What is an SPT Test?

A standard penetration test (SPT) measures soil resistance at selected depths within a borehole. A drop hammer drives a steel sampler into the ground, and the test records the number of blows required to achieve a specified penetration.

The test produces both a penetration measurement and, where material is recovered, a soil sample. These provide evidence to assess ground conditions beneath foundations, retaining structures and earthworks.

Two soil layers can have similar descriptions yet respond differently to loading. Their natural structure, packing, and stress history influence behaviour in ways appearance alone does not reveal.

SPT adds a measured response to the investigation, giving engineers evidence of the ground’s condition at selected depths. This helps determine whether assumptions based on existing records and initial observations reflect the conditions encountered on site.

The sampler is a hollow steel tube with a cutting shoe at its lower end. As it advances, soil enters the barrel. After withdrawal, the tube separates lengthways so the contents can be examined.

Recovered material lets an engineer describe features such as colour, particle size and composition. Samples may also undergo suitable laboratory classification tests, including particle-size analysis and plasticity testing.

Driving disturbs the soil’s original structure, but the material still provides useful identification evidence. Its description accompanies the resistance measurement taken at the same depth, linking a numerical result with the ground encountered.

The SPT N-value expresses the resistance encountered during the main test drive. It combines the blows recorded after the initial seating stage, which is excluded from the result.

In granular soils, higher counts generally indicate denser material. Comparing readings at successive depths helps engineers identify changes through the ground profile.

Interpretation considers the recovered soil, borehole observations and equipment used. Corrections can account for differences in hammer energy, allowing results to be used with appropriate design correlations.

The N-value therefore provides a practical link between field testing and assessments of ground behaviour for foundation design.

The N-value forms part of a wider record. Details typically include:

• Borehole reference and test depth: locate the measurement within the investigation.
• Incremental blow counts: show the effort required during seating and subsequent penetration.
• Distance driven: records full or partial penetration for each relevant interval.
• Sample recovery: identifies how much material was retrieved.
• Soil description: documents the character of the recovered sample.
• Equipment and observations: capture relevant test arrangements, interruptions or unusual resistance.

These details allow results to be traced back to their field conditions. Corrected values and the assumptions used to derive them belong in the accompanying interpretation.

Testing can target particular layers, including potential piling zones or deposits beneath shallow foundations.

For residential and commercial developments, readings can help assess changes in granular soil density beneath the proposed building footprint. Comparing results between boreholes supports decisions about foundation layout and the need for further investigation.

Infrastructure projects, such as bridge approaches and embankments, introduce different loading patterns. Here, tests at selected depths contribute to assessing the deposits affected by new foundations or placed fill.

The programme follows these engineering questions, with test depths chosen to gather relevant evidence as drilling progresses.

Successive readings help engineers build a picture of how resistance changes with depth. Lower counts within a described sand layer, followed by higher values below, may indicate a transition to denser deposits.

The pattern matters as much as an individual result. A single high count associated with gravel or a recorded obstruction needs a different interpretation from consistently elevated readings across several depths and boreholes.

SPT provides measurements at selected intervals. Where closer profiling is useful, cone penetration testing (CPT) can add detail between sampling depths.

Reading these datasets together helps define layer boundaries and assess their continuity across a site. The resulting ground model connects field observations with decisions about foundation levels, pile design and construction methods.