Geotechnical investigation starts with field tests that measure how the ground responds to loading. Penetration tests push a sampling tool into the soil and record the resistance, then correlate that resistance with engineering properties such as density index, consistency, and bearing capacity. These tests are useful for general exploration of erratic soil profiles, for finding the depth to bedrock or a hard stratum, and for getting an approximate indication of strength in cohesionless soils where undisturbed samples are difficult to obtain.
Two penetration tests dominate practice: the standard penetration test and the cone penetration test. Both rely on the large body of experience behind them, which is why the results are trusted across decades of foundation design. The measured resistance can also be checked against full-scale behavior, the same way a load test on piles verifies the capacity assumed in design.
This article covers what the standard penetration test measures, the equipment and procedure, the corrections applied to the blow count, and how the corrected N value is interpreted on site and in the design office.
What Is the Standard Penetration Test?
The standard penetration test, usually shortened to SPT, determines the penetration resistance of soil. It belongs to the penetrometer family of tests, and an empirical correlation is derived between the soil properties and the penetration resistance. The test is performed in a clean borehole 55 to 150 mm in diameter, with casing or drilling mud used to support the sides of the hole.
A thick-wall split-tube sampler, 50.8 mm outside diameter and 35 mm inside diameter, is driven into the undisturbed soil at the bottom of the hole. The drive weight is 63.5 kg, falling freely through 75 cm, and the minimum open length of the sample should be 60 cm.
Equipment Used in the SPT
- Split spoon sampler, 50.8 mm OD and 35 mm ID, with a minimum open length of 60 cm.
- Drive weight of 63.5 kg with a 75 cm free fall.
- Borehole 55 to 150 mm in diameter, kept open with casing or drilling mud.
- Standard guide and anvil assembly that keeps the hammer fall consistent.
Where the SPT Is Used
SPT results feed foundation design for buildings, bridges, retaining walls, and embankments. The test works well in cohesionless soils such as sands and gravels, and it doubles as a sampling method because the split spoon returns a disturbed sample for classification. The same penetration-resistance logic appears in other construction materials; the Marshall stability test measures how compacted bituminous mixes resist deformation under load.
Procedure for the Standard Penetration Test
The SPT procedure is controlled and repetitive so results stay comparable between holes and between sites:
- Advance the borehole to the test depth and clean out the loose material at the bottom.
- Lower the split spoon sampler to the bottom of the hole and allow it to sink under its own weight.
- Seat the sampler 15 cm with blows of the hammer falling through a height of 75 cm.
- Drive the sampler a further 30 cm, or stop at 50 blows, recording the blows for each 15 cm of penetration.
- Report the penetration resistance N as the blows required for the second and third 15 cm of penetration.
- If the total drive is less than 45 cm, report the resistance for the last 30 cm of penetration.
The first 15 cm of the drive is the seat drive, which pushes the sampler through disturbed material at the bottom of the hole. Only the blows for the final 30 cm count toward the N value. In very soft subsoil, the sampler may sink under its own weight; those conditions are recorded rather than forced, because the N value loses meaning when the soil cannot offer measurable resistance.
The N Value
The N value is the sum of the blows for the second and third 15 cm increments, typically reported as blows per 300 mm. It is a raw field number that needs correction before it is used in design. Field testing discipline matters here just as much as in other site tests; a surface regularity test answers a different question about pavement finish, and the two tests have no interchangeable roles.
Recording Soft Ground
When the sampler sinks under its own weight, the record should note the condition explicitly. Many codes report such strata as an N value of zero or record the self-weight penetration, which tells the designer the soil is too soft for direct bearing.
Corrections to the N Value
The raw N value is corrected before it is correlated with soil properties. Two corrections are standard practice: the correction for overburden pressure and the correction for dilatancy or submergence. Selecting the right test method is as important as running it correctly, whether the job is checking a gravity pipeline with an air test and a water test or deciding which SPT correction applies to a saturated sand.
Correction for Overburden
Dense sands at depth show higher N values partly because the confining pressure increases with depth, not because the soil is stronger. The overburden correction normalizes the measured N to a standard effective overburden pressure so values from different depths can be compared. Overburden pressure correction formulas by different workers, such as those by Peck, Hansen, and Thornburn, are applied when the effective overburden differs from the reference pressure used to build the correlation charts.
Correction for Dilatancy
Fine sands below the water table can dilate during driving, which raises the blow count. When the N value exceeds 15 in saturated fine sand, the corrected value is often taken as 15 plus half the excess. This adjustment keeps the correlation charts from overestimating density in submerged fine sands.
Correlating N With Soil Properties
The corrected N value correlates with relative density, consistency, and bearing capacity. The table below shows the common interpretation bands for granular soils.
| N value | Relative density | Typical use in design |
|---|---|---|
| 0 to 4 | Very loose | Avoid shallow foundations; expect settlement |
| 4 to 10 | Loose | Consider deep foundations or ground improvement |
| 10 to 30 | Medium | Suitable for conventional footings with checks |
| 30 to 50 | Dense | Good bearing; economical foundations |
| Over 50 | Very dense | High capacity; pile driving may be hard |
The full details of the standard penetration test apparatus, procedure, corrections, and applications in soil investigation are covered in our dedicated reference. Engineers use these bands to pick allowable bearing pressures, estimate settlement, and judge liquefaction potential in seismic zones.
Bearing Capacity Estimates
Many empirical charts link corrected N to allowable bearing pressure for footings of a given width. The charts are site-calibrated, so the same N value can produce different allowable pressures in different regions. For cohesive soils, the same blow count is correlated with consistency terms such as soft, firm, stiff, and very stiff, which then guide allowable pressure selection.
Liquefaction Screening
In earthquake-prone areas, corrected N values are plotted against cyclic stress ratios to screen soils for liquefaction. The screening is a first pass; detailed analysis follows where the screening flags risk.
Advantages and Disadvantages of the SPT
Every field test carries trade-offs, and the SPT is no exception.
Advantages
- Simple equipment that is available in most parts of the world.
- Provides a disturbed sample for classification at the same time.
- Works in cohesionless soils where undisturbed sampling is impractical.
- Decades of published correlations for density, bearing capacity, and settlement.
- Rapid enough for routine boreholes on small and large projects.
Disadvantages
- Results depend on hammer efficiency, drill rod length, and operator technique.
- The disturbance caused by driving alters the sample and the surrounding soil.
- Correlations are empirical and regional, so blind extrapolation is risky.
- Energy losses in the rod string can vary the blow count between rigs.
- Penetration in gravelly or cemented soils can damage the sampler tip.
The SPT answers strength questions in soil; other standard tests answer different questions. When a full materials investigation is required, the Los Angeles abrasion test on aggregates complements geotechnical work by checking whether the rock used in fills and pavements will survive handling and traffic.
Efficiency and Practical Use of the SPT
The efficiency of the SPT is usually measured against a theoretical hammer energy of 4740 joules per blow, which is the energy of a 63.5 kg weight falling 75 cm. Field hammer systems deliver a fraction of that energy, typically 45 to 85 percent, and the N value is adjusted by the ratio of delivered to reference energy so results from different rigs can be compared.
Standardized Reporting
Reports should record the hammer type, the anvil arrangement, the rod length, the borehole diameter, and the sampling depth alongside the N value. Without that context, two identical N values can mean very different things. Automated hammer systems with trip or rope mechanisms deliver different energy, which is why the energy ratio is reported with the N value.
Penetration Tests Across Materials
Penetration resistance is not limited to soil testing. The bitumen penetration test grades the hardness of bitumen by measuring how far a standard needle sinks under a fixed load, while the SPT measures how many blows a soil column resists. Both tests convert a physical measurement into a classification number that engineers can compare.
The habit of standardized testing extends from soil to pavement materials. Abrasion testing of aggregates tells you whether a crushed rock will survive traffic loads, and together with SPT results it completes the picture of site and material behavior for a road project.