Variability of geotechnical parameters of lateritic gravels overlying contrasted metamorphic rocks in a tropical humid area (Cameroon): implications for road construction

The variability of the geotechnical properties of lateritic gravels developed on metamorphic rocks from the same clay protolith and of a nearby degree of metamorphism was studied for road construction purposes. Chlorite schist-derived lateritic gravels are more quartzose, more plastic (Plasticity Index PI = 27%) and have a weak Californian bearing ratio (CBR = 50%) compared to those developed on mica schists (PI = 18%, CBR = 62%). The principal components analysis results confirmed by those of agglomerative hierarchical clustering reveal the existence of two groups of materials, the lateritic gravels on chlorite schists (skeleton = 48.27%, mortar = 36.31%, fines = 27.63%, PI = 29%, grading modulus (Gm) = 1.88, maximum dry density (MDD) = 2.00 g/cm³, CBR = 44%) and the mica schist-derived lateritic gravels (skeleton = 34.82%, mortar = 25.45%, fines = 17.90%, PI = 18%, Gm = 2.22, MDD = 2.13 g/cm³, CBR = 65%). Chlorite schist-derived lateritic gravels are mainly characterized by granulometric and plasticity parameters, while those developed on mica schists are characterized by physical and compaction parameters. The mica schist-derived lateritic gravels are usable as sub-base layers for any type of traffic and as base course for low volume traffic. On the other hand, those developed on chlorite schists can be used as a sub-base for any type of traffic and require some treatment prior to a possible use as base layers. The higher the degree of metamorphism of the parental rock, the better the geotechnical properties of the lateritic gravels generated.