Characterization of Subsurface Using Vp/Vs and Poisson’s Ratio Prior to the Foundation of Critical Civil Structures

Geotechnical investigation, site characterization and assessment of foundation rock play a vital role in design of foundation for any major important civil engineering structures viz. Nuclear and Thermal Power Plants, bridges, dams, etc. Geophysical techniques, complement and supplement to geotechnical investigations, provide compressional and shear wave velocities (i.e. Vp and Vs) of the subsurface layers in rapid and cost-effective way, using seismic method. Acoustic velocity (Vp and Vs) versus depth provides detailed picture of the subsurface. As ratio of Vp and Vs (i.e. Vp/Vs) is better indicator of lithology than individual velocity values i.e. Vp and Vs, objective of this paper is to highlight nature and behaviour of Vp/Vs and Poisson’s ratio, calculated using Vp/Vs, within near-surface layer and rock under different geological and structural condition, useful in characterization of subsurface. Vp/Vs and poison’s ratio, used in this paper, was calculated using digitized value of Vp and Vs w.r.t depth from real-time examples, selected on the basis of literature survey where cross-hole seismic survey was conducted prior to the foundation of critical civil structures. The discussion on subsurface characterization and assessment of foundation rock using Vp/Vs and Poisson’s ratio in this paper may be useful to geotechnical engineers and professionals dealing with design-foundation of important civil engineering structures.