Determination of Empirical Relations between Geoelectrical Data and Geotechnical Parameters in Foundation Studies for a proposed Earth Dam.

In order to establish empirical equations relating layer resistivity with geotechnical parameters for engineering site characterization, geotechnical tests comprising Standard Penetration Test (SPT), Atterberg Limit, Triaxial Compression, and Oedometer Consolidation Tests were conducted on the subsoil in six boreholes drilled to a maximum depth of 20m along a proposed dam axis. The locations of the boreholes coincided with VES stations previously occupied along the axis. Atterberg Limit Test was conducted on the disturbed soil samples while the undisturbed samples were subjected to quick (undrained) triaxial and oedometer consolidation tests. The SPT blows (N) required for the 300mm penetration following the first 150mm penetration below the bottom of the borehole was taken as the penetration resistance of the soil. The geoelectrical data and geotechnical parameters were subsequently correlated. The depths to geoelectrical interfaces (x) correlate strongly with the actual depths (y) obtained from drilling while the subsoil resistivity (⍴) weakly correlates with SPT, N values. The regression equations are: y = 0.977x – 0.035 and N = 0.001⍴ + 21.89, while the coefficients of correlation are r=0.95 and r=0.21 respectively. There exists a positive correlation (r=0.59) between layer resistivity and bulk density (γ). The empirical equation: γ = -8E-06⍴ + 1.735 shows increase in resistivity with increasing bulk density/subsoil compaction. Plasticity index (PI), cohesion (C) and coefficient of compressibility (Mv) decrease with increasing resistivity. While PI and C correlate strongly, Mv only correlates fairly. The empirical equations and the coefficients of correlation (r) are: PI = 29.04–0.02ρ (r=0.92), C = 304.9ρ -0.21 (r=0.98) and Mv = 3E-09⍴ 2 – 3E-05ρ + 0.182 (r=-0.77), respectively. The empirical equations established can be used to estimate geotechnical parameters from which relevant engineering deductions can be made.