Geological identification, geotechnical and mechanical characterization of charnockite-derived lateritic gravels from Southern Cameroon for road construction purposes

Geological analyses have been associated with geotechnical and conventional mechanical tests to study charnockite-derived lateritic gravels from southern Cameroon for road construction purposes. The results show that the studied lateritic gravels consist of quartz (9.0-31.4 wt%), kaolinite (15.2-27.7 wt%), muscovite (13.2-30.2 wt%), goethite (10.4-24.0 wt%), hematite (7.4-14.9 wt%), gibbsite (9.0-15.8 wt%), ilmenite (2.8-3.3 wt%) and anatase (4.3-5.5 wt%). These ferrugino-silico-aluminous materials are moderately to highly altered, averagely indurated and constituted of either “true” laterites, lateritic materials, or non lateritic materials, depending on the site. Values of the chemical index of alteration (98-100) and that of the relative potential index of lixiviation (77-94%) consistent with the mineralogical determinations indicate that kaolinite is the only clay mineral in these materials thus excluding any swelling phenomenon. Geotechnical and mechanical tests conducted showed that those materials present: fine particles (16.20-44.10%), plasticity index (26-55%), Californian bearing ratio (31-68%), compressive strength (0.88-1.20 MPa), indirect tensile strength (0.07-0.15 MPa). Results of geotechnical and mechanical tests combined with that of geological analyses showed that the studied lateritic gravels are suitable as subbase layers for all volume traffic and, as base layers only for low volume traffic in road construction. Nevertheless, one should be very cautious when (1) using silica/sesquioxide ratio, (2) deducting the swelling potential εs from clay fraction vs. plasticity index diagram or from the liquid limits and the percentage of fines. These relations are not verified with southern Cameroon lateritic gravel materials.