The impact of roads on the socioeconomic development and progress of any country cannot be quantified. A geotechnical investigation of subsoils along Isinbode-Ara road, stretching in southeastern to northwestern direction of 7 km distance within Ekiti State, Southwestern Nigeria, was carried out. This investigation follows the British Standard Institution, such as moisture content, particle size distribution, specific gravity, Atterberg limit, compaction, consolidation and California bearing ratio. Results of investigation showed that the moisture content, specific gravity, liquid limit, plastic limits and plasticity index ranged from 7.2-25.9%; 2.64-2.77; 24.0-61.1%; 19.2-26.2% to 4.35-38.90%, respectively. Grain size distribution showed the fine and coarser fractions range from 13.0-66.5% to 32.6-84.8%, respectively. The maximum dry density and its optimum moisture content ranged from 1.48-2.07 g/cm 3 to 11.3-30.3%, respectively. Soaked CBR results ranged from 3 to 44%. Two classes of subsoils, namely A-2-4 and A-2-6 (granular materials) and A-6 and A-7-6 (clayey soils), were identified and rendered suitable and unsuitable road construction materials, respectively. This investigation revealed that the subsoils are poor road construction materials due to its fine fractions and plasticities, which should be put into consideration during its foundation design and construction stages.

Information on plasticity and consolidation characteristics of soil materials have been found to be profoundly important for preliminary structural planning and designs since these properties influence soils' compressibility and settlement characteristics and, by extension, the performance of any engineering structure on the soils. The dependency or relationship of consolidation characteristics with plasticity characteristics of remolded migmatite-gneiss-derived laterite soils from southwestern Nigeria was statistically evaluated. A total of eight (8) bulk disturbed soil samples were subjected to Atterberg limits tests and one-dimensional consolidation tests following British Standard BS 1377 methods. Further, the results obtained were subjected to regression analysis. The plasticity index (PI), linear shrinkage, and shrinkage index values ranged from 8.0 to 33.65, 4.3 to 10.7, and 19.0 to 50.2, respectively. The average coefficient of consolidation (C v ) and compression index (C c ) values ranged from 0.02184 to 0.03290 mm 2 /min and 0.0203 to 0.0812, respectively. The correlation coefficient of compression index values with plasticity index and shrinkage index values is 0.90 and 0.72, respectively. The coefficient of correlation (r) of the relationship between consolidation coefficient values and consolidation pressure (AP) values of 20-160 kPa (loading session only) is 0.97. The coefficients of correlation (r) of average consolidation coefficient values with linear shrinkage, shrinkage index, and plasticity index values are 0.60, 0.73, and 0.88, respectively. Hence, coefficient of consolidation and compression index have better correlations with plasticity index over shrinkage index and linear shrinkage. Further, the correlation coefficient of plasticity Index (PI) with all the consolidation coefficient values, derived at each stage of effective vertical consolidation pressures (AP) of 20-160 kPa (loading and unloading), is 0.79. Based on these results, correlation equations for predicting consolidation coefficient and compression index were proposed. This evaluation has aided better understanding of the relationship between consolidation and plasticity characteristics of migmatite-gneiss-derived laterite soils and can be applied to several other basement terrains.

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Failure of highway pavement and collapse of building in basement complex of Nigeria is often related to the instability of the residual. This study evaluated the strength characteristics of gneiss-derived residual Soils as materials usable for road pavement structures. A total of eleven soil samples derived from granite gneiss were subjected to laboratory geotechnical analyses based on standard practices. The geotechnical analyses reveal the soils’ natural moisture content, specific gravity, grain sizes, consistency limits, shearing strengths, maximum dry density, and optimum moisture content. Based on AASHTO classification, the soil samples are classified as A-7-6, A-6, and A-7-5. The results of the laboratory analyses revealed that the natural moisture content and specific gravity ranged from 8.30 to 22.70% and 2.6 to 2.8 respectively. Particle size analysis reveals that the coarse contents of the soils ranged from 28.8% to 59.8% and amount of fines ranged from 40.2 to 71.2%. The liquid limit ranged from 31.3% to 68.3%, plastic limit ranged from 20% to 28.0%, plasticity index ranged from 4.8% to 38.90% and linear shrinkage ranged from 5.7 to 13.6%. The maximum dry density ranged from 1481 kg/m3 to 1921 kg/m3 and optimum moisture content ranged from 15.2% to 27.6%. Undrained triaxial shear strength (Cu) ranged from 43.0 Kpa to 250.3Kpa, angle of friction ranges from 11.7 to 29.30, and unconfined compressive strength ranged from 153 to 356.5Kpa. The results indicate that the residual soils are poor sub-grade and foundation materials due to their high amount of fines, linear shrinkage values, plasticity, and swelling potential, as well as low maximum dry density.

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