High-resolution geophysical imaging and characterization of severe landslides vicinities at South-Eastern Nigeria for uni-vario-seasonal degradation monitoring and civil engineering remediation.
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Date
2021
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Abstract
This research work used high-resolution geophysical methods to explore the near surface of
severe gully-erosion sites. The erosion that mesmerized the environment as landslides is the
major landscaping challenge facing Anambra Basin in the South Eastern Nigeria. The
geophysical techniques used include electrical resistivity tomography (ERT), induced
polarization (IP), vertical electrical sounding (VES), and geotechnical analysis to study the
subsurface conditions of severe gullies. Principally Wenner, Schlumberger, and dipole-dipole
are the geophysical tomography technique used during the survey, depending on the peculiarity
of each selected site. Geotechnical data analysis was used to confirm the results of vertical
electrical sounding at a specific location, with the aid of resistivity formations. The
geodynamics of the sites as related to rocks’ susceptibility to failure, and the mechanisms of
slope failure was investigated, and foundation depths of the immediate surroundings of the
Nanka gully were studied using geotechnical data. The surrounding and the subsurface of the
eroded portions were monitored through imaging and analysis across the basin to measure
hydrological contribution to the gully risks and other prevailing factors. The geoelectrical data
was acquired with the ABEM Terrameter SAS 4000 and the ABEM LUND ES464 electrode
selection system (using resistivity method) and processed with the RES2DINV software to
produce 2D subsurface images. The VES resistivity curve matching was developed by a partial
curve matching technique; and interpreted by Minitab 18 software to produce subsurface
contour maps. In addition, the subsurface contour maps were qualitatively analysed with
mapped surface geology and information on current geological typical rocks. In the
geotechnical analysis, Spangler and Handy sampling techniques were used to collect eighteen
(18) samples from dug gully walls, and the laboratory tests were to ascertain the soil properties
index. The results of the models indicate that the study area is mainly clayey and sandstone
formations that exhibited low resistivity values corresponding to the shale layers and
groundwater zones. Many features that may lead to slope failure are present in the study areas,
such as fractures, boulders, weak zone, and saturated zone. The results also showed that the
soils in the study areas are friable hence easily washed off whenever there is storm water runoff
from the surface, making the landslide active over the years despite every protective precaution
put in place. In conclusion, this research work has identified lithologies, structural
deformations, and distinguishing clayey zones from water-saturated zones, proving that the
geophysical technique is the most successful tool in the landslide investigation.
Description
Doctoral Degree. University of KwaZulu-Natal, Pietermaritzburg.