The longitudinal section of a road alignment shows the variation between the natural terrain elevation (ZTN) and the project’s red line (ZPr). Based on the longitudinal section ( Figure 2 ), the cut and fill have been marked out using the elevation of the natural terrain and the elevation of the project’s red line ( Figure 2 ).

Excavated areas have a natural terrain elevation (ZTN) higher than that of the project’s red line (ZPr): ZTN - ZPr > 0. They represent masses of earth that need to be removed to a specific depth in order to level or lower the ground level. Eleven cut zones (1 to 11), of varying lengths, were identified as part of this study ( Table 1 ). In addition, the elevations of the natural terrain and the project were taken with an application length of 40 m during the project.

For data acquisition, the configuration used for electrical resistivity tomography is the dipole-dipole ( Figure 3 ) with an inter-electrode spacing of 10 m. This configuration comprises current (AB) and potential (MN) dipoles, which are usually placed linearly on the ground ( Figure 3 ). The length of the tomographic profile depends on the length of the studied cuts. The entire length of the cut is studied, so as to obtain a 2D profile superimposed on the longitudinal section of the cut. Electrical resistivity tomography measurements were carried out on eleven (11) cuts from the Gagnoa-Guéyo-Bamenadou road layout.

The approach consists of determining the bedrock profile, if any, from the tomographic profiles carried out on each cutting. Characteristic tables have been drawn up for each studied cut. These tables make it possible to determine the nature of the investigated cut (NDB) and the thickness of the material (rock or loose material), by calculating the difference between the altitude of the bedrock (ZRX) and that of the project red line (ZPr):

NDB = ZRX − ZPr. (1)

There are two possible cases:

- NDB > 0, we have a rocky cut;

- NDB < 0, we have a soft cut.

To calculate the volume, the cut section obtained is multiplied by the planned road width. The cut section is delimited by the profile of the natural terrain and the project red line. By superimposing the project’s red line and the 2D sections of the electrical resistivity tomographs, we were able to determine, where they exist, the rocky sections and calculate the volumes.

The Gagnoa-Guéyo-Baménadou axis is a two-lane road with a pavement width of 2 × 3.5 m (7 m) and shoulders of 2 × 1.5 m (3 m).

Since the total width of the road is 10 m, the volume of cut (V) can be expressed for each profile (P) or abscissa by:

$V=l\times L_A\times h_i$ (2)

with:

l: road width (m);

L_A: application length (m);

h_i: Cut thickness at the profile (i) (m).

Figure 4 shows a tomographic section of DB 1 located between PK 26320 and PK 26520. The section highlights the vertical and horizontal variations in geoelectric signatures observed along the cut. Indeed, depending on the variation in color scale, three electrical signatures can be identified in this 2D inverse section of DB 1. A conductive layer (blue) with resistivity ranging between 115.6 and 270 Ohm·m. An intermediate resistivity layer (green) with a resistivity between 270 and 490 Ohm·m. Finally, a reddish resistive layer with resistivities extending from 490 to 1344.5 Ohm·m.

Moreover, an analysis of the volume measurement shows that this excavated material is not rocky, but rather entirely soft, with an estimated cumulative volume of 5980 m³ ( Table 2 ).

Located between PK 32560 and PK 32800, the 240 m-long tomographic profile of DB 2 reveals three (03) geoelectric units ( Figure 5 ). A conductive unit marked by blue coloration, with resistivity values ranging from 149.5 to 500 Ohm·m. This formation mainly forms the superficial part of the profile, although in places it lies below the project’s red line. Next to this unit is an intermediate resistivity layer (green) with resistivity values oscillating between 500 to 1150 Ohm·m. Beyond this unit, a brightly coloured signature indicates the presence of a resistive layer (red) with resistivities ranging from 1150 to 4540.4 Ohm·m. This formation is mostly found at greater depths and could therefore correspond to the basement.

On Figure 5 , the project’s red line is above the basement roof profile. The cuts therefore consist of soft rocks. It is therefore not rocky. An analysis of Table 3 shows that the cut zone has a volume of 12124 m³ of soft rock to be removed.

On the tomographic profile of this site (between PK 36720 and 37040), the electrical signatures can be broken down into three entities according to variations in coloration ( Figure 6 ). Firstly, conductive formations (in blue) with resistivities ranging from 182.9 to 590 Ohm·m. These are mainly located below the project’s red line, except for a few at the beginning and end of the profile. The profile then shows the presence of intermediate layers with greenish hues and resistivities vacillating between 590 to 1480 Ohm·m. Finally, there are resistive formations with more pronounced electrical signatures (vivid colorations), with resistivities ranging from 1480 to 64754.4 Ohm·m. This resistant ensemble is superficial between PK 36720 - 36880 and more deeply rooted between PK 36880 and 37040. It is also crossed by the project’s red line along the entire length of DB 3.

Moreover, the volume calculation analysis shows that DB 3 is made up partly of soft formations and partly of rock over most of the cut. Volume calculations give a cumulative volume of 11888 m³ of rocky cuts and 12272 m³ of soft cuts ( Table 4 ).

The tomographic profile of DB 4 between stations PK 66520 and PK 66800 shows two major units of differing geoelectric nature ( Figure 7 ). A conductive unit whose coloration varies in the order of blue, characterized by resistivity ranges between 29 and 295 Ohm·m. This unit occupies almost the entire site and is located below the project’s red line. This is followed by a moderately resistant layer with greenish to yellowish hues of resistivities between 295 and 1998 Ohm·m. This represents the dominant formation of DB 4.

Moreover, no bedrock was detected within the 30 m depth of investigation, so the studied cut is essentially soft materials.

This is also evidenced by the volume estimation analysis ( Table 5 ). This shows that the cumulative volume of material to be removed is around 11132 m³ of the soft formation.

Analysis of the tomographic profile of this study section between PK 80840 and PK 81080 shows three major geoelectric signatures from surface to depth ( Figure 8 ). Firstly, a conductive unit marked by a blue hue, with resistivities varying from 95 to 410 Ohm·m. This unit mainly occupies the superficial part of this study section and lies mostly above the project’s red line. Secondly, an intermediate layer with a greenish signature and resistivities ranging from 410 to 1100 Ohm·m marks the transition from superficial formations to deeper units. Finally, a resistive entity with reddish staining, with resistivity between 1100 and 5991.6 Ohm·m represents the deepest formations. The upper part of this formation is in places above the project’s red line, indicating the presence of rocky materials.

The bedrock roof levels show that DB 5 is both soft and rocky formations throughout the section, with a cumulative volume of 15156 m³ of soft material and 2966 m³ of rocky material ( Table 6 ).

The same approach was applied to all other cut sections (DB 6 to DB 11). Analysis of the tomographic profiles of these different cuts also revealed these three geoelectric signatures. Conductive units with lowest resistivity values ranging from 37 to 217 Ohm·m (shades in the order of blue) ( Figure 9 ). In most cases, these units occupy the profiles’ superficial parts, intersecting by the project’s red line. Alongside them are intermediate resistivity units (300 - 1000 Ohm·m) with greenish hues ( Figure 9 ). Finally, there are resistive units with high resistivity values (>1000 Ohm·m) and reddish signatures. In most cases, these are rooted and lie below the project’s red line, except at DB 7.

By analyzing variations in bedrock and the project’s red line, different types of cuts were observed throughout the project ( Table 7 ). Soft cuts are the most widespread along the profiles, with a cumulative volume ranging from 88 to 19897 m³. These volume studies show that DB 7 contains the most soft material to be excavated, while the site with the least soft material to be removed is DB 6.

As for rocky cuts, they were mainly found at DB 7 and DB 8, with a cumulative volume of 270 m³ and 1000 m³, respectively ( Table 7 ).

The electrical tomography profiles carried out on each cut to determine the profile of the bedrock roof, if any, have shown in this study that the subsoil of this road project comprises three main horizons of differing geoelctric nature. By correlating these entities with the geology of the region in particular, and that of Côte d’Ivoire in general, the low-resistivity units correspond to the lateritic cuirass that forms a superficial overlay on the surface. Because of their ferromagnesium content, these formations, which result from weathering processes, are conductive and therefore of low resistivity. Medium resistivity units are associated with alterites and the fractured fringe of the bedrock. Lastly, the resistive units beneath the surface coverings and alterites correspond to the granitic basement.

This lithological layout has been described in numerous studies carried out in Côte d’Ivoire. Indeed, research by Sombo et al. (2017) in several localities in the Ivorian basement domain has shown that in the crystalline basement zone, a lithological alteration profile includes lateritic cuirass, clayey-sandy alterites and a fractured fringe of the basement.

Also, in all the tomographic profiles, the granitic basement appears in the first 30 meters of depth on the Gagnoa-Gueyo-Baménadou axis. This corroborates the results of the simplified geological map of Côte d’Ivoire, which shows a predominantly granitoid geology ( Figure 1(b) ). Thus, this work shows that electrical resistivity is a key parameter for lithostratigraphic differentiation of subsoil formations in geotechnical work, as attested by the work of Akingboye et al. (2020) , Maślakowski et al. (2014) and Fadele et al. (2013).

In the same vein, the use of 2D and 3D electrical tomography enabled Ayolabi et al. (2012) to solve geotechnical problems in the Gbagada province of Lagos, Nigeria. They were able to highlight different subsoil units based on variations in resistivity. Low resistivity sets are associated with wet formations, while high resistivity values are associated with dry formations. Similar results were found by Akingboye et al. (2020) and Fadele et al. (2013) . In fact, the results of these authors showed that healthy bedrock had the highest resistivity, while medium and low resistivities were associated with the fractured part of the bedrock and superficial formations respectively.

In addition, the surface area and volume of soft or rocky cuts were calculated based on the rock top altitudes of each profile. The method used to calculate the volume of material takes into account the length of application of each profile (40 m in our case) and the width of the planned road (10 m in our case), as well as the thickness of soft or rocky cuts. The latter is determined every 40 m to ensure better superposition between the tomographic profile (10 m step) and the longitudinal section of the road (40 m step). This large discrepancy may result in the loss of some information about the bedrock profile. A tighter measurement interval for the longitudinal profile of the road may enable a better assessment of the cumulative volume. A study of the cuts using electrical tomography has enabled us to distinguish between rocky and soft nature of each cut along the road. The computer programs most often used, such as those presented by Bere (1989) for calculating earthworks on rural roads, do not distinguish between soft and rocky volumes. The use of electrical resistivity tomography coupled with the longitudinal section of the alignment, as demonstrated in this study, can therefore help to better establish a preliminary design, and finally to make the right provisions for the alignment of road sections.