Hydrological, mineralogical and geotechnical characterisation of soils from Douala (coastal, cameroon): potential used in road construction

The life span of pavements in coastal areas is diminutive due to high traffic levels. In this study, soils from different localities (Dibamba, Bekoko and Lendi) in the Douala coastal area were investigated, for their hydrological, mineralogical and geotechnical properties. This is to determine their potential used as raw material in the sub-base or base layer for road construction. Hydrological analysis of data for the past twenty years revealed that the average annual temperature stood at 27.5 °C with average precipitation of about 433.1mm. Average relative humidity was 82% throughout the year. XRD/XRF analysis were carried on the different soil samples and revealed quartz ranges from 52 -72%, kaolinite (20-35%); goethite (4-5%), anatase (3-4%) and feldspar (0-4%). The mineralogical content of these soils was similar to silico-aluminous-ferruginuos-titanous soils characterized by high silica content (43.52-66.4%), moderate alumina (13.5-31%), low content of Fe₂O₃ (2.5-3.2%) and TiO₂ (1.2-2.02%). The silica/sesquioxide ratio of the various sample ranged from 1:1 to 1:3 indicating that the samples from Bekoko are non-lateritic, those of Dibamba are lateritic while the soils from Lendi are true laterite soils. The laterites are suitable for the sub base layer of road pavements. Specific density of investigated samples ranges between 2.12 and 2.32 g/cm³ a result of low degree of laterization. The percentage of clay in the fine fraction for samples from Bekoko, Dibamba and Lendi stood at 24.5%, 19.0% and 14.9% respectively. These values are lower than 35% which is the standard value recommended for sub-base material by the CEBTP. The Atterberg limits range from 11.50 to 22.4% hence samples from Lendi are the low plasticity while those of Bekoko and Dibamba have a moderate plasticity with low swelling potentials. Soil samples from Dibamba and Bekoko were of the A-7-6 group while Lendi soil samples were of the A-7-5 group which correspond to clayey soils and hence classified as poor materials for road construction according to American Association of State Highway and Transportation Officials (AASHTO) classification. The Unified Soil Classification System (USCS) revealed that, the majority of samples collected in the study area belonged to the SM group (silty sand) except DIB1 and LEN1 samples which pertain to SC group (clayey sand). The average of the maximum dry density (MDD) values stood at 1.9 g/cm³ with an average moisture content of 11% corresponding to the lower limit recommended for the use as sub-base materials. The value of CBR range between 8.6 and 28,2% which correspond to class S4 and can be used as raw material for the foundation layer of low traffic (T1) in the CEBTP standard. If those materials are treated with coarse gravel, cement and/or bitumen, they can be used on sub-base and base layer of high traffic (T2/T3).