GEOTECHNICAL STUDY OF SOME GULLY EROSION SITES AT IHITEOWERRI, SOUTH EASTERN NIGERIA.

*Mbonu, E. J., Okeke, O.C., Akujieze, C. N., Imasuen, O.I.S and Obayanju, T. A

Department of Geology, University of Benin, Benin city, Nigeria

ABSTRACT

Gully erosion propagation is associated with hazards which include destruction of arable soil and infrastructures. This study is aimed at evaluating the interplay between the soil geotechnical parameters, geomorphology and hydrogeology of the study area in the gully formation and associated hazards. The study area was situated on a steep drainage slope range from (20⁰ to 32⁰). The lithology is mainly thick homogenous loose uncompacted and cohesionless lateritized sediments, with brownish colour. Three gully erosion sites (Abor, Ogbaka and Umuchukwu) were identified in the study area. Nine soil samples were collected from different depths of the three gully sites and their geotechnical properties including grain-size distribution, atterberg limit, compaction test, specific gravity, and shear strength were analyzed. The results indicate that the soil sites are predominantly fine to medium grain, with percentage fraction range from 61.3% to 77.2% and silt/clay range from 13.1% to 26.9%.  Coefficient of uniformity ranges from 4.0 to 13.3 indicating medium graded. Liquid limit ranges from 20.52% to 33.7%, with moderate plasticity range from 11.50% to 29.04% and plasticity index range from 1.44% to 16.24%, slightly dry. Maximum dry density ranges from 1.72 Mg/m³ to 1.91 Mg/m³, Optimum moisture content range from 8.8% to 12.6%, with medium Porosity range from 8% to 12%, and low Permeability range from 1.6 x 10^-1 cm³/sec to 9.0 x 10^-2 cm³/sec. Specific gravity ranges from 2.50 to 2.56. Shear strength result show low Cohesion value range from 1.0KN/m² to 6.0KN/m², with friction angle range from 8.29⁰ to 25. 0⁰. The result revealed that gully erosion development and propagation in Ihiteowerri as a result of susceptibility of the loose cohesionless sediments to the high kinetic force of overland flow from the steep drainage slope.  The large lateral extent of the gully sites and deep gully incision was as a result of occasional fall by slumping of the near vertical gully slope side walls. From the study, integrated measures for erosion control by designing adequate drainage network and afforestation is recommended.

Keywords: Ihiteowerri, gully erosion sites, soil erodibility, geotechnical parameters, geomorphology, overland flow