher plasticity index, lower density of soil particle, high ignition loss and occasionally higher organic matter content. Not like other physical properties (plasticity index, fine fraction and density of soil particles), the values of ignition loss and organic matter were not clearly showed the differences between Kuroboku and Scoria layers. Therefore, in this study, the ignition loss and organic matter values were difficult to be used as the parameters of the characteristics for the slip surface.
Same with those analyses, the identification of the slip surface using particle size distribution and soil texture classification scheme were not specified. The particle size distribution (Fig. 4) confirmed that Kuroboku layers contain a higher fine-fraction than Scoria layers. However, the soil classification scheme (Fig. 5) in which almost all soil layers are located in the same group (SF to S-F). SF to S-F group showed that soil layers were dominated by sand and fine fraction. Therefore, it was difficult to be used as the parameters of the characteristics for slip surface because the results between Kuroboku and Scoria layers showed the similarity on this method.
However, the identification of the slip surface with a plasticity chart (Fig. 6) showed an interesting result. The plasticity chart showed that almost all soil layers are inorganic silts of high compressibility and organic clays. N2 Scoria layers are inorganic silts of medium compressibility and organic silts. The plasticity index values for Kuroboku layers are higher than for Scoria. The range of Kuroboku layers on the liquid limit started from 50–120% and its plasticity index from 10 to 35. Moreover, the liquid limit for Scoria ranges from 30–55% and its plasticity index is from 0 to 20.
The Casagrande plasticity chart shows that Kuroboku and Scoria generally have different values. Even though the upper part of the N3-4 Kuroboku at Takadake 2 were plotted between Kuroboku and Scoria, it is still evident that the Kuroboku layers and Scoria layers have different characteristics. Figure 6 shows that the slip surface layers (located in the lower part of N3-4 Kuroboku) have a high value of plasticity index and high liquid limit, whereas the range between the layers was excessively wide for liquid limit value. Therefore, from Fig. 6, we infer that the plasticity index may be a useful parameter to help identify the slip surface.
Furthermore, the plasticity index and fine fraction were combined into one chart (Fig. 7), and show clearly that Kuroboku layers and Scoria layers plot in two distinct groups. Scoria layers have low plasticity index (0–20) and a low fine fraction (10–30%), while Kuroboku layers have a high plasticity index (10–35) and a high fine fraction (20–45%). This relationship shows that the plasticity index is directly proportional to the fine fraction.
Figure 7 shows that soil layers from each landslide site were fitted with trend lines and were separated in Kuroboku and Scoria layers group. The Scoria layers plotted outside the trend lines, whereas almost all the Kuroboku layers were plotted within the trend lines. The slip surface layers also plotted within the trend lines and have the highest values of fine fraction and plasticity index (Fig. 7). This chart showed clearly the different characteristics between Kuroboku and Scoria layers, further, the characteristics of slip surface layers also was obvious on this chart. Therefore, this chart enabled to help identify the slip surface layers on Takadake at Aso volcano.