Standard Penetration Test (SPT)

Standard Penetration Test (SPT) is the most-widely-used field test in Indian geotechnical practice for in-situ characterisation of soil strength and density. Standardised in IS 2131:1981 (revised 2024), the test consists of driving a split-spoon sampler (50 mm OD, 35 mm ID) into the soil at the bottom of a borehole using a 63.5 kg hammer dropped from 760 mm height. The number of hammer blows required to advance the sampler the second and third 150 mm increments (combined) is recorded as the N-value. Lower N-values indicate looser cohesionless soils or softer cohesive soils; higher N-values indicate denser/stiffer materials.

SPT N-values are correlated to virtually every geotechnical design parameter. For sandy soils, N is correlated to relative density (loose: N < 10; medium: 10-30; dense: 30-50; very dense: >50), angle of internal friction (φ), modulus of elasticity, and allowable bearing pressure (Terzaghi-Peck, Meyerhof, Bowles correlations). For cohesive soils, N is correlated to undrained shear strength (cu ≈ 5N to 12N kPa, with substantial scatter) and unconfined compressive strength. For Indian projects, IS 6403 Cl. 5 provides tabulated bearing capacity values directly from N-values, which most routine designs use without explicit shear-strength computation.

The primary limitations of SPT are well-recognised: significant scatter in N-values due to hammer-energy variability, sampler condition, and operator technique; inadequate for soils with gravel >50 mm size; not usable in saturated dense sands where dilation gives spuriously high values. Modern Indian practice supplements SPT with: Cone Penetration Test (CPT, IS 4968) for continuous profile, Pressuremeter Test for in-situ deformation modulus, and laboratory triaxial testing of UDS samples for high-importance structures. For routine residential and commercial work, 3-5 boreholes with SPT every 1.5 m depth, terminated 5 m below founding depth or in firm strata for ≥ 3 m, remains the Indian standard practice.