Geological and Geotechnical Evaluation of Gully Erosion at Nguzu Edda, Afikpo Sub-Basin, Southeastern Nigeria

A detailed geological field mapping of Nguzu Edda has revealed that gully erosion and landslides have remained active over the years, posing serious threats to human life, agricultural land, infrastructure and socio-economic activities within the area. Consequently, twelve true representative soil samples from different locations within gully sites have been analyzed based on ASTM and British Standards, to evaluate geologic conditions and geotechnical parameters that influence the gullies problems. The results of the laboratory tests revealed that the soil at the gully sites are predominantly sands (57~99.5 % with mean of 69.7%) with low amount of fines (silt; 0.3~ 22% with average value of 14.6% and clay: 0.2 ~ 21% with average value of 13.9%). The liquid limit (LL) ranged from 15 ~ 31.50 % with mean of 23.32% and the plasticity index (PI) ranged from 2.92~14.42% with mean of 8.95%. The moisture content, permeability, bulk density, porosity, organic matter and pH have average values of 25.31%, 6.04×10-4 cm/sec, 1.55 kg/m3, 40.40, 1.26% and 5.12, respectively. The cation exchange capacity (CEC) is low, which could be attributed to low organic matter, pH and non active clay minerals. With regard to the aforementioned characteristics, the soils at gully sites are loose sands with low amount of fines fraction, hence cohesionless, and are easily exposed to agents/factors of gully erosion. Thus, provides insights about the vulnerable causes of gully erosion and landslides problems that are prevalent in the area. This research addresses, to a great extent the effects of local geology, geotechnical properties of the underlying soil and associated human activities on the formation of gully erosion, in-turn results to landslides within in the study area. Engineering-geologic aspects of soil erosion control were also recommended. Keywords: Afikpo Sub-basin, Geologic conditions, Geotechnical analysis, Gully erosion and Erosion control measures