**5. Soil microstructure**

The soil microstructure investigation was carried out using a scanning electron microscope (SEM), and SEM images at different magnifications are shown in **Figure 8**. SEM photos show that the soil microstructure of the soil of the research area is an aggregation of soil particles with fractures and pores (see **Figure 8**). The percentage of soil aggregation compared to fractures and pores is 70:30. This Figure implies that soil aggregation is dominant against fractures and pores, causing the soil permeability of the volcanic residue in the research area to be small to relatively permeable.

From **Figure 8**, it can be seen that the soil aggregate is kaolinite in the form of platy, illite in the form of curved and spongy sheets, and halloysite in the form of


### *Tropical Volcanic Residual Soil DOI: http://dx.doi.org/10.5772/intechopen.98285*

**Table 1.** *Some soil samples and their physical properties.*

**Figure 8.** *a-b SEM tropical volcanic residual soil.*

tubes. The three minerals are bound/cemented by iron oxide. Plate-shaped particles will be susceptible to degradation due to mechanical efforts and forces acting in nature. The shape of the plate particles causes the shear strength of the volcanic residual soil in the research area to be not so large [58, 59].
