Assessment of Foundation Instability Using Integrated Geotechnical and Geophysical Techniques: A Case Study of Giza, Keana LGA, Nasarawa State, North Central Nigeria
Engineering structures such as buildings, in Giza community (central Nigeria) have developed severe cracks, undergone differential settlement, and failed in some parts. These have been a disturbing situation to its inhabitants. Integrated geophysical, geotechnical, and hydrogeological techniques were employed to investigate the causes of this menace. Vertical Electric Sounding (VES) was used to delineate the subsurface geo-electric layers. Soil samples were analyzed for grain-size distribution, Atterberg Limit, moisture content, specific gravity, compaction, coefficient of permeability, and undrainedtriaxial test. Static water level measurements were carried out during wet and dry seasons to establish the zone of groundwater fluctuation. VES results revealed that foundations within the cracked zone are underlain by incompetent, low resistivity (2.77 – 24.8 Ωm) saturated clay (0.5-3.1m thick). The non-cracked zone is underlain by moderately competent, clayey sands (3.5-6.9m thick). Geotechnical results of the cracked zone revealed high plasticity (29-51%), high moisture content (10.11-12.02%,), and low permeability (4.26×10-5-5.36×10-7m/sec) which impedes drainage. Maximum dry density ranges from 1.76 – 1.88g/m3 with corresponding optimum moisture content of 10.11 – 12.02%. Cohesion contributes more to the shear strength than angle of internal friction in the cracked zone, compared to the non-cracked zone, which shear strength is controlled by both. The groundwater fluctuation zone was found to be 1.1-6.2m and within the clay layer. The shallow zone of fluctuation saturates clays under foundation making it highly plastic. The soils experiences swelling and shrinking as water level rises and falls, which cause buildings to heave, crack, or settles differentially leading to failures.
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