Geotechnical characterisation of Obudu damsite, Obudu, south-eastern Nigeria

Abstract

The Obudu dam is being built across Abeb river in Obudu area of the Cross 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 part of the Obudu crystalline basement plateau which is a region of low seismicity. The terrain is smoothly undulating and low lying and was known to be composed of unclassified basement and decomposed bedrock (overburden). The present study was carried out to assess the suitability of the chosen dam axis based on the determination of the nature and geotechnical characteristics of the overburden and bedrock. The investigation included geophysical surveys, bearing capacity tests (cone penetrometer and standard penetration), classification and grain size distribution as well as tests for compaction, consolidation and compressive strength. The results show that the bedrock is heterogeneous, including gneisses sillimanite, biotite and granite types), dolerite, quartzite and pegmatite with an overall moderate strength (about 76.04 MN m −2 average) and fair rock mass rating (RMR). The bedrock along the dam axis is apparently lacking in major fractures which could lead to short circuiting of the future impoundment. The overburden (soil) comprises silt (MH, ML), silty clay (CL) and silty sand (SM) with a combined thickness of about 2–20 m, increasing away from the valley floor towards the shoulders. Generally the overburden is of suitable compressive strength (150–300 KN m −2), low to medium plasticity and swelling potential and low permeability (up to 1.41 × 10 −7 ms −1) which would ensure a tight reservoir. The material settlement is expected to be small and slow. Accordingly, excavation of up to 5 m, decreasing towards the valley shoulders has been suggested to expose the recommended bearing medium: fresh/competent bedrock and overburden in the valley floor 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 evaluation of material properties.