The rocks and soils underlying Lambata-Minna and Minna-Bida roads in central Nigeria were mapped with the view to determine their impact on the stability of the roads underlain by them. Vertical electrical sounding was done along the roads to determine the soil profile of the roads and statistics of the roads utilization was also done to infer if the roads are overused by vehicles. The geophysical studies revealed that the soil profiles of the two roads are composed of laterite, sand and clayey soils while the road statistics revealed that the traffic densities in the two roads are within the permissible limits. A total of 5 and 4 rock samples were collected from Lambata-Minna and Minna-Bida road respectively and each subjected to thin section, XRD and XRF analyses. The thin section and XRD were used to determine the mineralogical composition of the rocks while the XRF was used to determine the chemical composition of the rocks. Thirty five (35) and twenty eight (28) water samples respectively were collected from wells along Lambata-Minna and Minna-Bida roads and subjected to hydrochemical tests to determine the variations in the ionic concentration and physical properties of the groundwater with the underlying lithologies. A total of 47 and 60 soil samples respectively were collected along the same roads and each subjected to grain size distribution test using wet sieving to determine the soil group dominating the soil occurring on each lithologic unit. Thirty nine (39) and fifty (50) samples from the above were selected and subjected to Atterberg limit tests to determine their plasticity. Twenty two (22) and nineteen (19) samples from the above were subjected to compaction, permeability and California bearing ratio (CBR) to ascertain which of the soils are suitable for sub-grade, sub-base or base material. The field mapping, thin section and XRD results revealed that Lambata-Minna road is underlain by migmatites, gneisses, granites, marble, granodiorite and schist while Minna-Bida road is underlain by granites, migmatite, schist and sandstone. The chemical compositions of the rocks indicate they are mostly acidic rocks/protholith. The physico-chemical tests revealed that the groundwater occurring within the sandstone terrain of Minna-Bida road has the least ionic concentration and physical properties while that occurring within migmatite/schist terrain along Lambata-Minna road has the highest ionic concentration and physical properties. The grain size distribution test revealed that the soils occurring within the sandstone terrain are composed mostly of sandy soils (SW and SP) while those within the migmatite gneiss and granite terrain along Lambata-Minna road are gravely (GW and GP) soils. Permeability of the soils ranges from 4.73 x 10-4 to 9.78 x 10-3 cm/s. The compaction test revealed that the soils occurring within the sandstone terrain along Minna-Bida road has optimum moisture content (OMC) ranging from 9.4 to 18.0%. The OMC of the migmatite gneiss and granite terrain along Lambata-Minna road ranges from 18 to 27% while those within granite terrain ranges from 15 to 18%. The soaked CBR of soils within the sandstone terrain along Minna-Bida road ranges from 45 to 95% while those within the migmatite gneiss and granite terrain along Lambata-Minna road ranges from 0.9 to 70%. The unsoaked CBR of soils within the sandstone terrain along Minna-Bida road ranges from 70 to 144% while those occurring within the migmatite gneiss and granite terrain along Lambata-Minna road ranges from 5 to 70%. Results of the permeability tests revealed that permeability of the soils is generally low and does not vary with the different underlying lithologies. Grain size distribution, Atterberg limits, compaction and CBR reveal that soils underlying Minna-Bida road are generally more competent than those underlying Lambata-Minna road. The results also show that soils occurring within the sandstone terrain along Minna-Bida road are more stable than other portions of the studied roads and can satisfactorily serve as road sub-grade and sub-grade in their natural state. The consistent failure of the Lambata-Minna 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 sub-grade, sub-base or base material in their natural state.
1.1 Background Information
Engineering geologists and geotechnical engineers are integral parts of design team for virtually all civil engineering projects like roads and buildings that involve site characterization and geotechnical design. To understand the geologic conditions of a site for civil engineering project and their implication(s) in design criteria, a common understanding of the site geologic origin and geotechnical properties of the construction aggregates to be used in the civil engineering project is essential. Generally, the engineering geologist provides basic information for the planning of land-use and for the design, construction and maintenance of civil engineering works. Such information is needed to assess the feasibility of a proposed land-use which will in turn assist in the selection of the most appropriate type and method of construction in order to ensure the stability of the intended structure as well as aid in the performance of necessary maintenance. Engineering geological research and mapping are therefore mainly directed towards understanding the interrelationships between the geological environment and the engineering structure; the nature and the geological relationships of individual geological components; the active geodynamic processes and the possible effect(s) that can result from the changes being made.
In Nigerian highway construction, attention is mostly paid to the sampling and testing of the aggregates to be used as the base/sub-base and wearing surface of the pavements as well as those serving as sub-grade. However, irrespective of billions of Naira spent by Nigerian government on the design and construction of these roads, most of the constructed roads do not render satisfactory service. The cause of these road failures can be attributed to the neglect of the role of geologists/engineering geologists in highway design by Nigerian contractors/civil engineers. For example, in choosing the route for the highway, one of the important factors to be considered is geology (lithostratigraphy and hydrogeological conditions) of the intended route. Such information can only be confidently ascertained by a geologist/engineering geologist.
Research has shown that the stability of pavements constructed in tropical regions is controlled mostly by the geological/hydrogeological, soil, climatic and drainage conditions of the terrain as well as the design technique, type of aggregates used, construction procedure and age of pavement (Clare and Beaven, 1962; Tanner, 1963; Gidigasu, 1974, 1975, 1983; Okagbue and Uma, 1988). Works by Weinert (1968), Farquhar (1980), Okagbue and Uma (1988) have shown that geological conditions along a highway route are important factor to the proper performance of the highway. Since roads are built on and with geologic materials (rocks and soils), a good knowledge and understanding of these materials is therefore vital for the successful construction of roads.
1.2 DESCRIPTION OF STUDY AREA
1.2.1 Location and Accessibility
This study was done in North-central Nigeria in the area lying between longitudes 6o01’E and 7o00′ E and latitudes 9o05I N and 9o 36ˈN (Fig. 1.1). The study was centered along Lambata – Minna Road (LM-R) and Minna – Bida Road (MB-R). The Lambata – Minna Road (LM-R) is a trunk A road constructed/maintained by the Federal government that links most of the North western state to the Federal Capital Territory. The Minna – Bida Road (MB-R) is a trunk B road constructed/maintained by the Niger State Government. The Lambata – Minna Road (LM-R) covers a linear distance of 76 km and the Minna – Bida Road (MB-R) covers a linear distance of 84 km. Some important towns along the road traversed are Bida, Mina, Kataeregi, Mingida, Guto, Pita, Lambata and Gawu.