Tropical Soils: Considerations on Occurrence and Characteristics and Studies in Brazil

Abstract

Soil engineering is challenging as soils are found in different conditions of climate, topography and location and have properties that vary in depth and laterally. The tropical soils represent types of soils with similar characteristics and can be found in locations where tropical climate occurs. Inevitably, engineering projects must consider the particularities of these soils due to their genesis and the study of their properties, which requires investigations ranging from simpler methods to more complex field and laboratory tests, is essential to predict their behavior. In the case of lateritic soils, their differentiated characteristics impose additional challenges on designers since their classification does not fit perfectly into the classifications formulated for temperate climate soils. One limitation found in developing countries is that there are few or no geotechnical mappings and the knowledge of these soils are restricted at the central-south region of Brazil, where the major urban centres and the largest engineering companies and laboratories are located. Although studied by authors from all over the world, it is still possible to deepen the understanding of the properties of tropical soils and their application in the most diverse requests of civil engineering and others. This chapter was developed from bibliographic research.

1. Introduction

Soils are formed by the joint action of climatic factors, weather, relief, source material, and organisms. Source materials consist of rocks or other soils, under which the other factors work. Thus, the properties of a residual soil and the behavior it presents in the face of various requests will largely be determined by the rock or material from which it originated.

In tropical and intertropical regions, a diversity of climates and relief is observed, resulting in a very large variety of soils known as tropical.

Brazil, for its large size (more than 8 million square kilometers), presents geological, climatic, and relief diversity that has conditioned the formation of soils with various behaviors.

In general, the significant climatic factors for soil formation are mean precipitation and temperature, which condition the rates of chemical reactions, the rate of change of rocks as well as the mobility of elements along the profile. Formed from the leaching of bases and the concentration of oxides and iron and aluminum sesquioxides in this pedological evolution, we have the lateritic soils, which have properties differentiated from the soils formed in temperate climate.

Thus, considering the wide distribution of tropical climate in the world and its occurrence in much of the Brazilian territory, aspects on the genesis, importance, and properties of tropical soils in the country will be addressed in this chapter.

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].

Figure 1.

Climate map of South America [4].

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 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.

Figure 2.

Terms used in the description of tropical soils [18].

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].

3. Characterization and properties of lateritic tropical soils

Lateritic soils (later Latin: brick) are shallow soils, typical of well-drained regions of the tropical wet regions, and have peculiar characteristics associated with the laterization process being the most important from the technological point of view, soil enrichment of iron- and/or aluminum-hydrated oxides, and the permanence of kaolinite as predominant and almost always exclusive clay. These minerals give the soils a typical color: red, yellow, brown, and orange [7, 15].

The saprolitic soils (sapro, Greek: rotten) are those resulting from the decomposition and/or in situ breakdown of the matrix rock by the action of the timeless agents and that maintains the structure of the source rock. They can be considered genuinely residual, because the particles that make it up remain in the same place as the matrix rock and can be called young residual soils, in contrast to the lateritic surface mature soils [7].

Saprolitic soils form layers underlying the layer of lateritic surface soil (or possibly other soil) generally appearing on the surface of the soil due to soil erosion or excavation due to man-made works. These soils are more heterogeneous and consist of a complex mineralogy containing minerals still in decomposition phase.

The characterization and evaluation of the geotechnical properties of residual soils are a complex subject, and there is the need for studies on their peculiar behavior for different purposes such as foundation, roads, stability of taludes, construction of earthworks, among others.

Figure 3 illustrates some variations that can be found in the intemperism profile of tropical soils and that contribute to the complexity of the approach, and Figure 4 shows concretions formed by this material.

Figure 3.

Alteration profiles of tropical soils [16].

Figure 4.

Distinct mesoscopic aspects of the lateritic materials at the Rondon do Pará bauxite deposit. A. Contact between Belterra clay and Nodular Bauxite; B. massive bauxite; C. fragments of iron crust with hematite; D. ferruginous bauxite with oxyhydroxide clasts in a gibbsite matrix, strong goethitized; E: massive bauxite base with kaolinite; F. mottled zone in the basal clay. Approximate scale, drillcore HQ diameter of 9.65 cm (3.5″). Gbs-Gibbisite, Gth-Goethite, Hem-Hematite, Kln-Kaolinite [20].

Field investigations of residual soils often relate to heterogeneous soil profiles vertically and horizontally, great structural complexity, and expected metastability due to the process of leaching and chemical decomposition, the presence of rock blocks immersed in matrix, among other aspects [21].

Despite the difficulties of naming these soils, there is a relative consensus that their characterization is made by conventional criterion, which is chemical, that is, would be lateritic soils all those in which the silica/sesquioxides ratio is greater than or equal to 2, and it is deeply weathered soil [22].

Despite the conventional definition, it is found in many situations that the behavior of these soils cannot necessarily be addressed by the conventional geotechnical project due to one or more of the following reasons [21]:

1. Soil state is variable due to complex geological conditions.

2. Classical constitutive models do not offer an approximation of their true nature.

3. These formations are difficult for sample and the soil structure cannot be reproduced in the laboratory. As a consequence, mechanical behavior and geotechnical properties should be evaluated directly from in situ test data for most geotechnical design problems.

4. There is also a limited experience collected and reported and the finding that parameter values are outside the most commonly found ranges for sand and clay formations of sedimentary soils.

5. Deposits are often unsaturated and the role of matrix suction and its effect on soil permeability and shear resistance must be recognized and accounted for.

These difficulties of lifting and characterization are detached also by authors [23, 24, 25], and others, according to which the tropical grounds it has the reputation of there are “problematic soils” because of without being fitted in the classification systems usually used as they were developed for temperate climates; there is also the need to use adequate methods for them, since the destruction of the cement and its original structure compromises the analysis of its behavior.

Variability in its engineering properties implies, in many situations, a difficulty to meet traditional specifications or consecrated use. An example put forward by [8] is that these materials usually have gaps in the graduation curve (e.g., in the coarse sand fraction); high plasticity indices (PIs 15-20) and CBR values below the minimum 80% are normally specified. An interesting discussion about unconventional materials and their research can be seen in [26].

The geotechnical behavior of these soils is therefore influenced more by their unsaturated condition and by factors such as their structure, macro- and microporosity, anisotropy, and genesis than by their stress history [27, 28, 29].

3.1 The lateritic soils in Brazil

As mentioned above, studies on tropical soils often exhibit a higher degree of difficulty because of their mineralogical, textural, and structural variability, and this is not an exception in the Brazilian territory. Several researchers have studied the behavior of these soils both in the context of the execution of works and for experimental purposes, in universities and research centers. One aspect to highlight is the contribution of foreign companies and professionals in recent years to enriching the knowledge and discussion of engineering problems from the exchange of ideas [30].

Although the country has soil surveys developed by EMBRAPA (Brazilian Agribusiness Research Company) and other research bodies, these are mainly for use in agriculture, without the geotechnical focus [31].

In this context, the peculiarities of Brazilian soils (occurrence, constitution, formation, properties, indices, and environmental conditions) are therefore different from the conditions found in the temperate climate regions where the traditional soil classification systems were developed [4, 7].

It is thus observed that the physical, chemical, biological, pedological, and geomorphological processes vary throughout the area of occurrence of these soils and also in Brazil, where they are distributed over 80% of the territory, as shown in Figure 5 [32].

Figure 5.

Area of occurrence of laterite soils in Brazil represented by dark brown colors and hachuras [32].

Since knowledge of where a work will be deployed depends primarily on well-designed and developed local research, one of the important aspects for the development of an assertive engineering project is the description of the soil profile. Expedited forecasts of collapsible or expansive soil behavior could be inferred from pedological classifications, where in addition to soil identification and classification, information about soil genesis is provided. This is because there is a close dependence on the tropical humid climate of the changing soils in relation to the matrix rock, as, for example, granites decompose originating mycaceous soils with particles of clay and sand, and basalts change basically in clays [3].

Of course, the use of generic profiles is inadvisable and local research can in no way be replaced. Consequently, the use of the geological description of the soil profile in engineering projects is considered essential, and the ignorance of the soil profile leads the designer to make predictions with a degree of uncertainty above that tolerated in the standards. On the other side, when the origin was known and the characteristics of the whole region and of the profile in an individual place the foresight becomes more assertive, reducing risks, costs, and creating solutions more appropriate to each situation [33, 34].

The occurrence of porous layers of clay or sandy texture and materials with varying degree of intemperization is frequent in Brazilian soil profiles, requiring in some situations, analyses and more complex models regarding the geotechnical behavior of the soil. Figures 6 through 8 present some profiles of lateritic soils found in engineering works in Brazil.

Figure 6.

Soil profile along the tunnel, South Wing, Brasilia [4].

Figure 7.

Profile of shear module variation for Caxingui Shaft of Sao Paulo subway [35].

Figure 8.

Site characteristic profiles [15].

In relation to the research methods, one of the most used in Brazilian geotechnical engineering for underground research is the survey of simple recognition of soil with SPT test as highlighted [36, 37], its execution currently governed by the Brazilian NBR 6484:2020. Despite the existence of other methods, the tendency to use this remains, either for its lower cost compared with the other methods (CPT, DMT, geophysical, or other) or for the fact that it does not require specialized labor, for the empirical correlations that are established from or by which other methods are little publicized in the universities.

This tendency of using the sounding SPT can be observed in an abreve consultation to the site of the ABMS (Brazilian Association of Mechanics of Grounds) in which nearly 100 articles are listed that wrap different applications of the method in several types of work (https://philos.sophia.com.br/terminal/8530/) as in [32, 38, 39, 40, 41, 42, 43] and many other authors.

However, in some situations where the size of the work, geological complexity, or even academic studies so require, and other methods are used such as CPT (cone penetration test) or DMT (Dilatometer Marchetti Test) as well as the collection of undeformed samples for laboratório testing (triaxial compression, shear strength, deformation, modulus, and others) as exemplified by the work of [22, 30, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55].

3.2 Geotechnical properties of lateritic soils in some sites in Brazil

As previously mentioned, lateritic soils can present quite varied behaviors, which requires the implementation of projects that take into account their geotechnical properties, obtained from field and/or laboratory tests. The growing urbanization and verticalization of cities in the center-south region of Brazil, as well as the increase in infrastructure works, lead to increasing challenges in foundation design, since the foundation elements must consider the high loads to be distributed frequently. For this, the use of piles has been the most common foundation option, since in some situations, geotechnical limitations occur due to soil properties such as high porosity and/or the collapsible character or excessive settlements in the face of loads, which do not allow the use of direct foundation.

One example of this behavior is the soils of the city of Brasilia, located in the state of Goiás, which is located in an area of highly weathered tropical soil, with high levels of aluminum and iron. As a consequence of the high porosity of the cemented structure of this soil, known as “porous clay,” it presents great structural instability and can sorer that is highly unstable and can suffer changes in volume (collapse) due to changes in saturation and stress state. The possibility of sharp deformations must be considered since this type of soil covers 80% of the area of the municipality [53, 54, 55].

Figure 9 illustrates the SPT index strengths in the study conducted by [53, 54] and the strength parameters of unsaturated porous clay, according to Mohr-Coulomb criteria that can be considered as Cohesion angle ranging from 20 to 34 KPa, friction angle () between 25 and 28° and Young’s modulus varying 1–8 Mpa, Coefficient of collapsibility is 0–12%, Coefficient of permeability is 10⁻⁰⁶–10⁻⁰³ cm/s [55, 56]. The tests to evaluate the granulometry of the soil composed of sieving and sedimentation, in addition to the Atteberg limits test, allowed the classification of the soil as CH by the Unified Classification system, with a plastic Index of 12% and a natural unit weight around 15 kN/m³. This porous clay layer has a variable thickness of 20–30 m and NSPT indexes between 2 and 3 strokes with a deep water level, and in some cases reaching a depth of 5 m. Liquid limit LL = 50–80%, plastic limit PL = 35–50%, and water content w = 35–55%. The clay fraction, that is, the percentage of soil particles less than 2 μm lies between 70 and 55%. The percentage of fines (less than 60 μm in diameter,) varies from 70 and 80% [55].

Figure 9.

Stratigraphy and SPT-SPTT results [53].

Considering the characteristics observed in the drilling and laboratory tests, the authors state that a foundation option that has been used in the city of Brasilia and neighboring cities is the Alluvial Pile Anker, a new type of small diameter foundation characterized by fast execution, with technical and economic advantages over precast piles. It consists of drilling small diameter piles, where a 2 ½″ tube, 50 cm longer than the depth of the hole, with a cutting tip (Figure 10), is installed in the ground at very high speed, and the soil is drilled through rotation. The hole is filled with cement, and after it has been drilled, the capping is made with precast-reinforced concrete or steel sheeting on each pile. A gravel backfill is placed between the capstones at the same height, and a geogrid is placed over it, followed by a transition backfill that acts as a stress dissipator [53, 57].

Figure 10.

Alluvial Anker pile construction process [57].

It should be noted that in foundations embedded in lateritic or collapsible soils, the rigor in the design process and in the design should be greater, because the behavior of these foundations often differs from the classical models adopted and presented in the technical-scientific literature, being possible to observe a nonlinearity of the soil behavior due to variations in the soil parameters that control its behavior: modulus of deformation and shear modulus of the soil [58]. It is fundamental for pile foundation design like aspects such as the relative stiffness of the lateritic soil of the first layer when not saturated; the collapsibility of this soil, especially if the piles are totally embedded in this layer and the evaluation of the ultimate strength, due to its own executive process, be taken into consideration.

Studies conducted in collapsible lateritic soils in the city of Campinas, state of São Paulo, where most of the foundations employed are deep, with auger piles being the most commonly used, show the use of foundations executed as staked foundations, for example, a foundation element where the piles under the radier are interrelated may have greater efficiency in the reduction of settlements, because the greater contact of the surface foundation element contributes to the performance of load capacity and settlement reduction for the system [59, 60].

In the city of São Paulo, the construction of an extensive subway network allowed obtaining geotechnical parameters of soils existing in the São Paulo Basin from the study of 12 different sites, which demonstrated the heterogeneity of the profiles, comprising alternating layers of sandy clays and clayey sand with silt fractions. However, the horizontal stress index (Kd) revealed values greater than 2, confirming the overconsolidation of the variegated soils, which had been previously reported in the literature [22]. The in situ tests allowed obtaining information about the stress history of the sampled soils and despite the variability of the soils, authors point out that the corrections obtained are consistent with results presented in the technical literature, confirming the potentiality of the piezocone and dilatometer (DMT) tests [33].

It is noteworthy that lateritic soils can also present variation in their behavior as a function of matrix suction variation, and therefore, their geotechnical investigation should be careful [21, 26, 33, 58, 61, 62].

Furthermore, one must consider the resistance variation presented by laterite soils that vary considerably with depth, according to the influence of such factors as parent rock, depth of the water table, topography, degrees of decomposition, laterization, and desiccation, as well as mineralogical composition. Also in relation to mineralogy, if the clay present in the soil has the presence of iron oxide in ferric state, the soil is essentially stable and no changes are expected and therefore, standard tests can be employed for soil characterization [8, 32, 63].

4. Final considerations

Tropical soils occur over a large area of the planet, occupying about 40% of the surface. The tropical climate is responsible for the laterization process, which generates well-drained soils, porous, reddish in color, and with characteristics different from those of temperate soils and that may require, by their nature, solutions different from those proposed by classical soil mechanics.

Despite the importance of these soils, there is still no integrated database on their characteristics and behavior. In Brazil, residual and saprolitic soils are a challenge to engineering because they range from highly weathered and well-drained, porous soils in tropical and subtropical climates to thin and poorly developed soils in regions of the country where the drier climate predominates.

This chapter has aimed to approach some aspects of the soils in Brazil, without the pretension of exhausting the subject, which is of great interest to the country and others that are located in the tropical region.

There is a need for further investigation of soils not only in terms of fertility or application to agriculture, as is often the case, but also in terms of geotechnical aspects for the execution of foundations and roads, retaining structures, among others, so that field and laboratory tests can be conducted, not only in the south and southeast regions, but also in other regions, considering the constant expansion of the country’s infrastructure, including international partnerships.