What Is the SPT? A Practical Guide to the Standard Penetration Test

The Standard Penetration Test (SPT) is the most widely used in-situ test for site investigation worldwide. It recovers a disturbed soil sample and gives a simple resistance index, the N-value, in a single operation. For foundation designers from Houston to Hamburg, it is often the first number on the table.

How the test is performed

A split-barrel sampler is driven into the bottom of a borehole by a 63.5 kg (140 lb) hammer falling 760 mm (30 in). The crew counts blows for each 150 mm (6 in) of penetration, up to 450 mm. The N-value is the sum of the blows for the last two increments (300 mm); the first 150 mm is the seating drive and is disregarded.

Between tests the borehole is advanced by hollow-stem augers, wash boring or rotary methods. The recovered samples give the geologist a visual classification at every test depth.

What the N-value tells you

• Sands and gravels: relative density, from very loose to very dense.

• Clays and silts: consistency, from very soft to hard.

• Groundwater level: recorded during and after drilling, and critical for design.

Because hammer systems differ, results are commonly corrected for energy efficiency (N60) and overburden pressure before they are used in design correlations.

Why it remains the benchmark

It is quick, economical, and backed by decades of empirical correlations. Its weakness is repeatability: hammer type, rod length, borehole cleaning and operator technique all influence the count. Automatic trip hammers and well-maintained rigs significantly reduce that scatter.

Looking for SPT-ready drilling equipment?

Diamond Drill manufactures drilling rigs for geotechnical investigation. Contact our technical team to find the right setup for your projects.