1 Introduction

Soil is a critical resource worldwide, highly valued in agriculture and civil engineering for its role as an essential construction material [1]. Lateritic soils are the dominant soil type across Nigeria, serving as a critical construction material for various infrastructure projects [2]. However, classifying these soils using common systems like the Unified Soil Classification System (USCS) and the Casagrande Plasticity Chart can be challenging [3]. These widely used methods were primarily designed for temperate and transported soils, and their application to lateritic soils with unique characteristics can lead to inconsistencies. This review paper looks at the problems of using USCS and the Casagrande Plasticity Chart to classify lateritic soils in Nigeria. We examine cases or documented observations where the classification results from these methods contradict the actual properties or behavior of lateritic soils, which can affect the safety and performance of construction projects.

This review aims to appraise the limitations of the USCS and CPC for classifying lateritic soils in Nigeria and highlight the unique features of the soils that make their classification difficult with these prevalent methods. Identifying the limitations and understanding the weaknesses of these current systems can help with the development of a more appropriate classification methods and better ways to evaluate these critical construction materials. A systematic review was conducted using the PRISMA technique across two electronic databases, SCOPUS and Google Scholar, to identify previous relevant studies on lateritic soils. We focused on studies reporting the index properties and classification of lateritic soils using the USCS and CPC classification system. Data extraction and quality assessment were performed to ensure the reliability and validity of the included studies.

1.1 Lateritic soils/laterites

There is a considerable amount of discourse surrounding the terms 'laterite' and 'lateritic soil' among scholars. Reference [4] note that the classification of residual soils into two categories was established during the first international conference on residual soil: (i) Lateritic soil, a well-drained and leached soil devoid of primary minerals, containing hydrated iron oxide, and rich in kaolinite, with parent rock structures completely destroyed, and (ii) Saprolite or saprolitic soil, which contains a small amount of clay and primary minerals, and has a well-preserved parent rock structure. While lateritic soil is prevalent in tropical regions, saprolitic soil is prevalent in temperate regions. The term 'laterite' was initially introduced by Francis Buchanan in the year 1807. He used this term to describe the soft reddish rock that he encountered in Western India, which could be cut to produce bricks that would harden irreversibly when dried in the sun. Reference [5] describes laterites as “all the reddish, tropically weathered residual and non-residual soils including laterite rocks.” According to [6], laterites can be described as "highly weathered and altered residual soils, low in silica, that contain a sufficient concentration of the sesquioxides of iron and aluminum to have been cemented to some degree. Depending on the extent of the emplacement, the material could be described as lateritic or as laterite." References [7] and [8] define "laterite" soils "as profiles in which a laterite horizon is found, and "lateritic soils" as profiles in which there is an immature laterite horizon from which a true laterite horizon will develop if appropriate conditions prevail long enough".

Although the definitions and usage of the terms “laterite” and “lateritic” soils have been subject to arguments and criticisms by previous researchers, one undisputed fact is that laterites and lateritic soils are the result of intense weathering (laterization) under tropical and sub-tropical climatic conditions. Reference [8] observed that laterites/lateritic soils "genetically form a chain of materials ranging from decomposed rock through clays to sesquioxide-rich crusts generally known as a cuirasse or carapace". Therefore, we will henceforth refer to these soils as lateritic soil and adopt the definition proposed by ref. [9] .

Lateritic soils, transported soils, and soils from temperate/cold regions have distinct mechanical and physical properties [8, 10]. These properties are influenced by various factors, including the extent of weathering (termed laterization), the type of parent rock, the prevailing climate, the soil's position within the soil profile, and the topography [8, 11, 12]. Moreover, the geotechnical properties of lateritic soils exhibit significant variations not only across different tropical countries but also within the same country, if formed under different climatic conditions from the same parent rock [13]. Additionally, the geotechnical properties of lateritic soils vary with spatial location and depth, even if they are formed on the same parent rock [14,15,16]. Lateritic soils can be classified as cohesive and cohesionless soil based on their composition, with the cohesionless part consisting of silt, sand and gravel, and the cohesive portion including silt and clay sizes. The structural element in lateritic soils consists of less stable aggregation of coarse-grained particles of variable strength, which may break down in performance. The varying clay and silt content also renders most lateritic soil moisture sensitive, leading to strength deterioration [17]. Standard laboratory test methods may not always produce reproducible results when evaluating these soils as engineering materials [18].

In Nigeria, lateritic soils are the most prevalent soil type and can be found developing on various rock types, including basement complex and sedimentary rocks in diverse sub-climatic and drainage settings. They serve as the most widely utilized construction materials for earth dams, highways, embankments, airfields, and foundations to support structures. The utilization of lateritic soils is nearly indispensable for any construction project in Nigeria, as observed by refs. [19] and [2].

1.2 Soil classification

Soil classification systems (SCS) play a critical role in predicting the behavior and response of soils in various engineering and construction applications. As noted by ref. [20], “Soil classification enables the engineer to assign a soil to one of a limited number of groups, based on the material properties and characteristics of the soil”. Indeed, the properties and characteristics used for soil classification encompass various factors, such as textural, chemical, mineralogical, physical, and geotechnical attributes. Commonly, soil classification systems categorize soils into various behavioral categories based on their particle size distribution and Atterberg limits, which relate to their plasticity. There are several widely recognized soil classification systems in use, each with its own set of criteria and terminology. Some of the common soil classification systems include British Standards (BS), The American Association of State Highway Transportation Officials (AASHTO) classification, The Unified Soil Classification System (USCS) (ASTM D2487) and Australian Standards (AS).

In accordance with ASTM and AASHTO standards, soils classified as coarse-grained are defined as those in which no more than 35% and 50%, respectively, of the particles pass through the number 200 sieve (0.075 mm). Grain size distribution-based classification systems are commonly utilized for coarse-grained soils, encompassing materials such as gravel and thoroughly washed sands. In soils containing a substantial proportion of fines, specifically silt and clay particles, the nature and amount of clay minerals within the soil take on significant importance [21]. For fine-grained or cohesive soils, Atterberg limits or plasticity characteristics are primarily used to classify and predict their behavior. According to [22], "Atterberg limits are known to reflect the combined effects of soil constituents and their interaction with the pore fluid."

Table 1 displays the particle size ranges that are associated with gravel, sand, silt, and clay fractions. The table indicates that there are variations in the particle size range for gravel, sand, silt, and clay among commonly used standards. In the USCS standard, soils that are finer than 0.075 mm (sand particles) are not subdivided, but instead, the use of the Casagrande plasticity chart is relied upon for distinguishing silty and clayey soils.

Table 1 Particle size ranges of commonly use soil classification systems
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To determine the plasticity of fine-grained soils finer than 0.075 mm, the soil particles that pass through a 0.425 mm sieve are used. The plasticity characteristics of these soils are determined using the plasticity chart developed by Casagrande in 1948. This chart depicts the relationship between plasticity index values of the soil and their corresponding liquid limit values. The A-line on the Casagrande plasticity chart is used to differentiate inorganic clays located above the line from inorganic silts or organic soils located below it. As per the ASTM standard D2487 [23], soils above the A-line on the Casagrande plasticity chart, specifically inorganic clays, are categorized as either low or high plasticity. Similarly, soils positioned below the A-line, including both inorganic silts and organic soils, are also categorized as high or low plasticity. This classification is determined based on whether their liquid limit is greater or less than 50%.

1.3 USCS classification system and Casagrande Plasticity Chart

The USCS is a widely used classification system around the world, including among civil and geotechnical engineers in Nigeria, where it is mainly employed for classifying and predicting the behavior of lateritic soils. The system was developed during World War II for assessing soils for airfield construction, as noted by ref. [26]. As ref. [10] states, the system's almost universal adoption, with or without local modifications, is not surprising. The USCS categorizes soil into three main groups: “coarse-grained soil”, “fine-grained soil”, and “highly organic soil”. Within the coarse- and fine-grained groups, further subdivisions are made, although the basis of subdivision differs. Grain size distribution is exclusively utilized to subdivide coarse-grained soil into Gravel (G) and Sand (S) because the behavior of coarse-grained soil is mainly influenced by particle size. In addition to the group name (G or S), further descriptive information describing the group's characteristics is required, such as W for “well-graded sand or gravel”, P for “poorly graded sand or gravel”, M for “silty sand or gravel”, and C for “clayey sand or gravel”.

In contrast, fine-grained soils are categorized as either Silt (M) or Clay (C) based on their Atterberg limits or Casagrande plasticity chart. Reference [26] established a correlation between plasticity Index (IP) and liquid limit (WL) utilizing a wide variety of naturally occurring soils. This correlation assisted in the development of the Casagrande’s Plasticity Chart (CPC), in which the A-line is defined by the equation IP = 0.73(WL – 20). The A-line on the chart serves to discern inorganic clays from silts. Organic clays and silts may appear either below or above the A-line in this plasticity chart. When the liquid limit value is less than 50%, the silt and clay group are further classified as low (L) plasticity, and if it is greater than 50%, they are considered high (H) plasticity, respectively. The U-line on the chart represents the threshold of the interdependence between IP and WL for any known soil.

1.4 Implementation of Casagrande’s Plasticity Chart (CPC) to lateritic soils

To effectively apply CPC to lateritic soils in Nigeria, a careful selection of lateritic soils derived from various parent rocks was undertaken. This selection was based on the availability of documented Atterberg’s limits and particle size distribution data for these soils, as reported in the literature. Consistency limits, particle sizes, and the dominant clay minerals (if reported) of these lateritic soils are presented in Table 2. The procedures used to obtain the IP, WL and particle size of the lateritic soils in most of the reported work followed BS 1377:1990 [27], although some modifications were made where necessary. However, [11, 28] followed the procedures specified by Austrian Standards (1990) for determining the IP, WL and particle size of the lateritic soils. Kaolinite is the prevalent clay mineral in the lateritic soils, and most of them are well-graded, containing varying amounts of silt and clay particles. Based on the CPC (Fig. 1, Table 2), it can be observed that most of the lateritic soils fall above the A-line within the clay zone, indicating their classification as CH or CL soils.

Table 2 Properties and the classification of the lateritic soils
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Fig. 1
figure 1

Plasticity classification of the soils [26]

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It would be reasonable to anticipate that the classification derived from the CPC would align with the predominant constituent of the soil (either silt or clay size fraction) as defined by the nomenclature outlined in AASHTO or British Standard for particle size distribution of the soil fractions smaller than 0.425 mm. However, upon a thorough examination of the classification of lateritic soils using the CPC, the following observations become apparent:

  1. 1.

    To begin with, it was noted that lateritic soils containing lower amounts of clay than silt contents (or sand/ silt fractions) are typically designated as CL or CH soils. When their Atterberg limits are graphed on CPC, they fall above the A-line in the clay zone. In some cases, lateritic soils characterized by higher clay contents than silt contents (or a combination of silt and sand fractions) are typically classified as MH or ML soils. This pattern was discovered in nearly all of the data provided by refs. [15, 16, 29].

  2. 2.

    Similarly, the behavior or performance of lateritic soil in the field may not always correspond to their placement on CPC. In a study on the geotechnical causes of the failure of segments of the Lagos-Ibadan expressway in southwestern Nigeria, ref. [30] observed that "the soils beneath unstable pavements were less plastic than those from stable areas; although these differences are not statistically significant." A comparable observation was also noted by ref. [31] during their investigation of the underlying factors responsible for pavement failure along the Ilesha-Akure highway in southwestern Nigeria.

  3. 3.

    Furthermore, several of these classifications do not correspond with the physical observation or visual examination of the lateritic soils. It's interesting to note that in the study by ref. [11], the textures of the lateritic soils were mostly sandy, yet they were also categorized as CL or CH soils on the CPC. Similarly, [16] discovered that a significant portion of the lateritic soils, characterized by a sandy-silty texture, were primarily classified as CL soils on the CPC.

  4. 4.

    The use of CPC has been widely employed in the classification of fine-grained temperate and transported soils. However, it has been noted that this method has certain limitations when applied to lateritic soils, as discussed earlier. As pointed out by ref. [10], ‘the plotting of Atterberg limits of residual soils, such as lateritic soils, on plasticity charts should be considered a guide to likely engineering properties rather than a means of rigid classification’. Reference [30] have also proposed that the exclusive reliance on established standard specifications may not consistently be the most suitable approach for evaluating the suitability of soils for construction purposes. Therefore, challenges associated with the use of lateritic soils in construction may be attributed to the injudicious application of classification schemes principally designed for transported and temperate soils that are not suitable for residual lateritic soils.

Over the past decades, there have been attempts to develop classification systems specifically suited for different types of residual soils. This has become necessary due to the unique properties of these soils, which current classification schemes fail to account for. The Geological Society Working Party Report on Tropical Soils [32, 33] offers a compilation of classification schemes formulated between 1951 and 1986, each designed for distinct applications. Some of the classification schemes listed in the report are based on pedology and take into account pedological characteristics, but they may not be useful or relevant when classifying residual soils for engineering or construction purposes. Recently, [34] and [35] introduced a novel classification system for residual soils. This system relies on the mineralogical composition and soil structure (micro- and macro-structure), without taking into account their undisturbed state. The aim of this classification scheme, according to [10], is to identify groups of residual soils that exhibit similar engineering properties. It is believed to be a better alternative to using pedological terms, which are not effective in classifying residual soils for engineering purposes. However, a major disadvantage of this classification system is that geotechnical engineers often lack the necessary facilities to identify minerals [10]. Consequently, none of the classification schemes specifically developed for residual soils have gained global acceptance. This is attributed to the widely varying nature and properties of residual soils, which cannot be addressed by a single classification scheme. As noted by ref. [36], no appropriate classification system exists that can be used to effectively study residual soils, despite the efforts of geologists, pedologists, and engineers.

In light of the limitations of classification schemes developed for residual soils, it is important to emphasize the factors or properties of lateritic soils that hinder their proper classification using the Casagrande's plasticity chart. Such an understanding would facilitate a better appreciation of the soil properties and inform the criteria to consider when classifying or evaluating these soils.

1.5 Problematic behavior/properties of lateritic soils

As previously mentioned, the USCS (Casagrande's plasticity chart) classification system, originally developed for the classification of temperate sedimentary soils, has some shortcomings when applied to residual soils. Therefore, residual soils, including lateritic soils, often do not conform to this classification system. This section will outline some of the issues that may contribute to this problem.

1.6 Presence of coating and cementing agents

Lateritic soils, like many residual soils, display a distinct structure that arises from the presence of coating and cementing agents present in the soil. Typically, the structure of lateritic soils is concretionary, with ferric gels enclosing siliceous particles, and iron hydroxides absorbed by clay-mineral particles, completely covering them in some cases [8]. When lateritic soils are saturated with water, this structure can present challenges because the layer of hydrated hydroxide may inhibit proper soil saturation. In contrast, temperate soils typically exhibit a dispersed structure, where water films surround individual soil particles when soaked ([37], as cited in [8]. Consequently, evaluation methods or procedures used for assessing the textural and plasticity properties of dispersed soils may not be sufficient for lateritic soils that have a concretionary structure. Furthermore, the USCS classification system relies on the consistency/Atterberg limits and particle size of destructured soils, which cannot fully account for soils like lateritic soils that possess unique structures.

Iron and aluminum oxide under tropical weathering generally do not dissolve as they would in an acidic environment. Under tropical conditions, they often remain in situ. The plasticity of tropical residual soils can be influenced by the mode of occurrence of iron and aluminum oxides—whether they are present as “amorphous colloids” or exert an aggregating effect on clay minerals [38, 39]. Reference [38] noted that the extraction of iron oxide from tropical red clay led to an increase in the soil's WL. This observation suggests that the occurrence of aluminum and iron oxides in tropical residual soil can have a suppressive effect on its plasticity. In this scenario, the oxides exert an aggregating effect on the clay minerals existing in the soil. On the other hand, [40] stated that if these oxides are occurring in a residual soil as amorphous colloids, they will contribute to the plasticity of the soil. In this case, they possess a high water retention capacity owing to their large specific area. Recent research conducted by ref. [41] revealed that removing free iron oxides from lateritic soils leads to a reduction in soil particle aggregation, which in turn reduces the surface area available for interaction with water, thereby impacting the Atterberg limits. However, the effects of iron and aluminum oxides on the plasticity characteristics of lateritic soils from Nigeria have not yet been investigated.

1.7 Sensitivity of Atterberg limits to methods of sample preparation and manipulations

The determination of Atterberg limits of tropical residual soils is influenced by pre-test sample preparation, and duration and method of mixing. Consequently, these tests may yield inconsistent or significantly varied results, influenced by factors such as the extent of Pre-Test Drying (PTD) (whether natural moisture content, oven-dried, or air-dried), the duration of mixing, and the type of dispersing agents employed during testing [32, 38, 42,43,44,45,46]. Table 3 illustrates the impact of sample preparation methods, particularly the PTD method and duration, on the Atterberg limits of selected lateritic soils from Nigeria. The data in the table reveals a trend where the liquid limit (WL) and plasticity index (IP) of these lateritic soils decrease with an increase in PTD temperature and the duration of heating. Remarkably, the choice of PTD method notably impacts the WL and IP values. Specifically, Natural Moisture Content (NMC) or air-dried samples, where applicable, result in the highest WL and IP values, whereas samples subjected to oven-drying at 110 °C exhibit the least values. This shift in consistency limits due to variations in PTD conditions can have significant implications, potentially altering the classification of the soil. In a study conducted by ref. [46], it was noted that in lateritic soils derived from granite, the change in PTD temperature resulted in a shift in classification from CH soil to CI soil when subjected to oven-drying at 110 °C. Conversely, in lateritic soils derived from charnockite and quartzite, changes in PTD temperature did not lead to an alteration in the overall classification of all soil samples. However, a closer examination of the relative shifts in the positions of data points on Casagrande’s chart distinctly indicates that with an increase in PTD temperature, lateritic soils tend to display decreased plasticity.

Table 3 Effect of sample preparation on Atterberg limits tests
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Increasing the pre-test drying temperature, as suggested by refs. [42] and [48], results in the aggregation of soil particles. This agglomeration phenomenon diminishes the accessible surface area for water interaction within the soil. Consequently, the soil's capacity to absorb water decreases, resulting in a reduction in both the liquid limit (WL) and plasticity index (IP). These findings are in agreement with the outcomes of particle size distribution analysis conducted in the research by ref. [46], which indicated that the percentage of sand fractions increased while clay fractions decreased with elevated pre-test drying temperatures for lateritic soils. In addition, the impact of drying before testing, as illustrated by ref. [49], can be attributed to heightened cementation arising from the oxidation of aluminum and iron sesquioxides. Furthermore, the duration of mixing plays a pivotal role, in line with the observations of [3]. Longer mixing durations correspond to higher liquid limits and plasticity indices. This rise in values is likely attributable to the extensive breakdown of bonds between clay clusters and within peds, resulting in a heightened proportion of fines in the soil. Importantly, these changes are irreversible, with drying, in particular, leading to permanent alterations in the soil's plasticity characteristics.

However, [50] proposed a method for testing the susceptibility of lateritic soils to pretest sample preparation. According to him, this involves testing the soil samples at various drying temperatures and periods of mixing. He further stated that if there is insufficient time for such testing, the optimal approach is to assess the soil without drying below its moisture content and the period of mixing should be five minutes. Whether this procedure is applicable to lateritic soils from Nigeria is still an open debate.

1.8 Procedure of determining the WL and WP

In accordance with AASHTO and ASTM standards, fine-grained soils are defined as those in which over 35% and 50% of the particles, respectively, pass through the No. 200 sieve (0.075 mm). These soils are categorized based on their plasticity characteristics, particularly the Atterberg limits. It's essential to highlight that the tests for plasticity characteristics or Atterberg limits, which are employed to classify fine grained soils as either clay or silt, are usually performed on soil particles passing through the No. 40 sieve (with a sieve opening of 425 µm), rather than on particles passing through the No. 200 sieve. In many soil classification systems, the soil materials that pass through the #40 sieve but remain in the #200 sieve are identified as fine-medium grained sand. For example, ASTM and AASHTO classify this range of grain size as fine sand, while BS 1377 designates particles coarser than 75 µm but finer than 425 µm as medium-grained sand. When the coarse-grained and fine-grained portions of a soil sample, intended for classification, are approximately equal or in close quantity, the sample prepared for the WL and WP tests will include a significant proportion of coarse-grained particles. This raises a question about the appropriateness of including fine-medium grained sand in a test designed for the classification of fine-grained soils into clays and silts.

WL and WP demonstrate a distinct correlation with the size of soil particles, typically showing an increase as soil particles become smaller [51]. Consequently, it is anticipated that soil particles passing through sieve #40 will typically display lower WL and WP in comparison to soil particles passing through sieve number #200. Moreover, the IP is anticipated to be either equivalent to or marginally lower than that of soil fractions passing through sieve #200. When considering the position of soil fractions passing through sieve size 200 on the plasticity chart, it is expected that they will be situated to the right of the soil particles that pass-through sieve size 40.

This shift in position holds particular significance when the soil's classification, either CL or ML, lies precisely at or very near the boundary that separates the two classification categories. For instance, if a CL soil is plotted to the left of the division line where WL = 50% for the sieve number 40 material, it is anticipated to shift into the high plasticity clay zone (CH) when evaluating the IP versus the WL for the sieve number 200 material.

In research conducted by ref. [52], research aimed to compare the consistency limits of different fractions of lateritic soils that passed through sieve numbers 40 and 200, using three genetically distinct lateritic soil fractions. The findings from this research revealed that the lateritic soils exhibited a well graded nature, containing a substantial proportion of sand. They were predominantly categorized as inorganic clay with intermediate to low plasticity.

Analysis of the particle size distribution of fractions smaller than 425 µm indicated that these soil fractions contained a substantial proportion of medium to fine sand. The consistency limits of the lateritic soils examined revealed an interesting trend, as the soil particles passing through sieve #200 exhibited higher values compared to those passing through sieve number 40, as illustrated in Table 4. The rise in the consistency limits of soil particles smaller than 75 µm in relation to those smaller than 425 µm had a notable impact, resulting in changes in the USCS classification and the overall plasticity levels of the lateritic soils.

Table 4 Liquid limit, plasticity Index, and USCS classification of varied particle sizes in lateritic soil [52]
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1.9 Sensitivity of particle size distribution to methods of sample preparation

Pre-test drying and chemical treatments have the potential to induce alterations in the physical behavior, microstructure, and clay content of residual soils [38, 53]. Drying processes are known to modify clay minerals and facilitate the aggregation of fine particles, leading to the formation of larger particles that maintain bonding even upon wetting or subjected dispersion techniques [48, 54]. Consequently, the most frequently observed impact of drying is a reduction in the reported percentage of clay fraction and an increase in the percentage of sand fraction, respectively [3, 46]. As noted by ref. [33], clay soils often exhibit characteristics resembling silt or sand with reduced plasticity due to drying, although in some cases, the opposite outcome can occur. Thus, it is of utmost importance to avoid subjecting lateritic soil samples to drying when determining their particle size.

Table 5 presents the impact of PTD on the particle size of various lateritic soils. Past studies suggest that drying encourages the loss of “adsorbed and inter-particle water” [55]. This mechanism results in the aggregation of smaller/fine particles and inter-particle attraction [56]. Consequently, this process leads to an increase in capillary stress, facilitating close particle contact and the formation of strong “Coulombic and Van der Waals bonds” that are difficult to reverse [57].

Table 5 Effect of pre-test dry on particle size distribution
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1.10 Crushable particles

The particles composing lateritic soils are inherently crushable, a characteristic that can be attributed to the loss of strength resulting from chemical weathering or the weak induration of fine particles by a clay matrix or secondary cementing agents. This quality often creates the false impression that lateritic soils are rich in coarse fractions when, in reality, they are not [8]. Consequently, when subjected to high stresses, these weak particles are susceptible to breakage. These stresses may originate from either mechanical or thermal sources. The transformations or breakdown of particles due to mechanical and thermal stresses are referred to as mechanical and thermal instabilities, respectively [18]. Mechanical instability, as defined by ref. [18] can occur during the remolding and manipulation of lateritic soil from its in-situ state, leading to the destruction of structure and cementation. This, in turn, can affect properties such as compaction parameters, consistency limits, and particle size [38, 44]. According to [58], thermal instability in ‘soils can be seen in the sensitivity to drying and what has been described as elements of potential self-stabilization’. This phenomenon primarily influences the Atterberg limits and particle size distribution.

2 Concluding remarks

Lateritic soils are the result of extensive weathering processes occurring in tropical or subtropical regions. These soils exhibit a high degree of structure and are often cemented due to mineral deposition during or after the laterization process. However, the terminology used to describe lateritic soils varies significantly, both in usage and between different countries. Consequently, numerous inconsistencies and challenges exist in the classification and identification of these soils. Prominent and widely accepted classification systems, such as the USCS, which have been developed primarily for sedimentary and temperate soils, cannot be reliably applied to predict the behavior or performance of lateritic soils. The standard classification tests used to construct plasticity charts may yield inconsistent results with lateritic soils due to the influence of coating and cementing agents, the degree of pre-drying, chemical treatment, and mechanical instability. Hence, the plotting of Atterberg limits of lateritic soils on CPC should primarily be viewed as a guide to estimate their probable engineering behavior, rather than being considered as a rigid method of classification. At present, there is no universally accepted classification scheme capable of effectively characterizing residual lateritic soils, given their diverse and unique nature and properties.