The western end of West Africa was, at the end of the Tertiary and the beginning of the Quaternary, the scene of an important basaltic volcanism [1] . This part of Senegal has a wide variety of rock combinations (tuffs, dolerites, basanites and basalts) of different nature, petrographic, mineralogical, physical and mechanical characteristics. The Tertiary and Quaternary volcanic formations of the Cap-Vert peninsula have been the subject of previous researches that have identified, mapped and helped to understand their modes of emplacement [1] - [6] . However, there are fewer geomechanical characterization studies on the area. The Dakar region is located in the Cap-Vert peninsula and covers an area of 550 km², or 0.28% of the national territory. It is located between 17˚10 and 17˚32 west longitude and 14˚53 and 14˚35 north latitude. It is a region that is bounded on the east by Thiès and by the Atlantic Ocean in its northern, western and southern parts [7] . The developing city of Dakar, capital of Senegal has experienced a construction and infrastructure boom over the last two decades. The basanites are used both as building materials and as foundation support. Volcanic boulders are considered as one of the facies corresponding to the shallow geotechnical components in some municipalities of Dakar. This situation presents many challenges in the construction process as recognized by geotechnicians working in this area. These volcanic facies must meet specific criteria of properties and characteristics to be used as foundation material. The behavior of any civil engineering structure depends largely on the strength and deformation properties of the geo-materials. Sarr et al. (2020) [8] set the basis for the study of the behavior of foundations on these basanites and other rocks through a geomechanical study in the western part of the Cap-Vert peninsula. It is important, whatever the geomechanical use that one wants to make of them, to know the geotechnical parameters of the facies in place. This geotechnical knowledge will be achieved by determining the physical, mineralogical and mechanical properties of basanites. Figure 1(b) below shows the location of the sampling points after a core drilling campaign. All these samples were packaged, labelled and sent to the laboratory for analysis. This article presents the results of the study on the mineralogical, physical and mechanical characterization of Quaternary basanites from the Cap-Vert peninsula of Dakar, Senegal.

The study area is located in the Dakar region, specifically at the tip of the Cap-Vert peninsula. The Cap-Vert peninsula constitutes the western end of the Senegalese-Mauritanian coastal basin located on the western of the West African craton. The climate is a Sudano-sahelian type, with an alternating rainy season (from July to October) and a relatively cool, windy dry season (from November to June). Its temperature varies within 17˚C - 25˚C from December to April and 27˚C - 30˚C from May to November [7] . From a geomorphological point of view, the Cap-Vert peninsula is characterized by a West-east profile of two domes framing a depression. The head of the Cap-Vert peninsula (Dakar region) is dominated by two hills, the largest of which reaches 105 m, and the Horst de Diass, which forms a slightly accentuated relief of about 100 m in altitude and plunges northward toward Kayar. United by a depressed area, the Gabren of Rufisque of low altitude (20 to 30 m) [6] . Inside the peninsula, the relatively flat relief is formed by clayey sands and sand dunes or the volcanic plateau upon which, a part of the city of Dakar is built; making it difficult to access the volcanic formations. The geological history of the study area is integrated into the overall geology of the Cap-Vert peninsula, which was the site of Tertiary and Quaternary volcanism (Figure 1(a)). Tertiary volcanic formations are scattered throughout the Cap-Vert peninsula and the Thiès region, while Quaternary formations are confined to the tip of the Cap-Vert peninsula [4] [5] [6] . Three successive volcanic units are interbedded in the sands, from bottom to top:

· A lower volcanic unit consisting of tuffs and highly altered lavas;

· A middle volcanic set/unit, located in the median part of the sands. It is represented by flows of dolerite and basaltic;

· An upper volcanic set to which our study refers is located at the top of the sands. It corresponds to the Mamelles volcano and to the flows which came from it.

Laboratory tests such as physical and mechanical properties tests require good quality sample. The basanites tested are either altered, vesicular or sound from Quaternary volcanism.

Cylindrical samples of 80 mm and of the same height were prepared for petrographic analysis, porosity, dry density, methylene blue value and a slenderness of 2 for simple compressive strength according to the recommendations of the International Society of Rock Mechanics (I.S.R.M.). The thin plates sections are made in the laboratory of the Institute of Earth Sciences (IST) of Cheikh Anta Diop University of Dakar with several instruments dedicated to cutting, polishing and manufacturing. The process of realization of the thin plates, as follows:

· Cutting of the rock with a chainsaw equipped with a diamond saw

· Preparation of the glass slide by roughening one side of the slide and then labelling the slide

· Polishing of the pseudo-planar face of the sample to obtain a flat surface

· Drying of the sample on the hot plate

· Preparation of the resin to glue the piece of rock

· Put the resin on the glass slide then carefully place it on the sample to avoid bubbles and let it rest for at least 24 hours so that the glue set

· Cutting the basanite sugar to obtain a slide with a thickness lower than 100 µm

· Thinning of the slide by polishing with silicon carbide powder.

The dry weight of the rock was obtained by direct measurements of the volume and weight of the samples after drying in an oven at 105˚C for 12 hours to the guidelines of the NF P94 064 standard.

γ = w v (1)

With

g: Density (kN/m³)

W: Weight of dry material (kN)

V: Volume of the sample (m³)

The porosity was measured by the dry density and saturation method [3] [10] . The principle of this technique is to first weigh the sample; then dry it in the oven and finally soak it. It is then weighed when all voids are filled with water (by water saturation for 72 hours in a water bath set at 20˚C (Figure 2(b)). After saturation, the samples are quickly wiped with a clean cloth and weighed again.

n = 100 × m w − m s m s (2)

With

n: porosity (%)

m_s: mass of dry material (Kg)

m_w: mass of the saturated sample (Kg)

The methylene blue value is determined according to the guidelines of the NF P94 068 standard. It is used to quantify the state of alteration of the rocks made at the end of the petrographic study. The test consists in measuring by dosage the quantity of methylene blue which can be adsorbed by the material in suspension in water. This quantity is related by direct proportionality to the 0/5 mm fraction of the fine material. The test sample is 60 g and is introduced into a 100 ml solution of blue solution. The suspension of the material is subjected to a permanent agitation of 400 revolutions per minute. The rest of the test consists of injecting 5 ml of methylene blue and gradually testing the stain on the filter paper. The stain thus formed is composed of a central deposit of dark blue

colored material surrounded by a colorless wet zone. Proceed with successive injections in steps of 5 ml to 10 ml (depending on the blue solution) until a light blue peripheral halo appears. The test is then said to be positive from that moment on. The execution of the blue test is fast and does not require expensive specific equipment (Figure 2(c)). The methylene blue value VB is determined by the following relation:

V B = B m 0 × 100 B = V × 0.01 (3)

With

m₀: mass of dry basanite

B: mass of blue introduced (10 g/l solution)

V: volume of blue introduced

VB: is expressed in grams of blue for 100 g of dry material

In addition to the identification tests, the simple compressive strength and the Young’s modulus are determined according to the NF P 94 420 standard. The experimental device used is a 1500 kN press from TECNOTEST. The monitoring of the deformation during the test is carried out with the help of a displacement sensor equipped with a manometer of allowing to measure the axial displacement in the central third of the specimen.

R C = F A (4)

With

F: is the value of the applied force

A: is the section of the rock specimen

Several authors have carried out tests [11] [12] [13] on the influence of porosity on the physical and mechanical parameters of geometries. We have conducted similar studies between porosity and strength on about twenty basanite samples. The results were analyzed for healthy basanites, with an absence of vesicles (low porosity basanite) or with a low percentage of pores, and on basanites with vesicles (very porous to porous basanite).

Figure 5(a) and Figure 5(b) show that the compressive strength decreases with the porosity of the rock. The correlation coefficient is 0.5644 which corresponds to an average correlation for basanites with few or no vesicles. We notice a gradual decrease in simple compressive strength with increasing porosity. In the case of the vesicular basalts, with an alteration, one notices a big dispersion. The correlation coefficient is 0.2423 which corresponds to a weak correlation. This dispersion of results with a poor correlation between these two parameters is related to the neoformation of clays that come to fill part of the voids. They decrease consequently the porosity.

In this paper, the olivine basanites of the Cap-Vert peninsula of Dakar were geotechnically characterized. A series of physical (porosity, dry density and methylene blue value) and mechanical (uniaxial compressive strength and Young’s modulus) measurements were performed. The physical studies showed that the porosity of the rock is variable according to its texture, its degree of alteration and the presence of vesicles. As for the mechanical results which concerned the determination of the resistance to compression and the module of elastic deformation (Young’s modulus) on cylindrical specimens, these measurements revealed that the various series of basanites present a behavior of elastoplastic type. The simple compressive strengths show that the basanites are of low to medium quality. These results corroborate perfectly the structuring of the basanites of the study areas. Indeed, the results of the physical properties and the visual observations show that the altered basalts with vesicles (pores) lead to a low value of compressive strength and therefore affect the mechanical parameters. The analyses also show a decrease in compressive strength with porosity and alteration. This is related to the neoformation of the clay which gradually fills the voids.

Authors would acknowledge Technosol-ingénierie for its financial and technical support.

The authors declare no conflicts of interest regarding the publication of this paper.

Sawadogo, M. and Sarr, D. (2023) Physical and Mechanical Features of the Quaternary Basanites of the Cap-Vert Peninsula of Dakar (Senegal, West Africa). Geomaterials, 13, 124-138. https://doi.org/10.4236/gm.2023.134009

NF P94-064-2. (Mai 2001) Essais pour déterminer les propriétés physiques des roches Partie 2: Détermination de la masse volumique-méthodes géométriques et par immersion dans l’eau. Norme Française.

NF P94-420. (Mai 2000) Détermination de la résistance en compression uniaxiale. Norme Française.

NF P94-068. (1998) Mesure de la capacité d’adsorption de bleu de méthylène d’un sol ou d’un matériau rocheux. Détermination de la valeur de méthylène d’un sol ou d’un rocheux par l’essai à la tâche. Norme Française.