The Cone Penetration Test and the Standard Penetration Test are the two most widely used in-situ geotechnical tests in the world. Both measure soil resistance to penetration. Both feed into correlations for soil properties, bearing capacity, and liquefaction susceptibility. Both appear on the same geotechnical reports, sometimes from the same site. Yet they are fundamentally different tests — in how they work, what they measure, what information they produce, and where each is most and least reliable.

The question “CPT or SPT?” is one that every geotechnical engineer faces when planning a site investigation. The answer is rarely one or the other in isolation. Understanding the genuine strengths, limitations, and blind spots of each test — and knowing when a combined program gives you something that neither test alone can provide — is essential for designing an investigation that is both technically adequate and cost-effective.

This article makes the full comparison: measurement principle, data resolution, soil sampling, equipment and availability, repeatability, performance in specific soil types, suitability for particular design applications, cost, and decision rules for choosing between them.

The fundamental difference #

The SPT works by stopping drilling, lowering a split-spoon sampler to the base of the borehole, and driving it into the soil with a falling hammer. The number of blows to advance the sampler 300 mm (two 150 mm increments) is the N-value. The test takes several minutes, retrieves a disturbed soil sample, and produces a single data point — one N-value at one depth.

The CPT works by pushing a cone-tipped probe into the ground at a controlled rate of 2 cm/s, measuring the resistance to that penetration electronically and continuously as it advances. There is no hammer, no borehole, and no sampling. The test produces thousands of data points per metre — one reading every 1–2 cm — without stopping.

This difference in measurement principle — dynamic blow counting at discrete intervals vs static continuous push measurement — creates two tests with very different data products, very different error sources, and very different optimal applications. Neither test is universally superior. Each fills gaps the other leaves open.

Side-by-side comparison — the complete table #

Feature	CPT	SPT
Measurement principle	Static push — continuous electronic measurement of tip resistance (q_c), sleeve friction (f_s), and pore pressure (u₂ for CPTu)	Dynamic driving — blow count (N-value) to advance split-spoon sampler 300 mm at the base of a borehole
Depth profile type	Continuous — data every 1–2 cm	Discrete — one N-value per 1.0–1.5 m interval
Thin layer detection	Excellent — resolves layers as thin as 50–200 mm	Poor — thin layers between test depths are missed entirely
Soil sample retrieved	None	Yes — disturbed split-spoon sample for visual description and lab testing
Repeatability	High — automated push rate, no hammer variability	Moderate — hammer energy, driller technique, and equipment condition all affect N-value
Energy corrections required	None — push rate is controlled; cone area ratio a is calibrated per cone	Yes — five correction factors (C_e, C_r, C_b, C_s, C_N) required before use in correlations
Performance in clay	Excellent — continuous profile with q_t correction; s_u from N_kt method; CPTu pore pressure identifies soft zones	Moderate — N-value in soft clay is sensitive to disturbance; undisturbed tube sampling preferred for soft clays
Performance in sand	Excellent — D_r and φ’ from reliable correlations; continuous profile reveals layering	Good — N60 correlations well-established; energy correction essential
Performance in gravel/cobbles	Poor — cone refuses on individual cobbles; cannot penetrate dense gravel reliably	Good — split-spoon and rotary drilling can penetrate dense gravel
Pore pressure measurement	Yes (CPTu) — direct u₂ measurement; pore pressure ratio B_q aids classification	No — not measured
Speed	Fast — 20 m in 30–60 minutes including set-up	Slower — borehole advance between each test adds time
Soil classification	Behaviour-type classification (SBT) from q_c and R_f — no direct grain size or mineralogy	Direct visual and laboratory USCS classification from split-spoon sample
Liquefaction assessment	Excellent — continuous profile ideal for liquefaction; CPT method generally more reliable than SPT method for liquefaction	Good — SPT method well-established but discrete intervals can miss thin liquefiable layers
Cost per metre	Lower per metre — faster penetration rates and no borehole casing	Higher per metre — includes borehole advance, casing, and testing time
Equipment availability	Less universal — specialist rig required; fewer contractors in developing regions	Near-universal — standard drill rigs available almost everywhere globally
Applicable standard	ASTM D5778 / ISO 22476-1	ASTM D1586 / ISO 22476-3

Data resolution — continuous vs discrete #

The most consequential practical difference between CPT and SPT is data resolution — the frequency at which measurements are made through the soil profile.

The CPT provides a data point every 1–2 cm. A 20 m sounding produces 1,000–2,000 data points, each with q_c, f_s, R_f, and u₂. This resolution allows the CPT to detect and characterise soil layers as thin as 50–200 mm — a feat completely beyond SPT with its 1.0–1.5 m test spacing. A thin sand layer in a clay sequence — the kind of permeable layer that controls drainage paths in consolidation settlement or that represents a liquefiable zone in seismic assessment — may be entirely invisible to SPT testing if it falls between two test depths.

The SPT provides a data point every 1.0–1.5 m. A 20 m borehole produces 13–20 N-values. The profile is a series of dots, connected by assumption. Engineers drawing soil profiles from SPT boreholes must interpolate between test depths and infer the location and nature of layer boundaries from the sample description at the nearest SPT depth — which may itself be 0.5–0.75 m above or below the actual boundary.

In uniform, thick deposits — a single clay unit 15 m thick, for example — this difference matters very little. The SPT will adequately characterise the clay strength profile with tests every 1.5 m, and the CPT’s 1–2 cm resolution adds no design value that cannot be obtained from the coarser dataset. In heterogeneous deposits — alternating thin sand and clay layers, alluvial sequences with gravel lenses, estuarine sediments with variable organic content — the CPT’s continuous resolution is genuinely irreplaceable. The SPT simply cannot see what lies between its test intervals.

The soil sample — CPT’s critical gap #

The SPT’s split-spoon sampler does something the CPT fundamentally cannot: it retrieves a physical sample of the soil at each test depth. That sample, however disturbed, provides:

Direct visual identification: A geotechnical engineer can see, feel, smell, and describe the soil at each depth. Colour, texture, consistency, organic content, shell fragments, presence of gravel or fill — none of these can be inferred from CPT data alone. They require physical material.
USCS classification: The Unified Soil Classification System classification, with its ASTM D2487 group symbols, requires either direct observation of grain size or laboratory testing of the sample. CPT Soil Behaviour Type (SBT) classification is a proxy — it classifies how soil behaves during penetration, not its actual grain size or mineralogy. A highly overconsolidated clay may plot in the sand zone of the SBT chart. A calcareous sand may plot in the clay zone. SBT is a useful tool, not a substitute for a sample.
Laboratory testing input: Every geotechnical laboratory test — grain size distribution, Atterberg limits, Proctor compaction, consolidation, direct shear, triaxial — requires a physical soil sample. The SPT split-spoon sample is disturbed and unsuitable for tests requiring intact structure (consolidation, undrained triaxial), but it is fully adequate for classification testing (grain size, Atterberg limits, moisture content). Undisturbed Shelby tube samples are taken alongside SPT tests at selected depths specifically to obtain material for consolidation and shear strength testing. The CPT generates no material for any of this. See the laboratory testing guide for the full range of tests that require physical samples.
Contamination and environmental assessment: On brownfield sites where soil contamination may be present, the CPT produces no material for chemical testing. Physical samples from SPT boreholes are the only way to characterise contamination profiles for remediation design.

This is not a minor limitation of CPT — it is a fundamental constraint that makes the CPT unsuitable as the sole investigation method on most projects. The most capable and cost-effective site investigation programmes use CPT for spatial characterisation across the site and SPT boreholes at selected locations to retrieve samples for visual confirmation and laboratory testing.

Repeatability and operator variability #

The CPT is significantly more repeatable than the SPT, for a straightforward reason: the push rate is controlled mechanically at 2 cm/s and the electronic sensors measure continuously and automatically. The skill and attention of the operator affect test quality at the margins — filter saturation for CPTu, zero drift monitoring, rod deviation tracking — but the core measurement is mechanised and consistent.

The SPT is inherently more variable. The raw N-value depends on:

The type of hammer and its delivery mechanism (safety, donut, automatic, hydraulic) — which determines the energy actually reaching the sampler per blow
The condition of the cathead rope (for cathead-operated hammers) — wear and lubrication affect slip and energy loss
The driller’s technique — number of rope turns, release consistency, speed of release
Rod straightness and joint condition — bent or damaged rods lose energy before it reaches the sampler
Borehole cleanliness — loose material at the base of the borehole before the test is seated

The five correction factors (C_e, C_r, C_b, C_s, C_N) described in the SPT correction factors guide exist precisely to normalise for this variability. Properly corrected N₆₀ and (N₁)₆₀ values from different rigs using different hammers in the same soil should be comparable. In practice, the correction often relies on assumed rather than measured energy ratios — particularly the energy correction C_e — which limits the achieved standardisation.

Field measurements of SPT energy delivery have consistently shown coefficients of variation of 15–30% in repeated tests in supposedly uniform soil. Repeated CPT soundings in the same soil under controlled conditions show coefficients of variation of 5–10% for q_c — roughly three times more repeatable than SPT. This repeatability advantage of CPT is most valuable in liquefaction assessment, where the margin between liquefiable and non-liquefiable conditions is often narrow and where the reliability of the input data directly affects the safety of the conclusion.

Performance in fine-grained soils #

Both tests are used in fine-grained soils, but they measure very different things and have different reliability profiles.

CPT in clay #

The CPT excels in soft to firm clays. The continuous q_t profile — corrected for pore pressure using CPTu measurements — provides the undrained shear strength profile at every centimetre of depth via the N_kt method: s_u = (q_t − σ_v0) / N_kt. The CPTu pore pressure trace identifies consolidation state (normally consolidated clay generates high positive u₂; overconsolidated clay generates lower excess pore pressure), detects sand partings and drainage paths (u₂ drops to hydrostatic), and enables dissipation testing to estimate c_h for consolidation design. No other in-situ test produces this range of information from a single operation in soft clay.

The critical limitation of CPT in clay is that it measures behaviour, not soil type. A CPTu profile in soft clay produces a reliable s_u profile, but the N_kt cone factor must be calibrated against laboratory or vane shear data from the same site — a standard value of N_kt = 14–16 introduces significant uncertainty in soft, sensitive, or unusual clays. For critical designs in soft clay — embankment stability, pile capacity in offshore soft sediments, consolidation settlement under large loaded areas — the CPT should be complemented by undisturbed Shelby tube sampling and laboratory testing to verify N_kt and obtain consolidation parameters (C_c, C_r, c_v) that CPT cannot measure.

SPT in clay #

The SPT is significantly less reliable in soft clays (N < 4) than in sands. The dynamic driving process disturbs the soil structure of sensitive clays, generates excess pore pressures that do not dissipate during the test, and produces N-values that reflect a complex mix of undrained strength, sensitivity, and disturbance rather than a clean measure of either. SPT-based s_u correlations in soft clay (e.g. s_u ≈ 6.25N from Terzaghi & Peck) carry very high uncertainty and should be treated as order-of-magnitude estimates only. In stiff to hard clays (N > 15), the SPT is considerably more reliable and is widely used for routine bearing capacity assessment.

Verdict in fine-grained soils: CPT is clearly superior for characterisation of soft to firm clays. SPT is acceptable for stiff clays and provides the samples needed for laboratory testing at all clay consistencies. A combined programme is almost always optimal: CPT for the continuous undrained strength profile, SPT boreholes at selected locations for sample recovery and laboratory testing.

Performance in coarse-grained soils #

CPT in sand #

The CPT is highly effective in sands. The continuous q_c profile provides relative density D_r from calibration-chamber-based correlations (Baldi, Jamiolkowski, Robertson & Campanella), friction angle φ’ from q_t-based methods, and elastic modulus E_s from q_c multipliers. The continuous profile detects the thin layers and lenses — loose zones within generally dense sand, dense layers within otherwise loose fill — that SPT’s discrete interval completely misses. For liquefaction assessment in clean sands, the CPT is the preferred test because of its continuous resolution and higher repeatability.

The CPT’s limitation in sand is that it retrieves no sample. Sand layers identified by CPT as loose (low q_c) must be confirmed by a borehole sample to determine their actual gradation, fines content, and whether they warrant further investigation. Fines content in particular is critical for liquefaction assessment — fine-grained sands and silty sands have different CPT responses from clean sands and require fines correction using I_c-based methods.

SPT in sand #

The SPT is well-established in sands. The (N₁)₆₀ value, properly corrected, is the basis of most North American practice for bearing capacity, settlement, and liquefaction assessment in sandy soils. The split-spoon sample provides grain size and fines content that are needed both for USCS classification and for the fines content correction in liquefaction analysis. In dense to very dense sand (N > 30), the SPT may underestimate relative density slightly because the test energy is partly absorbed by dilatant response, but the overall performance is adequate for design purposes.

Verdict in coarse-grained soils: CPT has a data resolution advantage in heterogeneous sandy profiles and in liquefaction assessment. SPT has the sample advantage for classification and fines content determination. Both are reliable tools for dense, uniform sands; the choice is driven by site heterogeneity, available equipment, and whether sample recovery is needed.

Gravels, cobbles, and difficult ground #

This is where the CPT’s limitations are most pronounced and the SPT’s advantages are clearest.

The CPT cone cannot penetrate dense gravel, cobble layers, or materials containing individual particles larger than approximately 20–30% of the cone diameter (7–10 mm for a standard 10 cm² cone). A single cobble — even a 100 mm diameter stone in otherwise penetrable fill — can cause the cone to refuse at that depth. Dense gravels (D₅₀ > 10 mm) generate tip resistances of 40–80+ MPa that exceed the capacity of most standard CPT rigs. The result is that on many urban brownfield sites, coastal sites with beach gravel, glaciated sites with till containing boulders, or sites with construction demolition fill, CPT cannot reach the design investigation depth at all.

SPT, combined with appropriate rotary drilling, can penetrate through dense gravels (using core barrel drilling rather than auger advance), cobble layers, and cemented materials that would refuse the CPT. The drill bit can grind through obstructions; the SPT sampler, or a rock core, can then characterise whatever lies beneath. This penetration capability in difficult ground is one of the main reasons SPT drilling rigs remain the most commonly deployed site investigation equipment globally despite the CPT’s technical advantages in softer materials.

In karst terrain — where sinkholes and dissolution features in limestone produce voids — CPT refusal at unexpected shallow depths may indicate the void edge or infill rather than the competent rock head. SPT drilling can confirm the depth of actual rock and distinguish between rock head, karst infill, and void. CPT provides no way to make this distinction.

Liquefaction assessment #

Both CPT and SPT have well-established simplified procedures for liquefaction assessment. The SPT-based procedure (Seed & Idriss, 1971; Youd et al., 2001; Boulanger & Idriss, 2014) has the longest history. The CPT-based procedure (Robertson & Campanella, 1985; Robertson & Wride, 1998; Boulanger & Idriss, 2016) is increasingly preferred by practitioners where CPT data is available.

The CPT has three advantages in liquefaction assessment:

Continuous profile detects thin liquefiable layers: A 200 mm loose sand layer between two denser zones — invisible to SPT — can be identified and characterised by CPT. Thin layers are disproportionately important in liquefaction because they can initiate failure at much lower factors of safety than the surrounding material.
Higher repeatability reduces uncertainty in the factor of safety: Liquefaction assessment is a threshold problem — FS_L above or below 1.0 determines whether a layer is predicted to liquefy. With CPT’s lower coefficient of variation, the margin of uncertainty around the FS_L = 1.0 boundary is narrower, giving more confident conclusions.
More reliable fines correction: The I_c-based fines content correction in CPT liquefaction procedures is derived from continuous data and does not require a physical fines content measurement from every SPT interval. Where CPT and laboratory fines content data are available together, the calibration is more robust.

The SPT has one important advantage in liquefaction assessment: the split-spoon sample provides direct fines content measurement by laboratory grain size analysis. Fines content above 5% requires a correction to (N₁)_60cs (clean-sand equivalent) in the SPT liquefaction procedure, and knowing the actual fines content rather than inferring it from I_c reduces this source of uncertainty.

The preferred approach for liquefaction-critical projects is to use CPT as the primary dataset for continuous FS_L profiling and SPT boreholes at selected locations for laboratory fines content confirmation and cross-checking of the CPT interpretation.

For the full SPT liquefaction procedure, see Liquefaction potential from SPT — Seed & Idriss method.

Cost and speed #

On a per-metre basis, CPT is generally less expensive than SPT for investigations in soft to medium soils, for three reasons:

Speed: A CPT sounding to 20 m takes 30–60 minutes including set-up and extraction. A comparable SPT borehole to 20 m with tests every 1.5 m takes 6–12 hours. A single CPT rig can typically complete 3–6 soundings per day; a single SPT rig typically completes 1–2 boreholes per day to the same depth.
No borehole: CPT requires no drilling, no casing, no drilling fluid disposal, and no backfilling. These are significant cost components of SPT boreholes, particularly on sites where groundwater management, environmental protection, or confined-space working adds cost.
Data density: For a given site area, CPT provides far more data points per day of field work, which means a smaller number of soundings may be needed to achieve the same spatial characterisation as a larger number of SPT boreholes.

However, per-sounding mobilisation costs for CPT (specialist rig, operator, and data processing) are often higher than for SPT. In regions where SPT rigs are abundant and CPT rigs are scarce, day rates for CPT may exceed SPT despite the higher penetration speed. Total investigation cost always depends on the number of test points, their depth, the site logistics, and the prevailing market for each equipment type in the project location.

Equipment availability #

SPT equipment is available virtually everywhere in the world. Standard rotary drilling rigs — which perform SPT testing as a standard part of their capability — are operated by geotechnical contractors on every inhabited continent and in nearly every country. In regions with limited geotechnical industry infrastructure, SPT is often the only in-situ test available.

CPT equipment is less universally available. It is standard practice in the Netherlands (where it was invented), Scandinavia, Belgium, Australia, and increasingly across North America, the UK, and other developed geotechnical markets. In many developing regions — parts of Africa, the Middle East, South and Southeast Asia — CPT rigs may need to be mobilised from distant locations or imported, making them prohibitively expensive relative to locally available SPT rigs for routine projects.

This is not a reflection of CPT’s technical quality — it is a market development issue. As regional geotechnical industries mature, CPT availability tends to increase. In practice, for any project in a region where both tests are available and cost-competitive, the choice should be made on technical grounds. Where only SPT is available, an SPT-only investigation with careful sampling at critical depths can still produce a technically adequate design basis.

Converting between CPT and SPT — q_c/N₆₀ ratios #

It is sometimes necessary to convert CPT data to equivalent SPT N-values (or vice versa) — for example, to apply an SPT-based bearing capacity method to a site characterised only by CPT, or to compare new CPT results with historical SPT data from the same site. The standard conversion uses the q_c/N₆₀ ratio, which is soil-type dependent:

Soil type	Typical q_c/N₆₀ (MPa per blow/300mm)	Reference
Silts and sandy silts	0.1–0.2	Robertson et al. (1983); Lunne et al. (1997)
Clean fine sand	0.2–0.4	Robertson et al. (1983)
Clean medium to coarse sand	0.4–0.6	Robertson et al. (1983)
Gravelly sand	0.5–1.0	Robertson et al. (1983)
Sandy gravel	1.0–1.8	Robertson et al. (1983)
Soft clay	0.1–0.15	Robertson et al. (1983)
Stiff clay	0.15–0.3	Robertson et al. (1983)

These ratios carry significant uncertainty — the q_c/N₆₀ ratio for a specific soil depends on grain characteristics, mineralogy, and stress state, as well as soil type alone. A 2025 Hierarchical Bayesian study using 581 paired CPT-SPT observations from eight global regions confirmed significant regional variability in the ratio, even within the same USCS soil group — reinforcing that cross-conversion between CPT and SPT is an approximation, not a precise translation. Use CPT-to-SPT conversion only when no direct alternative exists, and apply appropriate uncertainty factors to converted values used in design calculations.

Decision guide — when to choose each test #

Choose CPT when: #

The site contains soft to medium clays, loose to medium-dense sands, or interbedded fine-grained sequences where thin-layer detection is important
Liquefaction assessment requires a continuous depth profile — particularly where thin liquefiable layers are a concern
Rapid spatial characterisation across a large site is needed and cost per sounding drives the programme
Offshore investigation is required — CPT from a seabed frame is the practical standard for offshore site investigation
High repeatability is important — for example, where monitoring ground improvement by comparing before and after soundings
Dissipation testing is needed to estimate c_h for consolidation design

Choose SPT when: #

The site contains gravels, cobbles, boulders, cemented soils, or fill with rubble where CPT refusal is likely
Physical soil samples are required — for visual description, laboratory classification, contamination screening, or undisturbed sampling for consolidation and shear strength testing
CPT equipment is unavailable or uneconomical in the project location
The investigation requires penetrating through rock or very hard strata that a CPT cone cannot advance through
The project involves karst terrain, old mine workings, or other conditions where void detection by drilling response is needed

Choose both when: #

The site involves multiple soil types, some of which resist CPT and some of which benefit from CPT’s continuous profile
Design is sensitive to thin layers that SPT would miss but sample recovery is also required for classification or testing
Liquefaction assessment requires both the CPT continuous profile and direct fines content measurement from samples
CPT results need verification against a physical sample to confirm SBT classification is geologically reasonable
The investigation involves both preliminary characterisation (well-suited to CPT) and detailed design support (which may require laboratory testing from SPT samples)

The combined program — why both tests together outperform either alone #

The highest-quality site investigations on technically demanding projects — offshore wind foundations, high-rise building rafts in variable ground, liquefaction-critical infrastructure, or complex contaminated brownfield redevelopments — almost invariably combine CPT and SPT rather than relying on either alone.

The typical combined investigation layout uses CPT soundings on a regular grid to map spatial variability across the site quickly and cost-effectively. CPT provides the continuous stratigraphic profile at many locations. SPT boreholes are then positioned at a smaller number of locations — typically one borehole per three to five CPT soundings — specifically chosen to recover samples from critical layers identified by CPT, to penetrate through CPT refusal zones, and to provide undisturbed tube samples from the key compressible or weak layers for laboratory testing.

This combined approach gets the best of both tests: the spatial coverage and thin-layer resolution of CPT, combined with the physical samples, laboratory data, and difficult-ground penetration of SPT. The two datasets also validate each other — CPT-based SBT classification compared against USCS classification from the SPT borehole confirms that the CPT interpretation is geologically realistic, and CPT-derived bearing capacity estimates compared against SPT-derived estimates from the same location provides a useful consistency check.

How DartiGeo handles both CPT and SPT data #

DartiGeo is one of the few geotechnical software platforms that integrates both CPT and SPT processing — alongside borehole logging, laboratory testing, and foundation design — in a single project file. This integration directly supports the combined program approach described above:

SPT data is entered in the field tests module alongside borehole layer descriptions. Correction factors (C_e, C_r, C_b, C_s, C_N) are applied automatically. N₆₀ and (N₁)₆₀ feed into the 200+ correlations library and SPT-based bearing capacity methods.
CPT data is imported directly from Excel (raw q_c, f_s, u₂ columns). The Robertson SBT classification, normalised Q_tn and F_r plots, soil parameter estimation, and CPT-based bearing capacity and settlement are all run automatically.
Both datasets exist in the same project file as the borehole logs and laboratory test results. Foundation design calculations can draw on SPT-based methods and CPT-based methods (Robertson & Campanella) simultaneously, allowing direct comparison of results derived from the two test types.
Reports from both SPT and CPT analysis modules are generated in the same professional PDF, Word, or Excel format, suitable for inclusion in the final geotechnical report as a single consistent document set.

Download a free 14-day trial of DartiGeo →

Frequently asked questions #

Is CPT more accurate than SPT? #

“More accurate” depends entirely on what you are measuring and in what soil. For characterising soft to medium clays and loose to medium-dense sands with a continuous depth profile, CPT is more repeatable and provides more reliable data at every depth — this is well-established. For determining actual soil type, grain size, or obtaining material for laboratory testing, CPT provides no data at all while SPT provides a sample. For penetrating gravels or cobbles, CPT fails outright while SPT succeeds. A more useful framing is that CPT and SPT have different accuracy profiles for different soil types and different engineering questions — which is exactly why the best investigations use both.

Can I convert CPT results to SPT N-values? #

Yes, approximately. The Robertson et al. (1983) q_c/N₆₀ ratios are widely used and are the standard approach for CPT-to-SPT conversion. The ratio depends strongly on soil type (grain size and fines content) and ranges from about 0.1 MPa/(blow/300mm) for silts to 1.8 for gravelly sands. A 2025 regional study using Hierarchical Bayesian methods across eight global regions confirmed significant regional variability in these ratios even within the same soil type, reinforcing that CPT-to-SPT conversion is an approximation. Use converted values as a secondary check, not as the primary basis for design if the original CPT data is available and an appropriate CPT-based design method can be applied directly.

Why do some countries predominantly use CPT and others SPT? #

The dominant test in each country largely reflects which test was adopted as standard practice first, before the alternatives were widely available, and the legacy effect of decades of local correlations, design methods, and standards calibrated to that test. The Netherlands developed CPT in the 1930s and built its entire geotechnical design framework on CPT data — so Dutch practice is CPT-centric globally. North American practice standardised SPT early and built AASHTO, USCS, and major geotechnical textbook correlations around N-values — so North American and countries influenced by US practice default to SPT. Neither heritage is wrong; both produce adequate foundations and stable earthworks when applied correctly. The technical literature increasingly recognises CPT as the more reliable test for characterisation, while acknowledging that SPT’s global ubiquity makes it irreplaceable in many contexts.

Can CPT detect sinkholes or underground voids? #

CPT can indicate the presence of anomalous low-resistance zones that might suggest loosely infilled karst or void edges — a sudden drop in q_c to near zero with no corresponding soft clay SBT classification is a potential indicator. However, CPT cannot definitively confirm a void: the cone may refuse on the vault of a cavity before reaching the void itself, or may push through collapsed infill without detecting the void geometry. Sinkholes and underground voids are best detected by a combination of methods: ground-penetrating radar (GPR) and electrical resistivity tomography (ERT) for imaging, followed by targeted drilling to confirm void presence, dimensions, and infill material. CPT alone should not be relied upon for void detection on karst or undermined sites.

If I can only do one test, which should I choose? #

If you can only do one, the choice depends on your specific project constraints. If the ground is likely soft to medium (soft clay, loose sand), site access is good, CPT equipment is available, and you do not need physical samples for regulatory or contractual reasons — choose CPT for its continuous profile and repeatability. If the ground might contain gravels, fill with rubble, or cemented layers; if physical samples are contractually or regulatorily required; or if CPT equipment is unavailable in your region — choose SPT. If you genuinely cannot determine which applies, a small number of SPT boreholes with Shelby tube sampling at critical depths will provide design-adequate information in almost any site condition, even if less efficiently than a combined program.

CPT vs SPT — Which Test Is Right for Your Site Investigation?