**Abstract**

In West Lampung, Sumatra, Indonesia, tropical volcanic residual soils are formed from weathering of volcanic breccias in hydrothermal alteration areas with a thickness of up to 20 m. This soil has the characteristics of clayey silt, low to high plasticity, brownish-red color, has the potential to swelling, easily eroded, and slide when it is saturated, and contains the minerals kaolinite, halloysite, illite, dickite, nacrite, montmorillonite, despujolsite, hematite, and magnetite. The results showed that this soil can cause corrosion of steel and is widely used by the community as a medium for growing plants and vegetables and as a foundation for infrastructure (for example, houses). The volcanic residual soil of the research area had Low Rare Earth Element (LREE) potential and specific uses. The soil with characteristic low plasticity has Liquid Limit (LL) brine value <50% will be suitable for agriculture purposes, building foundations, and earth construction. At the same time, the other category is soil with intermediate to high plasticity characteristics, which has an Liquid Limit (LL) brine value >50%, was more ideal for the primary forest.

**Keywords:** Corrosion, eroded, LREE, soil plasticity, West Lampung

## **1. Introduction**

The soil (Greek: pedon; Latin: solum) is a part of the earth's crust made up of minerals and organic matter. The soil originates from rocks' physical and chemical weathering and has particle sizes ranging from clay to boulders. In general, the soil is divided into two types: residual soil and sedimentary soil [1].

Residual soil is soil formed directly from the rock source due to rock chemical weathering, dominated by the hydrolysis process (reaction between silicate minerals and acids) due to high rainfall and temperature conditions. The soil is above and in contact with the source rock or, in other words, does not undergo a transport process [1–4]. Based on the degree of rock weathering, this soil is within or equal to zone VI [5–8] (**Figure 1**).

Residual soil is mainly formed in tropical climates and has high rainfall and is formed in areas with many quaternary volcanic rock formations, such as Indonesia (tropical volcanic residual soil) [9–12]. In Indonesia, these deposits occupy 53% of the land area and form the basis for constructing various types of infrastructure [9, 13–15].


#### **Figure 1.**

*Rock weathering profile and residual soil formation (Modification from [7]).*

Geologically, the residual soils have a unique mineral composition of clay. Allophane, halloysite, kaolinite, illite, and montmorillonite are some minerals of clay found in the residual soil [13, 16]. The formation of residual soil will produce its own physical and engineering characteristics [10, 17]. Rahardjo et al. and Huat et al. [10, 18] have revealed that the soil has complex and diverse properties. Depending on the characteristics and history of eruptions, residual soils, in particular volcanic residual soils, may be layered or massive, coarse-grained or fine-grained, bonded or unbound, cracked or not broken. Volcanic residual soil can cover flat or sloping areas, reaching thicknesses of up to tens of meters. This soil covers an essential part of the world's surface, including the formation of slopes, areas occupied by urban settlements, structures, and infrastructure, and can create environmental problems, such as landslides, erosion, and land degradation [19–30].

The remainder of this chapter deals with genetics, characteristics, advantages and disadvantages, land suitability, soil improvement, and comparison with tropical volcanic residual soils. This research was conducted in West Lampung Transect Road, West Lampung, Lampung Province, Sumatra, Indonesia (**Figure 2**).

**Figure 2.** *Research area observations and geological map. (Modified from Soehaimi et al. [31]).*
