The Standard Penetration Test (SPT) is the most widely used in-situ geotechnical test in the world. Developed in the United States in the 1920s and standardised under ASTM D1586 and ISO 22476-3, the SPT provides a penetration resistance measurement — the N-value — at discrete depth intervals throughout a borehole, alongside a disturbed soil sample from a split-spoon sampler. This combination of quantitative data and physical sample has made the SPT the backbone of routine site investigation programs on every continent.
This guide covers the complete SPT workflow: test procedure, hammer energy and its effect on results, how to apply the five standard correction factors to reach the design-ready (N1)60 value, the most commonly used correlations for estimating soil engineering properties, bearing capacity methods based on SPT data, and how specialist software automates the entire process.
SPT procedure step by step #
The SPT is performed at the base of a borehole at specified depth intervals — typically every 1.0 or 1.5 meters, or at every change in material. The procedure follows these steps:
- Advance the borehole to the required test depth using the appropriate drilling method (hollow stem auger, mud rotary, etc.) and clean all loose material from the base of the hole.
- Attach the split-spoon sampler to the drill rods. The standard split-spoon sampler has an outer diameter of 50 mm (2 in), an inner diameter of 35 mm (1⅜ in), and a length of 600 mm (24 in).
- Seat the sampler by driving it 150 mm (6 in) into the soil at the base of the borehole. This seating drive is not counted in the N-value — it advances the sampler past any disturbed material at the base of the hole.
- Drive the sampler through two further consecutive 150 mm increments using a 63.5 kg (140 lb) hammer falling freely through a drop height of 760 mm (30 in). Record the number of blows for each 150 mm increment separately.
- Calculate the N-value as the sum of blows for the second and third increments (i.e. the blows required to drive the sampler from 150 mm to 450 mm below its initial position). The seating drive blows are discarded.
- Record refusal if 50 blows are required for any single 150 mm increment, or if 100 blows total are applied without achieving 450 mm of penetration. The test is terminated and “REF” is recorded on the borehole log.
- Extract the sampler and remove the soil sample from the split-spoon barrel. The sample is visually described, classified, and placed in a labelled glass jar for laboratory testing.
Tests are performed at the specified depth intervals until the borehole reaches its target termination depth.
Key SPT measurements recorded on the borehole log #
| Measurement | Recorded as | Notes |
|---|---|---|
| Test depth interval | e.g. 3.0–3.6 m | Top of sampler seating to bottom of second drive |
| Seating drive blows | e.g. 4 | First 150 mm — discarded from N-value |
| Second drive blows | e.g. 7 | Counts toward N-value |
| Third drive blows | e.g. 9 | Counts toward N-value |
| N-value (field) | N = 16 | Sum of second + third drive blows |
| Sample recovery | e.g. 380 mm / 450 mm | Length of soil retained in sampler |
Hammer types and energy delivery #
The most significant source of variability in SPT results between different rigs, countries, and eras is the energy actually delivered to the drill rods by the hammer — which is not the same as the theoretical potential energy of the falling weight. All SPT correlations in geotechnical literature are calibrated to a reference energy efficiency of 60% of the theoretical free-fall energy (Er = 60%). Field measurements have shown that actual energy delivery varies substantially depending on the hammer type used.
The four main SPT hammer types #
| Hammer type | Release mechanism | Typical energy ratio (Er) | Primary use region |
|---|---|---|---|
| Safety hammer | Cathead and rope (2 turns) | 55–70% | North America (older rigs) |
| Donut hammer | Cathead and rope (2 turns) | 30–60% | Japan, Asia, some older rigs globally |
| Automatic (trip) hammer | Mechanical trip or hydraulic release | 80–100% | North America, Europe (modern rigs) |
| Hydraulic hammer | Hydraulic cylinder | 85–95% | Europe, Middle East (modern rigs) |
Because an automatic hammer delivers significantly more energy per blow than a donut hammer at the same nominal drop height, the same soil at the same depth will produce a lower raw N-value when tested with an automatic hammer. Without energy correction, N-values from different rigs are not directly comparable and should not be used together in the same analysis.
This is why correcting raw N to N60 — normalised to 60% efficiency — is not optional. It is the minimum required step before any SPT correlation is applied.
N-value correction factors — Ce, Cr, Cb, Cs, CN #
The raw field N-value must be corrected before it can be used in engineering correlations. Five correction factors are applied to convert the raw N to the fully corrected value (N1)60, which is normalised to both 60% hammer energy and 100 kPa effective overburden stress.
The (N1)60 formula #
The corrected SPT N-value is calculated as:
(N1)60 = N × Ce × Cr × Cb × Cs × CN
where N is the raw field blow count and each correction factor is described below.
Correction factor reference table #
| Factor | Symbol | What it corrects for | Typical range |
|---|---|---|---|
| Energy ratio | Ce (or Er/60) | Hammer energy efficiency relative to the 60% reference. Ce = Er/60, where Er is the measured or estimated energy ratio of the hammer system. | 0.5–1.6 |
| Rod length | Cr | Energy loss due to wave propagation in very short rod lengths (<10 m). Rods shorter than 3 m lose significant energy; corrections are greatest at shallow depths. | 0.75–1.0 |
| Borehole diameter | Cb | Larger boreholes (150–200 mm) reduce confinement around the sampler relative to the standard 65–115 mm diameter. Cb = 1.0 for standard diameters. | 1.0–1.15 |
| Sampler liner | Cs | Standard split-spoon without liner gives slightly lower N than with liner. Applied where liner is present or absent in non-standard configurations. | 1.0–1.2 |
| Overburden stress | CN | Normalises N to a reference effective vertical stress of 100 kPa (approximately 1 atm), because the same soil at greater depth produces higher N-values purely due to confining pressure. CN = (Pa/σ’v)0.5 with CN ≤ 2.0 (Liao & Whitman, 1986). | 0.5–2.0 |
N60 vs (N1)60 — when to use each #
N60 (energy-corrected only, not overburden-corrected) is used in correlations that already embed overburden effects — primarily bearing capacity methods (Terzaghi & Peck, Meyerhof) and some undrained shear strength correlations for clays.
(N1)60 (both energy- and overburden-corrected) is used in correlations for relative density, friction angle, elastic modulus, and all liquefaction assessments. It is the standard value for sand characterisation.
Applying an overburden correction to a correlation that already accounts for overburden — or vice versa — produces incorrect results. Always check the original reference for which N-value the correlation was calibrated against.
SPT correlations — estimating soil properties #
One of the primary values of the SPT is that N60 and (N1)60 can be correlated with a wide range of soil engineering properties through empirical relationships developed from large databases of field measurements and laboratory testing. These correlations are empirical — they carry inherent uncertainty — but they are fast, inexpensive, and widely accepted for preliminary design and routine investigations.
Different correlations apply to coarse-grained soils (sands and gravels) and fine-grained soils (clays and silts). Using a sand correlation on clay, or vice versa, produces meaningless results.
Correlations for coarse-grained soils (sands and gravels) #
| Soil property | Common methods / references | Input N-value |
|---|---|---|
| Relative density (Dr) | Skempton (1986): Dr² = (N1)60 / 60 for clean sand; Meyerhof (1957); Kulhawy & Mayne (1990) | (N1)60 |
| Friction angle (φ’) | Peck, Hanson & Thornburn (1974); Wolff (1989): φ’ = 27.1 + 0.3(N1)60 − 0.00054(N1)60²; Robertson & Campanella (1985) | (N1)60 |
| Elastic modulus (Es) | Bowles (1996): Es = αN60 where α depends on soil type (silty sand to gravel); Webb (1970); D’Appolonia et al. (1970) | N60 |
| Shear wave velocity (Vs) | Imai & Tonouchi (1982); Seed et al. (1983); multiple regional correlations | N60 or (N1)60 |
| Shear modulus (Gmax) | Derived from Vs: Gmax = ρVs² | Via Vs correlation |
Correlations for fine-grained soils (clays and silts) #
| Soil property | Common methods / references | Input N-value |
|---|---|---|
| Undrained shear strength (su) | Terzaghi & Peck (1967): su ≈ 6.25N (kPa); Stroud (1974): su = f₁·N₆₀; multiple correlations with PI correction | N or N60 |
| Unconfined compressive strength (qu) | qu ≈ 12.5N (kPa) approximate; various regional correlations | N |
| Pressuremeter modulus (Ep) | Ménard & Rousseau (1962); various correlations by soil type | N60 |
| Consistency description | Standard consistency vs N-value table (Terzaghi & Peck, 1967) | N (uncorrected) |
Consistency and density descriptions from N-value #
A widely used preliminary guide for interpreting raw N-values in the field is the Terzaghi & Peck (1967) classification:
| SPT N-value (field) | Sand — relative density | Clay — consistency |
|---|---|---|
| 0–4 | Very loose | Very soft |
| 4–10 | Loose | Soft |
| 10–30 | Medium dense | Firm to stiff |
| 30–50 | Dense | Very stiff |
| >50 | Very dense | Hard |
These field descriptions are qualitative guides only and should not be used as substitutes for corrected N-value correlations in design.
Bearing capacity from SPT #
Estimating the allowable bearing capacity of shallow foundations directly from SPT N-values is one of the most common applications of the test — particularly for preliminary design and on smaller projects where a full foundation analysis from laboratory parameters is not justified by the project scale.
All SPT-based bearing capacity methods produce allowable bearing pressures (qa) limited by a settlement criterion, not by shear failure. They implicitly include a settlement check (typically 25 mm for Meyerhof; 25 mm for Terzaghi & Peck) and a factor of safety against shear failure. The resulting qa is the net allowable bearing pressure to be applied to the foundation design.
SPT bearing capacity methods included in DartisTech software #
| Method | Reference | Settlement basis | Applicable soils |
|---|---|---|---|
| Meyerhof | Meyerhof (1956, 1974) | 25 mm | Sands and gravels |
| Terzaghi & Peck | Terzaghi & Peck (1948, 1967) | 25 mm | Sands |
| Teng | Teng (1969) | 25 mm | Sands |
| Modified Meyerhof | Bowles (1996) modification of Meyerhof | 25 mm | Sands and gravels |
| Burland & Burbidge | Burland & Burbidge (1985) | Settlement-based | Sands and gravels |
| Anagnostopoulos et al. | Anagnostopoulos et al. (1991) | 25 mm | Sands |
For critical or complex projects, SPT-based bearing capacity estimates should be validated against full bearing capacity analysis using shear strength parameters from laboratory testing. The foundation design guide covers the complete theoretical bearing capacity methods in detail.
Liquefaction assessment from SPT #
The SPT is the most widely used tool for preliminary liquefaction screening of saturated cohesionless soils under earthquake loading. The standard simplified procedure is that of Seed & Idriss (1971), with significant subsequent refinements by Seed et al. (1985), Youd et al. (2001), and Boulanger & Idriss (2014).
The procedure compares two ratios at each test depth:
- CSR (Cyclic Stress Ratio) — the earthquake-induced cyclic shear stress normalised by the effective vertical stress. Calculated from the peak ground acceleration (amax), total and effective overburden stresses, and a stress reduction factor (rd) that decreases with depth.
- CRR (Cyclic Resistance Ratio) — the resistance of the soil to liquefaction at the same effective stress. Estimated from (N1)60cs, the clean-sand equivalent corrected N-value (with a fines content correction applied where the fines content exceeds 5%).
A Factor of Safety against liquefaction (FSL) is calculated as CRR / CSR. FSL < 1.0 indicates that liquefaction is predicted at that depth under the design earthquake. The analysis is performed depth by depth throughout the saturated soil profile.
SPT-based liquefaction assessment is a screening tool. Sites with FSL close to 1.0, or where consequences are significant, warrant further investigation including CPT-based methods and specialist dynamic analysis.
SPT limitations and best practice #
The SPT has well-known limitations that every engineer interpreting the results should understand:
- High variability: Even with energy corrections applied, SPT N-values are subject to significant variability between rigs, drillers, and borehole conditions. Coefficients of variation of 15–30% are common in the same soil stratum.
- Disturbance in soft clays: The dynamic driving process severely disturbs soft cohesive soils, producing N-values that bear little relationship to the actual undrained shear strength. In soft clays, undisturbed Shelby tube sampling and vane shear testing are more reliable.
- Refusal in dense gravel or rock: The SPT cannot penetrate very dense gravels, cobbles, or weathered rock reliably. Refusal at shallow depths may indicate a localised hard layer rather than the underlying competent stratum.
- No direct measurement of drainage: The SPT does not control or measure drainage during the test. The N-value integrates both drained and undrained response in an uncontrolled way.
- Empirical correlations are not universal: All SPT correlations are empirical relationships developed from specific regional soils and geological environments. Local calibration against laboratory data significantly improves reliability.
Best practice recommendations:
- Always measure or document the hammer type and energy ratio on every borehole log.
- Apply all five correction factors before using N-values in any correlation.
- Run multiple correlations and compare results rather than relying on a single method.
- Supplement SPT with Shelby tube sampling and laboratory testing in critical zones, particularly soft clays and loose sands below the water table.
- Consider CPT profiling on the same site to provide a continuous depth profile that SPT cannot deliver. The CPT guide covers CPT interpretation in detail.
How DartiGeo and Dartis SPT handle SPT processing #
DartisTech offers SPT processing capabilities in two products: as a standalone module in Dartis SPT, and fully integrated within the comprehensive DartiGeo platform alongside borehole logging, CPT interpretation, laboratory testing, and foundation design.
Both products automate the SPT workflow in full:
- Automatic correction factor application: Enter the hammer type, rod length, borehole diameter, and depth profile once. The software calculates Ce, Cr, Cb, Cs, and CN automatically at each test depth and produces the corrected N60 and (N1)60 values throughout the borehole.
- 200+ published correlations: The full library of empirical correlations is applied simultaneously, intelligently filtered by soil type (coarse-grained vs fine-grained) at each depth interval. Results are displayed alongside the published equation so they can be checked and referenced in reports.
- Correlations with depth plots: Corrected N-values and correlation results can be plotted as variation-with-depth profiles, giving a clear visualisation of how estimated soil properties change through the soil profile.
- Bearing capacity estimation: All six SPT-based bearing capacity methods are run for the specified foundation geometry, producing qa values with full calculation transparency.
- Professional reports: All inputs, correction factors, correlation results, and bearing capacity estimates are compiled into a formatted PDF or Word report at the click of a button — with the published equations included for each correlation, meeting client and regulatory expectations for full calculation documentation.
In DartiGeo, SPT data entered in the borehole logging module feeds directly into the SPT correlations module with no re-entry, and results flow forward into the bearing capacity and settlement analysis module. The entire workflow — from field data to final foundation design report — runs in a single platform.
Frequently asked questions #
What is a good SPT N-value for foundation design? #
There is no universal “good” N-value — the adequacy of a particular N depends on the foundation size, load, and the design method applied. As a rough guide, uncorrected N-values below 5 in sand indicate very loose conditions that typically require ground improvement before shallow foundation construction. Values of 10–15 in sand are generally adequate for light to medium structures on conventional spread footings. For clay, N-values are less directly useful — consistency (stiff, very stiff, hard) and undrained shear strength from laboratory testing are more reliable design inputs.
What is the difference between N, N60, and (N1)60? #
N is the raw field blow count as recorded during the test — uncorrected for any factors. N60 applies the energy ratio correction (Ce) and, where relevant, the rod length (Cr), borehole diameter (Cb), and sampler (Cs) corrections, normalising to a hammer energy efficiency of 60%. (N1)60 applies the additional overburden correction (CN), normalising to an effective vertical stress of 100 kPa. (N1)60 is the standard input for sand property correlations and liquefaction assessment. Using the wrong N-value in a correlation that was calibrated against a different one is one of the most common errors in SPT interpretation.
What is SPT refusal and what does it mean? #
SPT refusal is declared when 50 blows are required to advance the sampler through any single 150 mm increment, or when 100 blows total are applied without achieving 450 mm of penetration. It is recorded on the borehole log as “REF” at the test depth. Refusal most commonly indicates very dense granular soil, weathered rock, a large cobble, or intact bedrock. It does not necessarily mean the borehole must be terminated — rotary drilling can continue through a refusal zone — but the N-value at refusal is recorded as 50/Xmm (where X is the actual penetration achieved with 50 blows).
Is the SPT accurate in clay soils? #
The SPT is significantly less reliable in clay than in sand. Dynamic driving severely disturbs soft to firm clays, particularly below the water table, often producing N-values that overestimate true undrained shear strength. In soft clays (N < 4), SPT results should be treated with considerable caution and supplemented with undisturbed Shelby tube sampling and triaxial or vane shear testing. In stiff to hard clays (N > 15), SPT results are more reliable and correlations to undrained shear strength are more widely used in practice.
Why do SPT results vary between different drilling rigs? #
The primary cause of variability between rigs is hammer energy delivery. A safety hammer operated by a skilled driller with a clean cathead rope may deliver 60–65% of theoretical energy; a worn rope or poor technique drops this to 50% or less; a modern automatic hammer delivers 80–95%. Without direct energy measurement and correction, N-values from different rigs in the same soil can differ by 30–50%. This is why ASTM D4633 recommends measuring hammer energy on every investigation, and why automatic hammers have become the preferred choice for quality-conscious investigations.
How many SPT tests should be performed in a borehole? #
The industry standard is to perform SPT tests at every 1.0 m or 1.5 m depth interval throughout the borehole, and at every observed change in material. In practice, many investigation programs specify 1.5 m intervals as a cost-effective compromise between resolution and testing time. For critical structures, liquefaction assessment, or sites with complex stratigraphy, 1.0 m intervals are preferable. CPT profiling between boreholes can fill the continuous-depth-resolution gap that the discrete SPT interval leaves open.
Related documentation #
- SPT procedure step by step — ASTM D1586 explained
- SPT hammer types — safety, donut and automatic compared
- SPT N-value correction factors — Ce, Cr, Cb, Cs, CN explained
- SPT correlations — estimating soil properties from N-value
- Estimating bearing capacity from SPT N-values
- Liquefaction potential from SPT — Seed & Idriss method
- SPT vs CPT — comparison and when to use each
- Borehole logging — complete guide
- Cone Penetration Test (CPT) — complete guide
- Foundation design — bearing capacity and settlement guide
