**8. Soil improvements**

Volcanic residual soil is unique for engineering purposes. For engineering purposes, much volcanic residual soil is compacted to increase soil strength and/or decrease permeability. This is done to increase density, reduce porosity, and shrink pores. Volcanic residual soil exhibits wretched incompetence, but this property can often be overcome by drying. Significant changes in cohesion and internal friction angles can be induced during drying to make volcanic residual soil suitable for engineering purposes.

One example is that a Road Engineer would dry volcanic residual soil in the sun to irreversibly reduce its moisture content or apply calcined lime (CaO), gypsum, cement, and/or bitumen to create an exothermic dehydration reaction [67, 96, 100].

Another way to improve the physical properties of volcanic soil is by using agroecological methods, which is not allowing volcanic residual soil to be bare, the goal is to prevent the soil from being easily eroded [67, 96, 101, 102]. Planting elephant grass is an agro-ecological effort. In addition to helping increase soil cohesion, elephant grass can also be used as animal feed.

### **9. Comparison**

In several years, Buurman, P. [103], Northmore, et al., [104], and Prasetyo and Gilkes [105], research on the characteristics of volcanic residual soils in Indonesia was conducted. Case studies involved soils on andesitic volcanic material between 100 and 1000 m altitude in West and East Java, West Sumatra acid volcanic tuffs, and South-East Sulawesi ultramafic rocks in Indonesia. Discussions are mainly on soil classification, soil genetic, engineering characteristics, and engineering use.

On West Java, Indonesia, Latosols in a toposequence between 40 and 1020 m altitude on andesitic rocks and derived sediments were classified according to Soil Taxonomy. Although soils can be classified according to Soil Taxonomy, several of the boundaries in Soil Taxonomy units are inconvenient for practical use. It is proposed to define Red and Yellowish Red Latosols. The Location of the research took place on the Salak and Gede Volcanoes, West Java. This area consists

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

of intermediary volcanic tuffs (andesite). Soil, which is the result of weathering of the source rock, contains halloysite as the main mineral that already shows some kaolinite characteristics (metahalloysite). This mineral is accompanied by fair amounts of interstratified illite-vermiculite, some illite-chlorite, goethite, and quartz.

The subsequent research is on the west and north to east slopes of the Lawu Volcano, East Java, Indonesia. The physical chemistry and mineralogy were studied of two sequences of soils: Andosols, Latosols, Mediterranean Soils, and Grumusols on the west slope and Mediterranean Soil on the north-east slope of the Lawu Volcano. Soils are developed from pyroxene andesite parent material on Upper Pleistocene and Holocene surfaces. Weathering gradually increases downslope. Andosols are the least weathered soils, while Mediterranean Soils at the lowest altitude are most strongly weathered. Downslope, free iron in soils, and particularly iron concretions in the sand fraction increase considerably, hence, perhaps the red color of soils at lower altitudes. Weathering with the prevailing high rainfall and constantly high (isohyperthermic) temperature produces deep soils with predominantly halloysitic mineralogy. Gibbsite is formed in medium acid soils. Smectite appears in neutral to mildly alkaline soils with a high supply of bases.

The presence was demonstrated of Oxisols on felsic and ultramafic parent materials on South-East Sulawesi. On ultramafic rocks, there was an association of Inceptisols, Alfisols, Ultisols, and Oxisols governed by topography. The rocks are mainly peridotites with a varying degree of serpentinization. They are separated from the main body by a fault zone with schists and phyllites. Montmorillonite minerals predominate in the clay fractions with additional vermiculite-illite, margarite, illite, and quartz following a high supply of bases—mainly magnesium—by weathering of peridotite. Montmorillonites form in contact with the disintegrating rock or in places with magnesium-rich groundwater. As soon as Mg becomes depleted, interstratified minerals form that finally change to kaolinite.

The soils studied in West Sumatra occur on Tertiary or Early Quaternary volcanic tuffs of dacitic and liparitic composition. The landscape is an undulating dissected peneplain, and erosion is only slight. The soil Colors were mainly strong brown and redder. Textures were very clayey. The soils were strongly desaturated. Clay fractions were invariably dominated by kaolinite, showed minor amounts of gibbsite, chlorite, goethite, and quartz.

Based on the literature review above, the soil in the research area has several similarities and differences in characteristics. The equation lies in the color reddish brown-brownish red, iron, and soils with predominantly halloysitic mineralogy. Its distinguishing characteristic is the content of other minerals. This difference is due to differences in geographical location and surrounding geological conditions. It is known that an area with mountainous conditions, tropical climates, bypassed by fault structures, and there is a manifestation of the geothermal/hydrothermal alteration environment, will produce soil with a sufficiently varied and concentrated clay mineral content, as well as minerals that are characteristic of the hydrothermal alteration region (e.g., Despujolsite) as in the research area.

### **10. Conclusions**

Tropical volcanic residual soils of the research area have several similarities and differences in characteristics compared to other volcanic residual soils in the Indonesian area. The equation lies in the color reddish brown-brownish red, iron, and soils with predominantly halloysitic mineralogy. Its distinguishing characteristic is the content of other minerals. This difference is due to differences in

geographical location and surrounding geological conditions. It is known that an area with mountainous conditions, tropical climates, bypassed by fault structures, and there is a manifestation of the geothermal/hydrothermal alteration environment, will produce soil with a sufficiently varied and concentrated clay mineral content, as well as minerals that are characteristic of the hydrothermal alteration region (e.g., Despujolsite) as in the research area.

The soil has a specific use. For the first cluster, the community can use the soil to growing some plants and vegetables because it is very fertile. The soil can be used for building foundations, but the soil can cause steel corrosion, requiring special attention when using it. Another particular concern is for the soil in the second cluster. This soil type has the potential to swelling and landslide during the rainy season. Soil improvements are needed when the community will use them. Finally, the soil in the research area has Rare Eart Element (REE) potential. Low Rare Earth Element (LREE) is the type that is likely to be in the soil.
