Geotechnical and environmental assessment of some gully erosion sites in Girei and environs, Adamawa State, North-Eastern Nigeria

Abstract

Gully erosion adversely affects farmlands and distorts ecological and social settings in many parts of Girei Local Government Area of Adamawa State. This paper assessed the geotechnical and environmental properties of gully erosion sites in the area to determine the causative factors. The study area experiences tropical climate with distinct rainy and dry seasons. It is underlain by Bima Sandstones; which contain quartz and feldspars, medium to coarse-grained, grade bedded and laminated. Thirty disturbed samples; collected at depths of 0.5m, 1.5m and 3.0m; from ten different gully sites were investigated. Laboratory tests results indicated that the gully sites soils are predominantly coarse sands (means: 45.05%, 44.7%, 47.6%) and fine sands (averages: 44.35%, 43.37%, 43.09%); with insignificant gravels, silts and clays percentages. The mean values of liquid limit, plastic limit and plasticity index are (25.07%, 23.63%, 22.85%), (16.16%, 14.67%, 13.98%) and (9.38%, 8.96%, 8.70%) respectively. The average natural moisture content values are 10.6%, 9.7%, 9.1% while that of  specific gravity are 2.63; at all depths of sampling. The average values of maximum dry density, optimum moisture content and bulk density are (1.85 kg/m³, 1.85 kg/m³, 1.82 kg/m³), (16.79%, 15.72%, 15.88%) and (1.74kg/m³, 1.66 kg/m³, 1.57 kg/m³) respectively. The mean values of angle of internal friction are 25.4°, 25.4°, 25.8°; while that of cohesion are 2.9 KN/m², 2.4 KN/m², 2.2 KN/m². The results indicated the soils contain low amounts of fines. They are loose, non-plastic and highly instable soils. Consequently, they possess high void ratio, infiltration rates, flow velocities, seepage pressure and internal erosion potential. Hence, they are liable to effects of gully formation. Irregular/intensive rainfall, drought/ desertification, intensive bush clearing, repeated cultivations, and bush burning caused gully erosion in the study area. Positive farming practices and tree/grass planting are recommended for mitigation.