**2. Tropical soils and Brazil**

The definition of tropical soil varies from region to region [1], but in general they are defined as those that occur in places that have tropical and humid climates.

The tropics are regions of the Earth located approximately in the middle of the globe between the latitude lines of the Tropic of Cancer and the Tropic of Capricorn and include the Ecuador line and parts of North America, South America, Africa, Asia, and Australia; tropical regions are home to about one-third of the world's population and account for 36% of the land mass.

Intemperism in the tropics can reach tens of meters below the surface, and the products of this process are complex and are not only of interest to geotechnical engineers; they are of great interest to other researchers [2]. It is possible to say that tropical soils are rather intemperated soils rich in iron oxides and aluminum; however, not all tropical soils can be included in this category, since they can originate from materials such as volcanic gray or form in regions of desert climate and thus exhibit different characteristics of the indicated [3].

In this sense, Brazil has 92% of Brazil's territory located in the Tropical or Intertropical Climate Zone, the remaining 8% are south of the Tropic of Capricorn and are inserted in the Temperate Climate Zone of the Southern Hemisphere. (**Figure 1**) in which the climate Aw is observed (the tropical savanna climate features distinct wet and dry seasons of relatively equal duration). Most of the region's annual rainfall is experienced during the wet season, and very little precipitation falls during the dry season [5, 6].

The tropical climatic conditions are constituted by rains concentrated in November and March and a dry period that goes from April to October with haste in general inferior to 60 mm in the dry periods. In the large area of central Brazil [5], Aw climate is markedly seasonal, with strong longitudinal gradient (east-west) of annual rainfall from 1,300 to 1,900 mm and an opposite gradient (west-east) in the rainfall seasonality.

The conditions found in the regions of wet tropical climate produce, in great part, peculiarities of the Brazilian grounds (incident, constitution, formation, properties, rates, and environmental conditions), which are different from the considered ones in climate regions seasoned for which there were developed the systems of classification of traditional grounds [7].

### **2.1 Lateritic soils: definition and origin**

The term laterita was used initially by Francis Buchanan in 1807 when, in travel to the west of India, he identified the use of a reddish ground that after drying was used like bricks in constructions of several sizes; the term laterita, however, includes a scale *Tropical Soils: Considerations on Occurrence and Characteristics and Studies in Brazil DOI: http://dx.doi.org/10.5772/intechopen.103947*

**Figure 1.** *Climate map of South America [4].*

bigger of materials and of behavior varied [8]. However, it is known that the laterita was already used like building materials before that and his importance attached for the production of foods and of construction in function of the vast area of incident in the world, they are studied by several authors around the world as in Brazil, Africa, India, Australia, and other places [9].

The laterização is a process that makes part of the evolution of the relief [10] and [11] in which it takes place to lixiviation of alkaline ones, magnesium, and partially of the quartz and the consequent layers, the formation of lateritas, what are the mixture of hydroxides of iron and aluminum in varying proportions plus add up titania and other residue left. They can constitute micro-collected or collected cement of few centimeters of diameter in the womb of the soil [12].

Depending on the degree of laterization, the materials can be presented under several forms of texturais what go from not consolidated soft clays that can be broken under pressure of the fingers up to materials having enough edurecidos. That led to the use in the concepts literature empiricos of degree of hardness as "hard" or "soft" [8, 13]. However, these expressions guard little relation with mechanical properties of interest of the engineering. Since the variety of lateritas and the changes in his conditions due to environmental factors, his agreement to classifications that use purely morphological concepts, will not always be possible [14].

Another aspect of the formation of these grounds is that the lixiviation of the bases and of the sílica, nevertheless, can be incomplete and the distinction between two types of grounds is difficult to be done; in spite of the properties of two types of grounds, it is similar in terms of properties for the engineering [15].

As for the time of formation of the grounds, lateríticos appreciate that takes place in nearly 104 years, but there are evidences of which this formation is quicker in rocks with less content of quartz like basaltos in granites or rich sediments in quartz [16, 17].

**Figure 2** illustrates the process of formation of the tropical grounds and the denominations used for the same.

As noted, the materials classified as lateritic owe their mechanical and hydraulic behavior to this process of "laterization" that promotes the leaching of basic cations and concentration of iron oxides and aluminum and additionally the predominance of clay minerals of the group of caulinites, low CTC. Studies have shown that soils formed under similar conditions tend to exhibit similar indices and engineering properties [19].

It is noted that Pedology, science that originated in the countries of the northern hemisphere, where soil formation processes are delayed due to cold winter or dry summers, meets challenges for soil description and classification in the tropics, including the difficulties of distinguishing soil from source material and the different horizons resulting from the intense pedogenetic processes of the tropical climate [10].
