ENGINEERING GEOLOGICAL CHARACTERISTICS OF RESIDUAL SOIL DEVELOPED ACROSS A MAJOR NONCONFORMITY AT LOKOJA AREA, NORTH CENTRAL NIGERIA

Abstract

The engineering geological characteristics of residual soils of Lokoja area was investigated to determine the influence of local geology on the index and strength properties of the soils with a view to determining the ultimate bearing capacity of the soils as foundation materials in civil engineering works. Thirty four samples, which are made up of twenty five disturbed and nine undisturbed samples, were collected from ten test pits at depths of 1.5m to 2.0m. The tests conducted on the disturbed soil samples were atterberg limits, grain size analyses, specific gravity, standard compaction, consolidation, quick undrained triaxial test and California Bearing Ratio tests. Those that were carried out on undisturbed samples were Natural Moisture Content, Bulk Density and Dry Density tests. The study area is underlain by migmatite-gneiss, diorite, pegmatite, quartz schist, undifferentiated older granite, mainly porphyroblastic granite, obajana gneiss member and the Cretaceous sediments which are part of the southern Bida Basin. The natural moisture content value obtained for the soil samples ranged from 5% to 21%, specific gravity (Gs) ranged from 2.62-2.78, liquid limit values ranged from 18.9 to 63.7, plastic limit values varied from 0 to 30.2, plastic index values ranged from 0 to 35.1 while linear shrinkage values varied from 0 to 17.8. Dominant soil types in the area were CL and ML groups based on USCS classification and A-3 soil group dominates based on AASHTO classification system. Grain size analysis shows that gravel fraction ranged from 0.0 to 60.0%, sand fraction ranged from 29.6 to 77.6%, clay fraction ranged from 0.2 to 45.1% and silt fraction ranged from 4.2 to 39.1%, The MDD ranged from 1,873kg/m3 to 2,189kg/m3 and the OMC varied from 8.8% to 19.2%, the final void ratio varied from 0.264 to 0.593, the coefficient of consolidation varied from 0.01049 to vii 0.02078, the rate of settlement ranged 0.1103 to 1.646, the compression index varied from -0.0051 to 0.1372, Swell index values ranged from -0.00889 to 0.06921, the coefficient of volume compressibility ranged from -0.03652 to 0.62586, recompression index varied from -0.004 to 0.082, and the coefficient of compressibility ranged from - 0.04616 to 1.02597, the angle of internal friction values ranged from 30.3o to 39.8o, the cohesion varied from 59.6KPa to 138.9KPa, CBR values ranged from 16% to 94% for unsoaked CBR and 7% to 91% CBR for soaked CBR (r = +77). There is a positive and strong correlation between clay content and settlement, cohesion and shear strength (r = +99), and there is also a positive but weak correlation between angle of internal friction and percentage sand (r = +42). However there is a strong but negative correlation between MDD and clay content (r = -74), CBR and clay content (r = -73), coefficient of consolidation and void ratio (r = -98). The test showed that the subsoils were generally made up of granular materials and have appreciable amount of sand percentage. This confirms that the soils ranged from good to excellent materials that are capable of bearing very substantial loads