The domain including the lateritic gravel formations studied is located in western Senegal and between 16˚30’ and 17˚10’ West longitude, and 14˚30’ and 15˚30’ North latitude.

The gravel lateritic soils studied come from the borrow pits of Lam-Lam, Mont Rolland, Pout, Ngoundiane and Sindia in the Thies region ( Figure 1 ).

Located east of Dakar, the Thies region covers 6,670 km² of the total 196,712 km² area of the country, and is made up of three (3) departments ( ANSD, 2021 ): Mbour, Thies and Tivaouane.

This region has on the whole a flat relief except for the Thies plateau that culminates at 105 m altitude, the Diass massif which reaches 90 m and the Thies basin which extends over 65 km² area. The relief is then marked by horsts and grabens.

The borrow pits of gravel lateritic soils present variable altitudes due to the mode of pocket operation for the proper supply of road construction sites.

Thies has two Atlantic facades and the climate of the region is influenced by sea currents. It is of the Sudano-Sahelian type in the South and South-East, and more Sahelian in the North and North-East while in the West it is sub-Canarian. This climate is tropical and characterized by a dry season from November to mid-June and a rainy season from mid-June to October, with average annual rainfall varying between 400 and 600 mm as indicated by the ANSD (2021) .

The vegetation of the Thies region is generally that of a tropical savannah with shrubs and little herbaceous plant species. In this region, especially in Pout, there are also fruit plants such as Adansonia digitata, Carica papaya, Mangifera indica, Citrus sinensis and Citrus reticulata. Calotropis procera, Euphorbia obtusifolia, Guiera senegalensis, Cassia occidentalis, Combretum micranthum and Boscia senegalensis are fairly well represented on the sites studied. However, the lateritic zones support very few fruit plants with the exception of Adansonia digitata and a few rare Mangifera indica.

The main factors in lateritic weathering are climate and vegetation as supported by Nahon and Demoulin (1971) as well as the petrographic nature of the bedrock, drainage conditions and ecosystem influences as noticed by Tardy (1993) .

The laterization process results, in accordance with Estéoule-Choux (1983) , to:

All the factors contributing to the rapid elimination of the bases, as the original poverty of the rock in minerals resistant to weathering and well-drained environment, are favorable to alteration. The more or less complete elimination of the bases determines the nature of the clay minerals formed ( Estéoule-Choux, 1983 ).

According to Schellmann (1981) , lateritic soils are products of intense weathering of rocks with a high content of iron and/or aluminum sesquioxide and a low silica content, as in a simply kaolinized rock. They consist mainly of quartz, kaolinite, hematite and goethite mineral aggregates.

As noted by Autret (1980, 1983) ; Bagarre (1990) ; Foko Tamba et al. (2022) ; Kamtchueng et al. (2015) ; Millogo (2008) , gravel lateritic soils are loose and located in tropical areas and consisting of a granular fraction of quartz gravels, pisolites or ferruginous nodules packed in a fine silty-clay matrix.

Many borrow pits of gravel lateritic soils are exploited in Thies where the geological setting has been favorable to their formation. According to Nahon & Demoulin (1971) , these gravel lateritic soils are dated to the Pliocene. This dating is based on a correlation established from Dakar where the same laterite type is located between the volcanic formations of the Tertiary and those of the Quaternary ( Roger et al., 2009 ).

Usually, a borrow pit of gravel lateritic soils presents pockets of different granular classes. The granularity of the best pockets is characterized by samples with more than 40wt% of particles greater than 2 mm and fines (mineral particles less than 0.080 mm in diameter) ranging from 12 to 25wt% ( AFNOR, 2005 ).

Climatic conditions, in particular temperature and precipitation were determining factors in the genesis of lateritic soils in the Thies region. It is thus important to locate the gravel lateritic soils studied in their regional then local geological context.

The geology of the Thies region is characterized by Meso-Cenozoic sedimentary formations with some outcrops of volcanic rocks of which the most important is that of Diack in the Miocene. Weathering has given gravel lateritic soils end-products which are sometimes covered by Quaternary sands.

Sedimentary deposits are present in this context, from the Triassic to the Paleocene, and include limestones, marls, clays and phosphates as substratum supporting sandstones and sands; these formations generally contain inclusions with varied morphologies. The chemical weathering is quite significant and produce gravel lateritic soils often covered by clayey sands or redden sands from the Ogolian ergs of the Pleistocene.

Tertiary volcanism occurred both in Dakar at Cap Manuel and in the Thies region. This volcanism is marked by basanite or dolerite basanite lava lakes at Diack (near Ngoundiane), a basanite sill at Lam-Lam (near Mont Rolland) and a basanite flow at Seune Sérère (near Pout).

The global tectonics of horst and graben is at the origin of the Senegalese-Mauritanian sedimentary basin in which most of the ferruginous soils of Senegal are found.

The structural scheme of the western part of the basin shows a dense network of submeridian faults east of Dakar (Rufisque Graben faults). To the east of this network, the Horst of Diass stands out. It is limited on its eastern edge by the fault of the Thies cliff ( Figure 1 ). This part is organized in Horsts of Dakar and Diass, and in Graben of Rufisque with a system of benches in Bargny, Sébikhotane and Pout to the west of the Diass Horst. These benches are consecutive to a regime of normal faults-oriented NE-SW.

The structure of the Horst of Diass is greatly underestimated, due to the lateritic overlay which masks tectonic accidents ( Roger et al., 2009 ).

Fine deposits characterize the Triassic stratigraphy in the western part of the Senegalese sedimentary basin. Medium to coarse sand deposits characterize the Maastrichtian ( Figure 2 ) whose summit is composed of ferruginous sands and nodules forming the dismantling facies ( Roger et al., 2009 ).

The Horst of Diass constitutes a high zone during the Cenozoic sedimentation, in the northern sector of the Tivaouane fault; the thicknesses are controlled by a set of synsedimentary faults.

The lateritic weathered soils of western Senegal are mainly located in the Thies region and particularly in Lam-Lam, Mont Rolland, Pout, Paki-Toglou, Bandia, Diass, Dougar, Kholpe, Ngoundiane and Sindia. There are also duricrust outcrops with sub-horizontal topography overhanging the cliff of Thies.

In the region, from North to South and from East to West ( Figure 1 ), the lateritic gravel samples come from five borrow pits, three on the Thies plateau (Lam-Lam, Mont Rolland and Ngoundiane) and two on the Horst of Diass (Pout and Sindia).

Mineralogical and geochemical characterization was carried out on pulverized samples of the 0/2 mm granular fraction taken from the Lam-Lam, Mont Rolland, Pout, Ngoundiane and Sindia borrow pits.

The mineralogical characterization was performed by physical methods, namely: X-ray diffraction for mineral identification ( Diop et al., 2023 ) and infrared spectroscopic analyzes for mineralogy confirmation, using Bruker Alpha platinium-ATR Fourier transform infrared spectroscopy at 32 scans in the range of 400-4000 cm-1. Their chemical composition was determined by chemical analytical methods after drying and sifting the soil samples. A control sample was used for the various experiments. The methods used are semiquantitative emission spectrographic, EDTA titration, colorimetric, flame photometry, emission spectrographic and flame atomic absorption ( Shacklette & Boerngen, 1984 ). The study is supplemented by a description of lateritic soil profiles (; Baize et al., 1990 ).

Six minerals forming gravel lateritic soils were identified by X-ray diffraction ( Figure 3 ). They belong to three mineral classes according to Strunz and Nickel (2001) .

Hematite or goethite can be masked depending on the reducing or oxidizing state of the environment respectively, the two most often coexist in lateritic soils in dry and draining conditions ( Diop et al., 2023 ).

X-ray diffraction is a crystallographic analysis, necessarily based on the geometry of minerals. Its results may therefore content a degree of error that can be avoided by infrared spectrometry. This method complements that of the X-ray diffraction and allow the detection of crystallochemical interatomic vibrations.

This analysis allows to determine the mineralogical composition of geomaterials through inter-mineral bonds ( Figure 4 ). Existing bonds with the elements of hydrocarbon calcite as well as the vibration of the bonds with stishovite are not detected. Raman spectrometer ( Diop et al., 2023 ) like that of Fourier Transform Infrared (FTIR), corroborates the X-ray diffraction results concerning the presence of characteristic minerals (quartz, kaolinite, hematite and goethite) in the analyzed lateritic soils. Such minerals were detected by Foko Tamba et al. (2022) ; Kessoum Adamou et al. (2023) ; Hyoumbi et al. (2018) in lateritic soils.

Laterization is a geochemical alteration characterized by new-formation, crystallization, and inheritance of minerals if the weathering process is not complete. This alteration has an impact on the composition and classification of laterites.

The two types of alterations affecting primary rock-forming minerals are biochemical and geochemical weathering. Quartz (inherited mineral), kaolinite (new-formed mineral), hematite and goethite (crystallized minerals) are all present in the gravel lateritic soils of Lam-Lam, Mont Rolland, Pout, Ngoundiane and Sindia.

Quartz, kaolinite, hematite and goethite are characteristic minerals of lateritic soils that form in tropical regions by geochemical weathering with good drainage ( Schellmann, 1981 ).

The chemical results of the gravel lateritic samples studied are presented in Table 1 .

The percentage of silica (SiO₂) varies from 5.07 in Sindia to 35.78wt% in Pout. The proportions of silica are 9.20 and 10.50wt% respectively in Lam-Lam and Mont Rolland both located in the same geological context. In Ngoundiane, silica reached 16.37wt%.

The percentage of alumina (Al₂O₃) is low in Ngoundiane with 2.77wt% and reaches 14.84wt% in Lam-Lam. The proportions of alumina are respectively 12.81, 8.87 and 11.64wt% in Mont Rolland, Pout and Sindia.

Ferric oxide (Fe₂O₃) is abundant in all the studied samples, ranging from 30.15 in Mont Rolland to 51.91wt% in Ngoundiane sample. The ferric oxide proportions are 41.41, 35.59 and 41.27wt% respectively in Lam-Lam, Pout and Sindia.

Alkaline base oxides (K₂O and Na₂O) are not abundant in the five borrow pits. They are contained in the soils in percentages ranging from 0.16 (Pout and Ngoudiane) to 0.31wt% (Mont Rolland) for K₂O and 0.59 (Mont Rolland) to 1.83wt% (Sindia) for Na₂O content.

Alkaline-earth base oxides (CaO and MgO) are significant in the soils studied. They are varying from 2.03 (Pout) to 13.13 (Mont Rolland) for MgO and 2.55 (Ngoundiane) to 10.24 (Lam-Lam) for CaO content. These results could demonstrate that the weathering process which affects mainly the alkaline and the alkaline-earth base oxides is not complete.

The amounts of the other oxides in these soils seemed to be closely linked to their bedrock composition as for P₂O₅ and SO₃ or for their relative humidity as for H₂O. These oxides are not involved in the classification of this soils and are increasingly released during the laterization process.

Some authors have classified laterites according to their chemical composition without taking into account their mineralogy.

Martin & Doyne (1927, 1930) propose a R_MD ratio-based classification (1):

$R_{MD}=\frac{\text{SiO2}}{\text{Al2O3}}$ (1)

R_MD < 1.33: corresponds to true laterites;

1.33 ≤ R_MD < 2: corresponds to lateritic soils;

R_MD ≥ 2: corresponds to non-lateritic soils.

Joachim & Kandiah (1941) define the R_JK ratio of silica to aluminum and iron sesquioxides defined in Equation (2), using the same classification limit values as Martin & Doyne (1927, 1930) .

$R_{JK}=\frac{{\text{SiO}}_2}{{\text{Al}}_2{\text{O}}_3+{\text{Fe}}_2{\text{O}}_3}$ (2)

These classifications are often used nowadays and are not unanimous of the fact that R_MD and R_JK ratios classify, in laterites, iron ores, bauxites and many ferruginous sandstones.

According to the values of the R_MD ratio defined by SiO₂/Al₂O₃, the gravel lateritic soils of Lam-Lam, Mont Rolland and Sindia are classified as true laterites, within the meaning of Martin & Doyne (1927, 1930) , R_MD is less than 1.33. In contrast, the Pout and Ngoundiane gravel lateritic soils are classified as non-laterite soils, R_MD exceeds 2 ( Table 1 ).

Considering the values of the R_JK ratio of Joachim & Kandiah (1941) defined by SiO₂/(Al₂O₃ + Fe₂O₃), the gravel lateritic soils of Lam-Lam, Mont Rolland, Pout, Ngoundiane and Sindia are all classified as true laterites, R_JK is less than 1.33. ( Table 1 ).

As part of this work, Gbaguidi and Diop propose the R_GD ratio, in Equation (3), which is defined by the weight percentage of aluminum sesquioxides over that of iron oxides. This ratio could complete the classification of Joachim and Kandiah (1941) and allows to classify laterites, previously identified by mineralogical analysis, in two groups, based on their chemical composition. It is defined as follows:

${\text{R}}_{\text{GD}}=\frac{{\text{Al}}_2{\text{O}}_3}{{\text{Fe}}_2{\text{O}}_3}$ (3)

R_GD ˂ 1: corresponds to ferruginous laterites;

R_GD ≥ 1: corresponds to aluminous laterites.

According to the values of the R_GD ratio defined within the framework of this study by Al₂O₃/Fe₂O₃, the gravel lateritic soils studied all correspond to ferruginous laterites, as the R_GD is less than 1 ( Table 1 ).

These various classifications, and the contributions and breaches of the authors, as well as the work of Bárdossy (1982) , Schellmann (1983) , Sinisi (2018) and Hyoumbi et al. (2018) have served Diop, within the scope of this work, in proposing a new Classification of tropical residual soils ( Figure 5 ). To avoid the drawbacks noted in the classifications of Martin & Doyne (1927, 1930) and Joachim and Kandiah (1941) by Florentin and Lhériteau (1952) , Autret (1980) , and Bourman and Ollier (2002) while being inclusive towards bauxites; Diop proposes a use of this classification diagram after mineralogical analysis and detection of mineral oxides and hydroxides of residual tropical soils. This diagram is dedicated to lateritic resources and not to the classification of non-residual formation such as ferruginous sedimentary rocks and/or those with a ferruginous matrix or cement.

It is a ternary diagram based on the chemical results of major elements in lateritic soils, providing contents of iron, aluminum and silicon oxides. This diagram distinguishes, on the one hand, the class of non-lateritic soils and, on the other hand, four other classes of lateritic soils, with two ferruginous and two aluminous, when the silica content is relatively less than 50wt% compared with the sum of iron and aluminum oxide contents.

These four classes are divided into two sub-classes, one representing lateritic soils and the other representing true lateritic soils, depending on whether the silica content is between 25 and 50wt% or between 0 and 25wt%, respectively. The intensity of weathering is proportional to the richness of iron and aluminum oxides in tropical soils, but inversely proportional to silica content. For true lateritic soils, weathering is very intense, for lateritic soils, it is moderate and for non-lateritic soils, it is weak ( Bourman & Ollier, 2002 ; Schellmann, 1981 ).

According to the Diop classification, the samples of Thies are all lateritic and ferruginous. The lateritic ferruginous soils, in decreasing order of iron content, include the laterites of Sindia, Lam-Lam, Mont Rolland and Ngoundiane. Among those samples, only the one from Pout belongs to the ferruginous lateritic soils ( Figure 5 ).

Furthermore, as shown in the diagram ( Figure 5 ), the significant concentration of iron and aluminum oxides ( Table 1 ) testifies to the lateralization of these soils ( Gidigasu, 1976 ; Meza-Ochoa et al., 2023 ; Schellmann, 1983 ).

Additional chemical analyzes carried out on the lateritic gravel samples from Thies gave the following results ( Table 2 ).

Cation exchange capacity (CEC) ranges from 13 for the sample from Sindia to 20 mEq/100 g for that from Mont Rolland. The soil saturation rates, S/CEC, deducted from the CEC are lower than 50%. Consequently, these soils are nodular ferruginous, with a great abundance of nodules in one or more nodular horizons containing 30 to 60wt% of ferruginous nodules formed in place;

The chemical characteristics of these soils, in a pedological consideration, correspond well to those of weakly acid ferruginous lateritic soils, rich in gravelly nodules but poor in organic matter. Based on the French soil reference system (), the Thies soils studied can be classified as Ferruginosols.

Table 2. Chemical characteristics of lateritic gravel horizon from the Thies region.

In a lateritic gravel soils profile, zones or horizons form layers distinguished by their specific properties ( AFES, 2008 ): color, structure, texture, chemical composition, root frequency, etc.

In geotechnical engineering, according to Ramanoarison (1985) , the typical lateritic profile ( Figure 6 ) consists of four zones of varying properties, from bottom to top.

The iron sesquioxide accumulations are more or less indurated and range from simple impregnation to the formation of scoriaceous or flaky duricrust. Lateritic gravel used in road construction is extracted from the accumulation zone in lateritic profiles.

Diop et al. (2023) presented the lateritic weathering profiles of Lam-Lam, Mont Rolland, Pout, Ngoundiane and Sindia. Excavation reveals mineable reserve with thicknesses that vary between 1.68 m (Lam-Lam) and 5.43 m (Mont Rolland). The overburden, or superficial leaching zone has a thickness varying from 0.57 (Mont Rolland) to 1.11 m (Ngoundiane).

The leaching zone is remarkable in the Lam-Lam and Ngoundiane profiles; however, it is thin in the profiles of Mont Rolland, Pout and Sindia ( Diop et al., 2023 ). The mineable reserve of gravel lateritic soils is highly heterogeneous, and its thickness varies according to the borrow pit considered: from 1.68 for Lam-Lam to 5.43 m for Mont Rolland. A borrow pit of lateritic gravel is mineable when the ratio of the discovery overburden to the mineable lateritic reserve is less than or equal to 0.50 ( Diop, 2022 ). The five borrow pits studied satisfy this condition, which is necessary but not sufficient, because the mineralogical composition and geochemical properties of extracted materials are decisive in building and road construction.

Mineable reserve colors were identified on the basis of the revised Munsell© Color (1994) . The yellow-reddish color is remarkable at Lam-Lam (5 YR 6/8) and Pout (7.5 YR 7/8). The color is yellow-reddish for Mont Rolland (7.5 YR 6/8) and dark yellow-reddish for Sindia (7.5 YR 6/4). Ngoundiane has a yellow-reddish color (7.5 YR 7/8) which turns yellow towards the base (10 YR 7/8). In an oxidizing environment, iron gives to laterites a yellow-reddish, ocher or red color while in a reducing environment the color tends to be yellowish.

The characteristic yellowish color of goethite is perceptible in the lateritic gravels studied as is that of the red hematite, which is consistent with the mineralogical and chemical composition of these materials ( Figure 6 ).