Characterisation of soils derived from different parent rocks from north central Nigeria

Abstract

Mineralogy and geotechnical properties of residual lateritic soils derived from sandstone and migmatite–gneiss (MG) were compared. The aim was to determine the influence of the parent rock geology of sandstone and MG on their engineering properties. This was done using the statistical method of Student's t-test. Thin sections show that the MG samples are rich in feldspars and micas with up to 45 and 23% modal estimates, respectively. The dominant clay mineral in the two soils was kaolinite, while the dominant oxides were silicon dioxide, aluminium oxide and ferric oxide. The silica/sesquioxide ratio of the MG-derived soils (MGS) ranged from 1·7 to 3·2 while those of sandstone-derived soils (SS) were 2·9–6·6. SS samples which contained essentially quartz grains exhibited better compaction characteristics, higher California bearing ratio and lower plasticity than MGS. Better engineering properties exhibited by SS can be attributed to the high quartz content present in the parent rock. Feldspars and micas present in MG are weathered into plastic and hydrophilic clay minerals. These are likely to have a negative impact on the engineering properties of the derived soils. Statistical treatment of all determined engineering parameters showed significant differences in all cases except in relation to specific gravity, permeability and compressibility.