Geotechnical as well as geophysical investigation methods were used to provide information for the assessment of the subsurface lithology along the proposed Nkari Dam Axis in South Eastern Nigeria. This paper reports on the geotechnical methods applied in the studies. Field tests; Dutch Cone Penetrometer (CPT), Standard Penetration Test (SPT), natural moisture content determination and borehole drilling for groundwater observation and lithologic identification were carried out. Disturbed and undisturbed soil samples were obtained from test pits and boreholes for different laboratory tests. Natural moisture content, specific gravity, bulk densities, Atterberg Limits, Consolidation settlement, Triaxial and Direct Shear tests, Unconfined Compression tests etc were determined. Assessment of data indicated deep-seated clay as dam foundation and potential watertight reservoir. The drilling records indicated maximum penetration depth of 5.0-6.9m. The subsurface consists of two main layers of clayey-silt to clayey shales. The top layer made of clayey-silt is about 2.3m thick while the underlying shale unit extends to a depth of 20m and beyond. This consolidated shales underlying the overburden, constitute the penetration refusals in the SPT boring process. A two-layer earth model fits the proposed dam site with the top layer having thickness less than 6m at the high grounds and averaging 2m in the valleys. The geological study prior to and during the present study has not indicated any faulting in the dam axis.