Geotechnical Site Investigation: Standard Penetration & Bearing Capacity
A field engineer's practical guide to interpreting borehole logs, corrected SPT N-values, water table fluctuations, and selecting shallow isolated footings vs bored cast-in-situ piles.
Ujjwal Yadav
APRIL 10, 2026
A structure is only as reliable as the ground beneath it. Foundation failures almost always stem from inadequate subsoil characterization.
Before any column layout is finalized, deep rotary boreholes must map soil stratigraphy, ground water table depth, and shear strength parameters (cohesion 'c' and internal friction angle 'phi').
Interpreting Corrected SPT N-Values
Raw field hammer blow counts must be corrected for overburden pressure (Cn) and hammer energy efficiency (Ce). For cohesionless sandy strata beneath groundwater, dilatancy corrections apply when N > 15 to account for pore pressure build-up during dynamic penetration.
Selecting Foundation Typology
When safe bearing capacity (SBC) exceeds 250 kN/m2 and settlements remain within 25mm, shallow combined or isolated footings are cost-effective. In soft clays or expansive black cotton soils, friction piles or end-bearing socketed piles into sound bedrock are indispensable.