The raw material (mining waste) was taken, in accordance with site practices, from the Hiré gold mine following the geographical location in Figure 1. This mining waste rock is transported to the laboratory for testing.

Figure 1. Geographic location of the Hiré mining waste rock sampling site.

This mining waste rock is transported to the laboratory for testing.

The methodological approach is based on:

Particle size analysis, which involves passing a representative borrowed sample of the waste rock through stacked sieves with opening diameters decreasing from top to bottom. The largest particles therefore remain trapped on the larger diameter sieves to produce the rejects or retained particles, while the finer particles move to the smaller diameter sieves and constitute the passing particles. When the masses retained on each sieve become constant, the sieving process is complete, and all the rejects are weighed. The mass of each reject is then compared to the initial total mass of the sample collected in the laboratory, which allows the calculation of the cumulative passing reject percentages. The results are plotted on a semi-logarithmic graph using an arithmetic scale ranging from 0% to 100% on the ordinate and modules expressed on a decimal logarithmic scale on the abscissa for small values ranging from 0.2 × 10⁻⁶ meters to 2 meters where they construct a granulometric curve according to in Figure 2.

Figure 2. Grain size distribution curve of waste soils [3].

From this grain size distribution curve, the HBR (High Research Board) classification of mine waste rock is performed, the designation of waste soils is identified, and the curvature coefficient Cc = (d30)²/((d60)d10)) and the uniformity coefficient Cu = ((d60))/((d10)) are calculated to characterize the rigid internal stability of mine waste rocks [4][5]Table 1.

Table 1. Soil designations according to soil type proportions.

Determination of the Atterberg limits (liquidity limit LL using the Cassagrand cup, plastic limit LP using a roller [6]. The plasticity index IP is the difference between LL and LP, i.e., the extent over which the mine waste soils are malleable. It characterizes the water content of the waste rock [7].Physical characterization using the modified Proctor method to determine the dry density and optimal water content, followed by the CBR test for the CBR at 95% of the OPM after 4 days of immersion in water, according to standards NF P 94-093 [8] and NF P 94-078 [9], respectively, for the degree of compaction and the CBR bearing capacity of the mine waste soils.

GranulometryoftheHiréminingwasterock

The size distribution of the different grains in the Hiré mining waste rock is shown in Figure 3. The distribution of the proportions of the different grains of the Hiré mine waste soils. The analysis shows that these Hiré mine waste soils are composed of 10% pebbles, 36% gravel, 44% sand and 3% silt. These soils have a dominant coarse fraction (sand and gravel) at 80% with 10% large elements then a small fine fraction (silt and clay) equal to 3%. This grain distribution shows the homogeneous distribution of the different grain sizes. The Hiré mine waste soils can be identified as pebbly gravelly sand with a trace of silt. The determination of the parameters Cc = 1.29 between 1 and 3 and Cu = 12.9 ˃ 6 suggest that the Hiré mine waste gravelly sands have well-nested grains, exhibit high internal stability and natural compaction.

Figure 3. Average particle size curve of Hiré mining waste.

Table 2 shows that depending on the granular composition, there is a low presence of fine matrix in the Hiré mining waste with 3% of silts while the coarse matrix (80%) is dominant and consists of 36% of gravels and 44% of sands. The large elements (pebbles) are in equal proportion to 10%.

Table 2. Grain size values of Hiré mining waste.

AtterberglimitsandWatercontent

Figure 4 represents the diagram for determining the liquidity limit (LL) of the Hiré mining waste gravelly sands which is a constant that allows the soil to pass from a plastic state to the corresponding liquid state on the semi-logarithmic scale at n strokes equal to 25. The water content corresponding to 25 strokes is the liquidity limit LL = 26%. The plasticity limit (PL) on the role of the fine fraction 10 cm long thinned and which breaks around 3 cm in diameter gave the laboratory results with WP = 25%, therefore PI = 5%. The plasticity index (PI) is low for these soils. The Hiré mining waste gravelly sands are moderately plastic and stable to water variation and they can be used as backfill in road technology.

Figure 4. Result diagram of the liquidity limit of the Hiré gravelly sands.

These mine waste rocks are type A-1b soils (gravelly sands) with a moderately plasticity index (PI) of 5% according to Table 3. These Hiré mine waste rocks are identified as pebbly gravelly sands with traces of silt. These gravelly sands have a semi-spread grain size; the Cu value is 12.4 (range 5 to 20) and the Cc value is 1.29 (range 1 to 3). These various results show that these gravelly sands (mine waste rocks) from Hiré have strong internal stability and are well graded. The optimal average dry density is 2.17 g/cm³ with an optimal average water content of 8.6% according to Figure 4. This optimal average water content made it possible to obtain, for each compaction energy at 4 days of immersion in water, the CBR bearing value curves as a function of the dry densities according to Figure 5.

Figure 5. Curve resulting from the variation in water content as a function of the dry density of sterile gravelly sands from Hiré.

Table 3. Values of the Atterberg limits of the Hiré mining waste rock.

The results in Figure 6 show that 95% of the California Bearing Ratio (CBR) of the Modified Optimum Proctor (MOP) after 4 days of immersion in water is on average 39.6 higher than 30. The Hiré mine waste gravelly sands, classified as S5 soil, are suitable for use as backfill for road engineering structures [5]. Indeed, S5 soils are lateritic gravelly type with a CBR greater than thirty. They are suitable for use as road bases.

Figure 6. CBR 95% MOP result curve for the Hiré mine waste gravelly sands.

The Hiré mine wastes are gravelly sand-type soils with some pebbles, including a trace of silt, i.e., 10% pebbles, 36% gravels, 44% sands, and 3% silts. The recommended particle size for the road surface is generally composed mainly of gravel and sand with a lower fraction of fine particles (d < 0.08 mm). A soil with a spread grain size will increase performance in terms of rigidity and resistance indicating a potential for internal stability of this soil to erosion according to [10]. Our work is consistent with that of [11][12]. According to [13], a soil with a CBR between 20 and 50 is a very slightly deformable support, which is confirmed by [14]. The results of physical tests on sterile gravelly sands from the Hiré gold extraction show that these mining wastes can be reused for embankments in road techniques for low to medium traffic, which remains in agreement with the work of [4]. The CBR at 95% MOP greater than 30 and Cc equal to 1.29 is between 1 and 3 and Cu equal to 12.9 greater than 6 would mean that the coarse matrix of the mining waste of Hiré is well graded which reflects an internal stability in terms of sufficient rigidity and our results are similar to those of the work of [15] on the valorization of laterites of Côte d’Ivoire. The absence of clay and the weak fine matrix shows that the sterile gravelly sands of Hiré do not retain water they are draining with a low water content 8.6% and a low plasticity index PI 5%. From a plasticity point of view, the soils studied are not too sensitive to water, therefore they could be useful in road construction. The distribution of grains in a soil influences the rigidity of this soil. Our results corroborate those of the work of [16].