Geological, Geotechnical and Geophysical Investigation of Gully Erosion in Ilorin, Kwara State Nigeria

Abstract

Geological, Geotechnical and Geophysical investigations were carried out on some selected gully sites in Ilorin with the aim of unravelling the underlying factors that causes the formation or aid the gully development. Reconnaissance assessment of the area revealed that the area is underlain by Basement Complex rocks. The soils of the area are products of insitu weathering of the underlying basement rocks. A total number of 39 soil samples were obtained systematically at three distinctive lithologies for geotechnical analysis. The test includes Sieve Analysis, Atterberg Limit test and Shear strength. The results of the laboratory test revealed that the soil at the gully site are predominantly Sands which range from 20-85%, with mean of 40.6%. Uniformity coefficient (cu) range from 3.5 – 18.9 and coefficient of curvature (cc) range from 0.5 – 3.4. This indicates that the soil from the gully sites are within the medium to coarse grain with very low percentages of fines (silts; range from 2-11% with average of 6.9% and clay 3-46% with average of 19.1%). The liquid limit (LL) ranges from 19.0-50% with mean of 32.8% and the Plasticity index (PI) range from 9.3-27.3% with mean of 15.7%. With regard to the aforementioned characteristics, the soils at the gully sites are loose sands with low amount of fine fraction, hence, cohesionless and are easily exposed to agents and factors of gully erosion. The results from the geophysical analysis shows that the geoelectric layers have high resistivity values which shows the presence of low plasticity and friable gravelly sand which constitutes part of the overburden across the study area. This research addresses to a great extent, the effect of local geology, geotechnical properties of the underlying soils, geophysical properties of the lithology associated with human activities on the formation of gully sites.