Aspects of Geology of Ganawuri Area of North Central Nigeria: Evidence from Field, Petrographic and Geotechnical Studies

Geological and geotechnical investigations involving detailed geologic mapping and composite soil sample laboratory tests respectively, have been carried out at Ganawuri, North Central Nigeria with a view to delineating the different rock units in the area and their Engineering significance. The area is underlain by the Crystalline Basement rocks composed of Granite Gneiss with late Diorite, Basalts and Pegmatite intrusions. The older units have been intruded by the Biotite Granite of the Younger Granite province, occurring as a ridge bordering the northeastern margin of the area. Structures observed in the area include fault/shear zones, pegmatite dykes, basaltic dykes and fractures. The dominant trend is in the NE-SW direction with local deviations in NNE-SSW, WNW-ESE, NW-SE and NNW?SSE. The joints are generally vertical with few dipping steeply to the SE or SW. Thus it is evident that the early NNW-SSE trending foliation in the granite gneiss guided the regional structural development in the area. This feature thus controlled all the other structures (anticline, shear zones, fractures and the river channel), all of which are oriented in the same direction. This dominant NE-SW trend may be associated with development of the Benue Valley of Nigeria. Composite soil samples were obtained at different points around the area and laboratory analysis carried out on them. Using the Unified soil classification system (USCS), soil sample I was classified as SM soils with zero plasticity, samples 2 and 3 soils were classified as SC, CL and SC, OH soils with medium and high plasticity respectively. Results also showed that while sample 1 soil has low potential expansiveness, samples 2 and 3 soils have medium and high potential expansiveness respectively. The Specific Gravity values range from 2.53 to 2.64 and decreases towards the Northeast. The Natural Moisture Content and Coefficient of Curvature variations in the area show similar trend. Both decrease towards the Southwest and have values ranging from 4 to 12.5% and 0.86 to 1.09 respectively. The Uniformity Coefficient increases towards the south and decreases towards the Northwest with values ranging from 11 to 19.5. The 3-D display of both the Maximum Dry Density (MDD) and Optimum Moisture Content (OMC) shows opposing trend in variations as MDD decreases towards the North while OMC increases towards the North. In both soil samples, the values of the Skewness lies between 0.5 to 1.0 and as such can be said to be positively Skewed (strongly fine Skewed). Moreover, the value of Kurtosis for the three samples lies between 1.11 to 1.66 and can be said to be Leptokurtic. They can be said to be well to moderately well sorted because of the range of values for sorting (Standard Deviation) lying between 0.35-0.71 indicating competence for engineering works. Based on the investigations carried out, it is suggested that further analysis through coring at the shear zones be carried out and consolidation grouting may be needed to reduce deformability and increase mechanical strength at these zones.