Residual Soils

Synonyms

Alteration products; Lateritic soils; Saprolites; Weathering products

Definition

Residual soil is the material resulting from the in situ weathering of the parent rock.

Residual soils are distributed throughout many regions of the world, such as Africa, South Asia, Australia, Southeastern North America, Central and South America, and considerable regions of Europe. The largest areas and thickness of these soils occur normally in humid tropical regions, such as Brazil, Nigeria, South India, Singapore, and the Philippines.

Characteristics

According to Duarte (2002), the diversity exhibited by residual soils is due, not so much to the lithology of the original rock, but mainly to external factors such as climate, topography, and vegetation cover; factors that provide distinctive weathering processes and, consequently, distinctive weathering products – the residual soils. At the first International Conference on Tropical Residual Soils, it was proposed to divide such soils into two classes (Brand and Phillipson 1985): (i) Lateritic soils are those that belong to a higher level, well drained and leached, in which the predominant clay belongs to the kaolinite group and contain hydrated iron oxides that give them a reddish color. Generally these do not include primary minerals, and the structure of the parent rock has been totally destroyed. (ii) Saprolite or saprolitic soils, sometimes referred to as young soil, are the residual soils that maintain relic structures from the parent rock, which generally are situated in the levels directly above the original rock, usually contain small amounts of clay minerals, and include primary minerals.

Lateritic residual soils predominate in tropical regions, within latitudes 30° N and 30° S, whereas saprolitic soils are common in temperate regions, for instance, in Portugal, France, Turkey, Piedmont (eastern USA), or in subtropical regions (e.g., Hong Kong and South Africa). The formation of saprolites, which is essentially related to granular rocks, includes primary and secondary minerals in its silt-clay fraction, the nature and quantity of which depends upon parent rock characteristics and on degree of weathering achieved.

The specific characteristics of residual soils in contrast to those of transported soils, are generally attributed either to the presence of clay minerals specific to residual soils (physical composition and mineralogical composition), or to particular structural characteristics of soil in its undisturbed in situ state, such as: (i) Macrostructure: includes the presence of unweathered or partially weathered rock, and relic discontinuities or other weakness planes and structures inherited from the original rock mass; Microstructure – includes rock fabric, interparticle bonds or cementation, particle aggregates, dimension and shape of micropores (Vaughan 1988; Duarte 2002; Wesley 2010). These specific characteristics influence the geotechnical behavior in situ, thus permeability is governed by the micro and macro-structure, as well as the strength and deformability of the residual soil masses (Townsend 1985; Blight 1997).

According to Gomes (1988), the clay of residual soils formed in temperate climates are intermediate, sharing characteristics both of soils from cold or desert climates, where physical weathering prevails, through the disintegration (mechanical breakdown) of phyllosilicates (mica and chlorite) from the parent rock, and those of tropical climates, where chemical weathering prevails, producing kaolinite, gibbsite or smectite, depending upon local conditions. In regions of temperate climate, soils can be derived from either mechanical weathering or chemical weathering. These soils show little evolution, since precipitation and temperature facilitate the moderate hydrolysis of silicates. In the weathering profiles, both neoformed and transformed clay minerals may be present (Fig. 1).

Residual Soils, Fig. 1

Weathering profile of a granitic massif in southern Portugal, under temperate climate, with a saprolitic residual soil cover of about 10 m thick (Photo by I. Duarte)

Cross-References

• Alteration

• Biological Weathering

• Chemical Weathering

• Classification of Rocks

• Classification of Soils

• Collapsible Soils

• Landslide

• Physical Weathering

• Sediments

• Soil Mechanics

• Soil Properties