Permeability Characteristics of the Foundation Materials of Gurara Dam, North Central Nigeria

Abstract

A comparison of refraction seismic velocities, electrical resistivity and in-situ permeability   values were  used in this study to evaluate the relationship between permeability values and geophysical parameters in the basement rocks in Gurara Dam area, Kaduna State, North Central Nigeria. Seismic refraction shootings, borehole drilling and rock coring enhanced the interpretations of the geo-electrical studies. The presence of low resistivity values (200-700 W-m) on the electrical resistivity soundings coincided with low velocity transmission of seismic waves (500-1,500 m/sec.) in the saprolite. The results of drilling, coring and water testing, show that the basement rock is fresh and moderately permeable. The Lugeon values from the water pressure tests performed in boreholes show local variations of the generally moderate overall permeability. The permeability values in the basement rock fractures which lie between the depths of 20 and 40m vary between 35 and 1,620 x 10^-7 m/s, meaning that the rocks require cement grouting for structural foundation works at these depths. The permeability values of rocks between 40 and 60m vary between 2.67 and 94.8 x10^-7 m/sec.

This study has established the relationship between low seismic velocity and low electrical resistivity and high permeability in the basement rock. Permeability increases down the profile in the up thrown side of the dam axis until where the fractures close up at depths, while it decreases downwards at the down thrown side. These areas need extra care in the construction stages, if the dam foundation is not to be “threatened”.

Key words;  Permeability, saprolite, fresh rock, grouting.