Geotechnical Assessment of Subsoil in Wuye District, Federal Capital Territory, Abuja, Nigeria for Construction Purposes

Abstract

A detailed assessment of the geotechnical properties of subsoil in Wuye area of Abuja, Nigerian Federal Capital Territory has been undertaken in this study. The analysis carried out on the disturbed soil samples include: Sieve Analysis, Liquid Limit, Plastic Limit, Plasticity Index, Linear Shrinkage Limit, Compaction test, California Bearing Ratio (CBR) test and soil pH, while for the undisturbed soil samples, hydrometer test, direct shear box test and consolidation tests were carried out. The average percentage of fines passing sieve 75 micron is 35.6%. The average liquid limit is 37.8%, the plastic limit is 24.97%, plasticity index is 14.4%, the shrinkage limit is 6.7% while CBR value is 21.11%. The pH of the soil is slightly acidic to neutral and soil acidity weakens foundation over time. The average clay fraction obtained from hydrometer analysis is 20.7%. The dominant clay mineral in the area is kaolinite and it is responsible for the low to medium shrinkage potential of the soil. The average settlement value of 42.2 mm falls within the acceptable settlement value of between 20 to 300 mm for foundations. The average allowable bearing capacity of 3250.3 KN/m3 obtained greatly exceeds the required allowable bearing capacity of 150.0 KN/m3 for shallow foundations. The results show that the study area can be broadly divided into two categories: areas underlain by suitable and competent subsoil that will support any super structure, occupying the north-west and south-east portion of the study area as well as those underlain by less suitable subsoil, covering the south-west and north-east part of the area. The study further reveals that the suitability and the competence of the subsoil is a function of their respective lithology. Based on the results of the investigation, it is recommended that high rise engineering structures should not be located on the regions with low soil bearing capacity in order to avoid failure. Replacement of less competent soil with more competent ones before any construction is advocated.