Geo-technical Investigation and Characterization of Sub-soils in Yenagoa, Bayelsa State, Central Niger Delta, Nigeria

Abstract

This study aims at establishing the sub-soil types and profile to ascertain the geotechnical characteristics of the underlying soils in parts of Yenagoa, Bayelsa State, Nigeria and recommend appropriate foundation design and construction of projects in the area. Four boreholes were drilled using hand auger and representative disturbed samples were taken at regular intervals of 1.0m depth, and also when a change in soil type was observed. Standard laboratory procedures were used in the analysis of the samples. Water levels recorded in the site BH-1 (1.50m), BH-2 (1.50m), BH-3 (4.00m) and BH-4 (1.00m), respectively. Geotechnical result reveal that the area is underlain predominantly by medium to firm silty clay (although) in BH-1, silty sand and medium to fine silty clay in BH-2 and medium to firm silty clay at the top, silty sand (middle) and fine to medium sand at the bottom of BH-3 and BH-4, respectively. Water level (WL) recorded in the site for BH-1 (1.50m), BH-2 (1.50m), BH-3 (4.00m) and BH-4 (1.00m), respectively. The soil material presented average Moisture Content (MC) of 27%; 35% average Liquid Limit (LL); 22% average Plastic Limit (PL); 13% average Plasticity Index (PI) with average Liquidity Index of 0.387. Average Hydraulic conductivity K (Permeability) obtained from grain size analysis for BH-3 and BH-4: 3.2 x 10^-3m/s and 1.8 x 10^-3 m/s respectively, with an average of K = 2.5 x 10^-3m/s. BH-3 and BH-4 were considered for sieve analysis only because of the appreciable sand presence which were not available in BH-1 and BH-2. The hydraulic conductivity values are high within the typical permeability values for fine to medium sand to gravel. The implication of high K values is that the aquifer system in the area is prolific. It is recommended that studies on the geotechnical characteristics of the area be carried out as it provides valuable data that can be used for foundation design and other forms of construction for civil engineering structures in order to minimize adverse effects and prevention of post construction problems.

Key Words: Geotechnical, Hydraulic conductivity, Sub-soil, Yenagoa, Niger Delta.