GEOLOGICAL AND GEOTECHNICAL ASSESSMENT OF SELECTED GULLY SITES, YOLA AREA, NORTHEAST NIGERIA

A geological and geotechnical assessment of selected gully sites in the Yola area, northeast Nigeria, is presented. The geotechnical results indicate a mean plasticity index of 6.0 percent and a mean silt/clay content of 4.0 percent. Maximum dry density ranges from 1.48 mg/m 3 to 1.56 mg/m 3 . An optimum moisture content ranging from 9.80 percent to 11.40 percent was recorded. The shear strength test indicated values of cohesion ranging from 1.60 mg/m 2 to 2.40 mg/m 2 in an angle of internal friction ranging from 26 o to 29 o . The soil is thus not compact but loose because it is highly weathered. The fractive force due to runoff and seepage flux are resisted mainly by the angle of internal friction, since the value of cohesion is low. The hydraulic properties of the upper aquifer were analyzed and used to infer the surface and subsurface processes that contribute to the formation and continued expansion of existing gullies in the area. A specific discharge value of 79.81 m/yr and an average linear groundwater velocity of 207 m/yr were recorded, indicating high pore pressures and seepage fluxes. From the geotechnical analysis recommendations for erosion control were suggested.