What is the standard weight of the hammer and its drop height?

A 63.5 kg weight falling freely from a height of 75 cm (Clause 2.1).

How is the N-value calculated?

It is the sum of the number of blows required to drive the sampler for the second and third 15 cm stages (total 30 cm) after the initial 15 cm seating drive (Clause 3.2.1).

What constitutes 'refusal' during an SPT?

Refusal is declared if blows exceed 50 for any 15 cm of penetration, or if 100 total blows are reached (Clause 3.2.1).

When is the dilatancy correction applied?

It is applied to N-values greater than 15 in saturated fine sands and silts below the water table (Clause 4.1).

IS 2131

: 1981

Method for standard penetration test for soils

CurrentEssentialTesting MethodGeotechnical · Soil and Foundation

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IS 2131 outlines the procedure for conducting the Standard Penetration Test (SPT) to determine the resistance of soils in a borehole. The resulting 'N-value' is a universally recognized empirical parameter used by geotechnical engineers to estimate bearing capacity, settlement, and liquefaction potential.

Describes the procedure for conducting the Standard Penetration Test (SPT) in cohesive and cohesionless soils for foundation design.

Quick Reference Values

Hammer weight63.5 kg

Height of free fall75 cm

Total penetration depth45 cm

Seating driveFirst 15 cm (blows discarded)

N-value measurementBlows for the last 30 cm of penetration

Refusal criteria50 blows for any 15 cm penetration or 100 total blows

Key Formulas

N = Blows for 15-30cm + Blows for 30-45cm

Ne = 15 + 0.5(N - 15) — Dilatancy correction for saturated fine sand/silt when N > 15

Practical Notes

Always discard the blow count from the first 15 cm of penetration as it represents disturbed soil (seating drive).

Ensure the drill rod is strictly vertical and the hammer fall is a free fall of exactly 75 cm; friction in the lifting mechanism drastically reduces actual energy delivered.

The water level inside the borehole must be maintained at or slightly above the groundwater table to prevent boiling or heaving, especially in sandy soils.