Caractérisation minéralogique, géochimique, géotechnique et stabilisation à la chaux des graveleux latéritiques de la zone d’Akonolinga (Sud - Cameroun)

Abstract

The purpose of this work is to evaluate the influence of metamorphism degree of schistose rocks on the geotechnical properties of lateritic gravels, and to study the behavior of these materials after lime addition, for road construction purposes. After a detailed macroscopic description of the studied materials, samples were run for petrological and geotechnical analyses. The mineralogy was determined by optical microscopy and X-ray diffraction. The major element concentrations were obtained by X-ray fluorescence and, FeO concentrations, by titrimetry. Geotechnical analyses focused on identification and compaction tests. The mechanical tests were carried out only on lime-lateritic gravel mixtures. The results obtained show that lateritic gravels developped on low metamorphism degree rocks (chlorite schists) are thicker and more quartzose than those derived from average metamorphism degree rocks (mica schsits). The chlorite schists and mica schist-derived lateritic gravels are made up of quartz, kaolinite, hematite, goethite and muscovite. Quartz content is higher in chlorite schists derived lateritic gravels than those from mica schists. Those materials are kaolinitized, however, chlorite schist-derived lateritic gravels present an average degree of lateritisation, while mica schist-derived ones are slightly to moderately lateritized. The behavior of geochemical parameters and geotechnical parameters is different in the studied lateritic gravels. In chlorite schist-derived lateritic gravels, the Californian bearing ratio parameter (CBR), is related to relative lixiviation potential index by an inverse linear relation. The CBR index therefore decreases with the increase of the leaching in these materials. The nuances highlighted in the pedogenetic evolution of the low and average metamorphism rock derived lateritic gravels are confirmed by the statistical Agglomerative Hierarchical Clustering analysis. Chlorite schist-derived lateritic gravels are mainly characterized by granulometric and plasticity parameters (skeleton = 48.27%, mortar = 36.31%, fines = 27.63%, plasticity index (PI) = 29%, grading modulus (Gm) = 1.88, maximum dry density (MDD) = 2.00 Mg/m3 , CBR = 44%), while those developed on mica schists are characterized by physical and compaction parameters (skeleton = 34.82%, mortar = 25.45%, fines = 17.90%, PI = 18%, Gm = 2.22, MDD = 2.13 g/cm3 , CBR = 65%). The mica schist-derived lateritic gravels are usable as subbases for any type of traffic and as bases for low volume traffic, while, those developed on chlorite schists can be used as subbases for any type of traffic. Chlorite schist-derived lateritic gravels, more weathered, require more lime (> 6 wt.%) for bases compared with mica schist-derived lateritic gravels (2 – 4 wt.%). The higher the degree of metamorphism of the parental rock, the better the geotechnical proprieties of the lateritic gravels generated.