Eights (8) mining pits, which varied in depth between 0.2 and 11 m, have been drilled in the study area. For each pit, bauxite samples of each litho-genetic type were collected at different depth levels. The bauxite samples were recorded and coded according to the pit number and depth of sampling. They were collected based on their color, structure and location within the study area. Figure 1 shows the pits, the litho-genetic type of bauxites in each pit and the sampling procedure, and Table 1 presents samples recorded for each bauxite type and the depth in which they were collected

In this subsection, we determined the water of absorption of the samples recorded in Table 1 . The water of absorption is a physical parameter that gives the value of a product’s susceptibility to let water pass in it once immersed in water. In this work, for all samples, the average weight of fired bauxite is measured, and that, when soaked in water for an hour, is also weighed [5] . We use the following relation computing water of absorption (WA).

$WA\left(\%\right)=\frac{Sw-Fw}{Fw}$ (1)

where Sw is the soaked weight, and Fw is the fired weight of samples.

Mechanical Characterization

Compressive Strength

The compressive strength is determined through uniaxial or triaxial compressive tests. Its value depends on the shape and dimensions of the specimen, its water content, and the loading speed. In this study, as in [12] , we used the ASTM C99/C99M-09 standards for this analysis using Model CAT C46L2 Compressive device. One sample of each bauxite type (see Table 1 ) was subjected to a load between 500 N and 2500 N at a uniform loading rate. The specimen from each bauxite type had dimensions of 1 cm × 2 cm. The compressive strength is the highest stress sustained by the material before failure; its value is computed by Equation (2).

$CS=\frac{P}{BW}$ (2)

where P is the failure load (maximum compressive force applied on Sample), B,

and W are the width and height of the rectangular specimen. Figure 2 shows the schematic representation of a compression test with a specimen of one of the bauxite types.

Dynamic Tensile Strength

As bauxites are often subjected to dynamic loading during mining operations (case of sangarédi bauxite deposits exploitation), in this study, we performed a dynamic tensile test by spalling using a modified Hopkinson bar to evaluate the tensile strength of all the bauxites types in Table 1 . The Hopkinson bar spalling test is dedicated to the evaluation of the dynamic tensile strength of concrete and rock-like materials. The basic principle of dynamic tensile testing by spalling is to obtain a dynamic tensile stress field within a specimen which is loaded by an incident compression wave by the reflection of this wave on a free surface. The dimensions of the specimen are 120 mm long and 46 mm in diameter. The set-up involves a gaz-launcher, a cylindrical projectile with a large radius spherical end cap and a Hopkinson bar ( Figure 3 ). The details of the design of this device can be found in [21] [22] . Tensile strength testing using a hardness tester was recently performed in [23] .

The gaz-launcher releases the projectile that impacts the bar. An incident compressive wave in the form of a pulse A is generated ( Figure 4 ). This pulse is propagated in the bar. When reaching the bar-sample interface, a part of the compressive wave is transmitted to the rock specimen B, while the other part of the wave is reflected in the bar because of the mechanical impedance discontinuity. The transmitted compressive pulse arrives at the free end of the specimen and

is reflected as a tensile pulse. When the reflected pulse exceeds the transmitted one in amplitude, a dynamic tensile loading develops in the core of the specimen, leading to its possible fragmentation [21] . Two strain gauges were placed directly on the bauxite specimen, respectively, at 100 mm and 50 mm from the free end of the sample ( Figure 3 ) in order to monitor the time evolution of axial strain and deduce the axial stress before tensile damage (according to elastic theory) at two different locations in the specimen ( Figure 3 ). A laser extensometer is also pointed directly toward the free back side of the sample to record the pullback velocity on the rear face.

Many approaches exist in the literature for the determination of the dynamic strength of specimen’s subject to dynamic tensile test by spalling. Here, we used the approach of [24] based on the use of a linear acoustic approximation proposed by [25] . According to this approach, the dynamic tensile strength is given by

$DTS=1/2\rho C_o\Delta V_{pb}$ (3)

where ρ is the material density, C_o is the one-dimensional wave speed of the incident wave, and ΔV_pb is the pullback velocity that corresponds to the difference between the maximum velocity and velocity at first rebound that is caused by tensile damage near the free surface. The compressive wave reaches first the gauge located at 100 mm from the free end and then the gauge at 50 mm from the free end, with a time shift measured so the speed of the incident wave can be estimated.

The results of the Physical and Mechanical Characterization of the bauxite type are presented in this section. Table 1 is completed by adding the water of absorption, compressive strength, and dynamic tensile strength ( Tables 2 - 6 ).

Figure 5 shows the effect of depth of sampling on water of absorption, compressive strength, and dynamic tensile strength of gelomorphic bauxite. From depth interval [0.2 - 1] m ( Figure 5(a) ), we observed an inverse relation between compressive strength and water of absorption (water of absorption increases when compressive strength decreases) until intersection, while from depth interval [1 - 4] m, the two parameters have the same evolution (both increase). At the end of the depth interval [4 - 6] m, they also have the same trends (both decrease). In Figure 5(b) , it is also observed that an inverse relation between dynamic tensile strength and water of absorption from depth interval [0.2 - 1.5] m. Unlike compressive strength, dynamic tensile strength keeps increasing constantly from depth interval [1.5 - 6] m.

In Figure 6(a) , an inverse relation is observed between compressive strength and water of absorption from all de depth intervals. An increase in water of absorption leads to a decrease in compressive strength, while a decrease in water of absorption leads to an increase in compressive strength. The relation between dynamic tensile strength and water of absorption shows the same trends ( Figure 6(b) ).

Figure 7(a) shows an inverse relation between compressive strength and water of absorption from de-depth interval [7 - 8] m. From depth intervals ([6.0 - 7.0] [1.0 - 2.0]) m, the same evolution is observed for the two parameters (increases), while from depth interval [4.0 - 5.0] [6.0 - 7.0] m, the same decreases is observed for the two parameters. At the end, again, an inverse relation is observed from depth interval [0.4 - 1.0] m. Figure 7(b) shows the same trends between dynamic tensile strength and water of absorption.

For the pseudomorph bauxites deriving from aleuro-algilites, compressive

strength and water of absorption have both a linear decrease over depth interval [6.0 - 7] and [5.0 - 6.0], while they have an inverse relation over depth interval [5.0 - 6.0] and [1.0 - 2.0] ( Figure 8(a) ). In Figure 8(b) , it is observed the dynamic tensile strength and water of absorption have trends contrary to those in Figure 8(a) . An inverse relation over depth interval [6.0 - 7] and [5.0 - 6.0] (a decrease of water of absorption leads to an increase of dynamic tensile strength). They have a linear decrease over depth interval [5.0 - 6.0] and [1.0 - 2.0].

Ferruginous laterites of the cover are a rock of the transition zone between topsoil and bauxite types. Because it is found on the first layers after the topsoil, it can only be collected from a depth interval of [0.2 - 1] and [0.3 - 1]. An inverse relation between compressive strength and water of absorption is observed from all the depth intervals. A decrease of water of absorption leads to an increase of compressive strength ( Figure 9(a) ). The dynamic tensile strength and water of absorption relation show the same behavior ( Figure 9(b) ).

From Tables 2 - 6 , we represent the comparison of the physico-mechanical characteristics of the different bauxite types and transition zone rock studied. Figure 10 shows this comparison.

As it can be observed, the Ferruginous laterites of the cover (transition zone rock) have the lowest water of absorption value. This is explained by the fact it is the rock that occupies the first upper layer after the topsoil, so it lets surface water pass toward the water Table. It contains greater porosity compared to bauxites. The Ferruginous laterites of the cover also have the highest compressive strength and dynamic tensile strength value (see Figure 10(b) & Figure 10(c) ). This is because of the inverse relation between water of absorption and compressive strength and dynamic tensile strength, as illustrated in Figure 9(a) & Figure 9(b) .

The Gelomorphic bauxites have the highest compressive strength value ( Figure 10(b) ), while Gelified bauxites deriving from aleuro-argilites have the highest dynamic tensile strength value ( Figure 10(c) ).