GEOTECHNICAL CHARACTERIZATION OF OBUDU DAMSITE, OBUDU, SOUTH-EASTERN NIGERIA

The Obudu dam is being built across Abeb river in Obudu area of the Cr oss River State (Nigeria). The earthfill dam will be approximately 18 m high with a crest length of 385 m. The dam site is located within pa rt of the Obudu crystalline basement plateau which is a region of low seismicity. The terrain is smoothly undulating and low lying and was k nown to be composed of unclassified basement and decomposed bedrock (o verburden). The present study was carried out to assess the suitabilit y of the chosen dam axis based on the determination of the nature and geotechnical characteristics of the overburden and bedrock. The invest igation included geophysical surveys, bearing capacity tests (cone pen etrometer and standard penetration), classification and grain size dis tribution as well as tests for compaction, consolidation and compressi ve strength. The results show that the bedrock is heterogeneous, inclu ding gneisses sillimanite, biotite and granite types), dolerite, quart zite and pegmatite with an overall moderate strength (about 76.04 MN m (-2) average) and fair rock mass rating (RMR). The bedrock along the d am axis is apparently lacking in major fractures which could lead to s hort circuiting of the future impoundment. The overburden (soil) compr ises silt (MH, ML), silty clay (CL) and silty sand (SM) with a combine d thickness of about 2-20 m, increasing away from the valley floor tow ards the shoulders. Generally the overburden is of suitable compressiv e strength (150-300 KN m(-2)), low to medium plasticity and swelling p otential and low permeability (up to 1.41 x 10(-7) ms(-1)) which would ensure a tight reservoir. The material settlement is expected to be s mall and slow. Accordingly, excavation of up to 5 m, decreasing toward s the valley shoulders has been suggested to expose the recommended be aring medium: fresh/competent bedrock and overburden in the valley flo or and flanks, respectively. While a detailed investigation of burrow areas was not part of the present study, a few potential sites in the dam axis and reservoir areas have been suggested based on the evaluati on of material properties.