Grain size distribution of eight soil samples recovered from boreholes drilled within the Yenagoa metropolis was determined by means of mechanical sieve analysis. From the distribution curves, grading characteristics: d10, d 25, d 30, d60and d75 and their derivatives such as the effective size, uniformity coefficient, coefficient of sorting, coefficient of gradation and porosity were calculated. The hydraulic conductivity of the unconsolidated aquifer materials was first evaluated using six empirical formulae on the basis of the grain size distribution and secondly by the constant head permeameter laboratory test method. Analyses of the results obtained using the various empirical formulae show that only Kozeny-Carman and Hazen formulae reliably estimated the hydraulic conductivities of the various soil samples as compared with constant head method results and were well within known ranges. The Slitchter, Beyer, Terzarghi and USBR empirical formulae, significantly underestimated the hydraulic conductivities of the samples and are probably not within the domain of applicability for the soils analyzed in the study area. Average hydraulic conductivity values determined using Kozeny-Carman equation, Hazen formula and the constant head permeameter test are 193.92 m/day, 102.37m/day and 171.93m/day respectively. The above values indicates that the groundwater yield of the aquiferous materials is adequate for municipal water supply.
Keywords : Grain size distribution; Sieve analysis; hydraulic conductivity; porosity; permeameter.