Vol.:(0123456789) 1 3 Modeling Earth Systems and Environment https://doi.org/10.1007/s40808-020-00741-w ORIGINAL ARTICLE Erodibility characteristics and slope stability analysis of geological units prone to erosion in Udi area, southeast Nigeria Uchenna C. Nebeokike 1  · Ogbonnaya Igwe 1  · Johnbosco C. Egbueri 1,2  · Stanley I. Ifediegwu 1 Received: 20 December 2019 / Accepted: 7 March 2020 © Springer Nature Switzerland AG 2020 Abstract In Udi province, gully erosion has caused several environmental damages such as loss of agricultural lands and natural veg- etation, loss of residential apartments, destruction of engineering structures, and disruption of wildlife ecosystem. In this paper, an attempt was made to assess the erodibility characteristics and slope stability of geological units prone to erosion in this area using integrated approach. Geotechnical analysis of soil samples revealed that they are susceptible to erosional pro- cesses. The soil samples were classifed as poorly graded sands and silty sands based on Unifed Soil Classifcation System. Grain size distribution analysis showed that the soils were almost devoid of clays, which could serve as binding materials. Atterberg limit test revealed that they were nonplastic. Compaction test indicated that the soils were loose and collapsible, with dry density ranging from 1.65 to 1.96 g/cm 3 . On the basis of permeability coefcients (which ranged from 6.81 × 10 –5 to 1.28 × 10 –4  m/s), the soils are moderately to highly permeable, indicating high infltration capacity. Shear strength analysis revealed that the soils have low cohesive capacity (ranging from 0 to 5 kPa) and relatively low angle of shearing resistance (ranging from 22° to 30°). Multivariate statistical analyses aided the establishment of the interrelationships between the analyzed geotechnical parameters. The obtained factor of safety of the gully slopes (in both saturated and unsaturated condi- tions) ranged from 0.81 to 0.90 and 1.02 to 1.41, respectively. This indicates that the gully slopes are typically unstable in saturated conditions and critically stable in unsaturated conditions. Keywords Erodibility characteristics · Gully · Slope stability analysis · Soil erosion · Statistical analysis · Udi province Introduction Land degradation, as a result of soil erosion, is one of the major geologic hazards threatening many lives and proper- ties in diferent parts of Nigeria (Igwe and Egbueri 2018; Egbueri and Igwe 2020). The most critical part of this men- ace is the formation of rills and gullies. Gullying has caused several environmental damages in this region. Such environ- mental problems caused by gullying in Udi include loss of agricultural lands and natural vegetation, loss of residential apartments, destruction of engineering structures like roads and drainage channels, and disruption of wildlife ecosystem (Figs. 1 and 2). Being geomorphic processes, rill and gully erosions play a critical role in the continuous modifcation and reshaping of the topographical confgurations of the study region. This has given the topography of the study region an undulating appearance. Water erosion occurs when the shear stress of overland fowing water becomes greater than the shear strength of the soil material over which it fows (Horton 1945). By this def- nition, the shear stress represents the ability of the fowing water to entrain and detach soil materials, while the shear strength represents the properties of the soil enabling it to resist the shear stress of fowing water. However, numerous other factors play signifcant roles in the initiation, develop- ment, and expansion of erosion gullies. Such factors that infuence gully erosion and gully slope movements include geological, meteorological, hydrogeological, geotechni- cal, topographical, geometrical, geochemical, biological, and anthropogenic factors (McCarthy 2007; Nwajide 2013; Emeh and Igwe 2017; Igwe 2018; Igwe and Egbueri 2018; Igwe and Una 2019; Egbueri and Igwe 2020). Among the listed factors, the geological, meteorological, geotechnical, * Uchenna C. Nebeokike nebeokikecharles@gmail.com 1 Department of Geology, University of Nigeria, Nsukka, Nigeria 2 Department of Geology, Chukwuemeka Odumegwu Ojukwu University, Uli, Nigeria