Geotechnical Characterization and Foundation Soil Assessment for a Proposed Earth Dam in Shendam, Central Nigeria

Abstract

A comprehensive geotechnical investigation was conducted along the proposed earth dam axis at Ngod'togut, Shendam, Central Nigeria, to evaluate subsurface conditions and the suitability of the soils for the foundation. Four boreholes were drilled to depths ranging from 0 to 6.6 m, and Standard Penetration Tests (SPT) were performed at 1.5 m intervals. Laboratory testing conducted in accordance with BS 1377 (1990) included particle size distribution, Atterberg Limits, specific gravity, natural moisture content, permeability, shear strength, consolidation, and chemical analyses (pH and sulphate). The subsurface profile consists predominantly of sand, silty sand, laterite, and weathered shale. Permeability values of the foundation soil range from 1.88×10−6 to 5.10×10−6 cm/s, indicating moderate drainage characteristics. Preliminary bearing capacity estimates range from 20 to 1000 kN/m² depending on depth and lithology. The coefficient of consolidation (Cv) ranges from 1.68 × 10-6 to 4.55 × 10–6 m2/yr, while the coefficient of compressibility (Mv) has a constant value of 3.11 × 10-4 m2/KN. The bearing conditions are generally adequate at depth. However, seepage control and slope stability analyses are necessary for final design validation. Recommendations include excavation of a cut-off trench to weathered shale, improvement of core material permeability, and detailed seepage and stability modeling.