Impact of geology on the Minna-Bida road,north-central Nigeria

Abstract

The rocks and solids underlying the Minna-Bida road in central Nigeria were mapped with the view to determine their impact on the stability of the road underlain by them. A total of 4 rock samples were collected from the road and subjected to thin section, XRD and XRF analyses. Ground water table of wells along the road were observed to determine the seasonal groundwater variation. Twenty-eight water samples were collected and subjected to hydrochemical tests to determine the variations in the ionic concentration and physical properties of the ground water together with the underlying lithologies. A total of 60 soil samples were subjected to grain size distribution test using wet sieving and fifty of the samples were collected and subjected to Atterberg limit tests. Twenty- two (22) and nineteen (19) of those samples were subjected to compaction, permeability, and California bearing ratio (CBR). The field mapping, thin section and XRD results revealed that Minna - Bida road is underlain by granite, migmatite, schist and sandstone. The chemical compositions of the rocks indicate that they are mostly acidic rocks/protholith. The physico - chemical test revealed that the groundwater within the sandstone terrain of the road has the least ionic concentration and physical properties. The grain sized distribution test revealed that the soil within the sandstone terrain are mostly sandy soils (SW and SP) while those within the migmatite gneiss and granite terrains are gravely (GW and GP) soils. Permeability of the soils range from 4.73 x 10-4 to 9.78 x 10-3cm/s. the soils occurring within the sandstone terrain has optimum moisture content (OMC) ranging from 9.4 to 18.0%, MDD ranging from 1.74 to 2.2, soaked CBR ranging from45 to 95% and unsoaked CBR ranging from 7 to 144%. within the crystoline rocks, MDD range from 1.73 to 2.08, OMC 12.24 to 19.7%, unsoaked CBR 35.90 to 121.8% and soaked CBR 24.19 to 115%. permeability of the soils is generally low and does not vary with the different underlain lithologies. Grain sized distribution, Atterberg limits, compaction and CBR reveal that soils underlain the sandstone are generally more competent than those underlain the crystoline rocks and can satisfactorily serve as road subgrade in their natural state. The consistent failure of the Minna - Bida road portion underlain by migmatite gneiss and granite is attributed to the fact that the soils occurring within those terrains have poor geotechnical properties to serve as either subgrade, subbase or base material in their natural state.