CPT vs SPT: Which Test Should You Specify?

When a project needs ground data in a hurry, the specification often includes one of two famous in-situ tests: the standard penetration test (SPT) or the cone penetration test (CPT). Both measure how the ground resists penetration, both are used to design foundations, and both have strong supporters. Yet they work very differently, suit different ground conditions and produce different kinds of data. Choosing between CPT vs SPT is not about which one is better in general, but which is better for your site, your design and your budget.

This guide compares the two tests side by side and offers practical guidance on which to specify, and when to use both.

A Quick Recap of Each Test

Standard penetration test (SPT). Carried out in a borehole. A split-barrel sampler is driven into the soil at the bottom of the hole with a 63.5 kg hammer falling 760 mm. The number of blows to drive the sampler 300 mm is the N-value. The test is performed at intervals, typically every 1.0 to 1.5 metres, and recovers a disturbed soil sample each time.

Cone penetration test (CPT). A cone-tipped probe is pushed hydraulically into the ground at a constant rate of about 20 mm per second, with no hammer and no borehole. Electronic sensors continuously measure the resistance at the cone tip (qc), the friction on a sleeve behind the tip (fs) and, in the piezocone version (CPTu), the pore water pressure generated (u2). The result is a continuous profile of ground behaviour with depth, recorded to a resolution of a few millimetres or centimetres.

The core difference is that SPT is an intermittent, dynamic, sample-recovering test, while CPT is a continuous, static, sensor-based test.

Key Differences at a Glance

Feature	SPT	CPT / CPTu
Principle	Dynamic driving of split spoon	Static push of instrumented cone
Data	One N-value per test depth	Continuous profile of qc, fs and u2
Sample recovered	Yes, disturbed	No
Needs a borehole	Yes	No, pushed directly
Repeatability	Moderate to poor	High
Resolution of thin layers	Poor (tests every 1 to 1.5 m)	Excellent (readings every 10 to 50 mm)
Speed	Slower, tied to drilling	Fast, often more metres per day
Best soils	Sands, gravels, weathered rock, mixed ground	Clays, silts, sands, soft and loose ground
Limitation in gravel and rock	Can work with cone variant, refusal common	Often refusal on gravel, cobbles and hard layers
Equipment	Drill rig, hammer, rods	CPT rig or truck, cone, data system
Groundwater information	Water level in borehole	Pore pressure data (CPTu), dissipation tests
Standards	ASTM D1586, ISO 22476-3	ASTM D5778, ISO 22476-1
Cost per metre	Usually higher once drilling is included	Often lower for deeper, continuous profiling

Where the CPT Excels

Continuous profiling. CPT does not miss thin layers. A thin soft clay seam or loose sand lens between SPT test depths can go undetected, but shows up clearly in a CPT trace. For settlement-sensitive structures, this is a major advantage.

Repeatability and objectivity. The cone and sensors are standardised and the push is mechanical, so results are far more consistent between operators, rigs and repeat tests than SPT results.

Speed and productivity. A CPT rig can often complete many tests in a day, and without the need for drilling and casing.

Soft and loose soils. In soft clays, silts and loose sands, SPT blows may be very low and give poor resolution. CPT resolves these soils well.

Rich interpretation. CPT data support soil behaviour classification, strength, stiffness, permeability trends, liquefaction assessment and pile capacity design using well-developed methods. Pore pressure data allow interpretation of drainage conditions and groundwater, with dissipation tests giving consolidation parameters.

Reduced disturbance. There is no borehole to disturb the ground, and no spoil to dispose of.

Cost efficiency on suitable sites. For deep profiling in soft or sandy ground, CPT is often cheaper per metre than drilling with SPT.

Environmental benefits. Less waste and a smaller footprint.

Where the SPT Excels

Works in a wide variety of ground. SPT can be carried out in gravelly soils, cemented layers, weathered rock and mixed ground where a CPT cone may meet refusal. This matters in many parts of Kenya, where weathered volcanic rocks, tuff, cemented soils and gravelly layers are common.

Provides a physical sample. The split spoon recovers a disturbed sample for visual description and laboratory classification, giving direct confirmation of what the ground is. CPT gives no sample, so soil type is inferred from the data.

Familiar and widely accepted. SPT has been used for decades, with a huge database of experience and correlations. Many local design methods, codes and clients’ specifications are written around SPT N-values.

Simple equipment. Drilling rigs are widely available, including in remote areas, whereas CPT rigs and trained operators may be fewer.

Integrated with borehole drilling. If boreholes are already being drilled for sampling, rock coring and groundwater installations, adding SPT is straightforward.

Access flexibility. Lighter drilling rigs can work on constrained sites where heavy CPT trucks cannot go.

Works at greater depths in hard ground where CPT may not penetrate.

Limitations Compared

SPT limitations:

• Intermittent data, so thin layers can be missed
• Considerable variability from hammer energy, operator and borehole conditions
• Disturbed samples only
• Poor resolution in very soft or very loose soils
• Slower and typically more expensive for deep, continuous profiling

CPT limitations:

• Cannot penetrate gravels, cobbles, boulders, cemented layers or rock, so refusal depth may be shallower than needed
• No soil sample, so soil classification relies on correlations and must be confirmed locally, ideally with some boreholes
• Requires specialised equipment and operators, and a rig capable of delivering sufficient reaction force
• Site access can be limited for heavy CPT trucks
• Correlations for local soils, such as black cotton clays and volcanic ash, need to be calibrated carefully
• Interpretation requires expertise

Neither test directly measures design parameters. Both rely on correlations that carry uncertainty, so local calibration and engineering judgement are always needed.

Soil Types and Typical Choice

Soft clays and silts (lake basins, river floodplains). CPTu is strongly preferred. It resolves layering and consolidation behaviour that SPT cannot.

Loose to dense sands (river and beach deposits). Both work well. CPT gives better continuity and liquefaction assessment, while SPT gives samples and is widely accepted for bearing capacity design.

Expansive clays (black cotton soils). CPT provides continuous strength profiles, but swell behaviour must be assessed through laboratory testing and sampling from boreholes and pits. SPT with sampling is useful, and trial pits are often essential.

Residual soils and weathered volcanic rock. SPT is often more practical because cone refusal can occur in cemented or gravelly layers. Combine with rotary coring for rock.

Gravelly soils and glacial or colluvial deposits. SPT, possibly with the cone variant, and drilling are usually necessary.

Rock and very hard ground. Neither test is appropriate. Use rotary coring and rock testing.

Mixed ground. A combination is often best: CPT for continuous profile in the softer layers, boreholes with SPT and sampling to confirm soil types and penetrate harder layers.

Project Type and Typical Choice

Low-rise buildings on competent ground. SPT with trial pits is often sufficient, particularly where shallow footings are expected.

Multi-storey buildings and heavy structures. Combine CPT or CPTu with boreholes and SPT. The continuous data improve settlement and bearing assessments, and boreholes provide samples.

Piled foundations. CPT is excellent for pile design in suitable soils, using direct CPT-based methods. SPT also supports pile design where CPT cannot penetrate. Investigate well below the proposed pile toe.

Liquefaction assessment in seismic areas. CPT is strongly favoured because of its continuous, repeatable data and well-established liquefaction methods. SPT is also used, particularly where CPT cannot reach the required depth.

Embankments and roads on soft ground. CPTu is ideal for identifying soft layers and consolidation parameters, supplemented by sampling for laboratory testing.

Tanks, silos and heavy industrial structures. CPT for continuous profiling, with boreholes and SPT for confirmation and sampling.

Dams and major infrastructure. Both, together with rotary coring, geophysics and laboratory testing, as part of a comprehensive programme.

Remote or constrained sites. Whichever equipment can be mobilised practically. Light drilling rigs for SPT are often more accessible.

Cost Considerations

Cost depends on site, depth, ground and provider availability, so compare quotations rather than relying on rules of thumb.

• CPT often has lower cost per metre where the ground is penetrable, because it is fast and needs no drilling. Mobilisation of the CPT rig is a significant fixed cost, so very small projects may not benefit.
• SPT is bundled with drilling cost. If you are drilling anyway for samples, coring or groundwater installations, the marginal cost of adding SPT is low.
• Combined programmes may cost more than either alone, but give the most reliable and defensible data for important structures.
• Refusal risk. If CPT refuses early, you may have to mobilise a drill rig as well, which costs more. A sound desk study reduces this risk.

Remember that the cost of the investigation is small compared with the cost of foundation problems, so choose the test that gives the data you need.

How to Decide: A Practical Checklist

Ask these questions before specifying:

1. What soils are expected? Soft clays and sands favour CPT. Gravels, cemented layers and weathered rock favour SPT and drilling.
2. How critical is thin-layer detection? If settlement sensitivity or liquefaction is important, favour CPT.
3. What does the design method require? Check whether the design codes, clients or consultants rely on SPT N-values or CPT parameters.
4. Do you need samples? If so, drilling with sampling is essential regardless of the in-situ test.
5. What depth is needed? Check whether CPT is likely to reach the required depth in the expected ground.
6. What equipment is available locally? Availability of qualified CPT contractors may be limited in some regions.
7. What is the site access like? Heavy CPT rigs need firm, accessible ground.
8. What are the budget and programme constraints? Consider mobilisation, productivity and risk of refusal.
9. How important is repeatability and data quality? CPT gives more consistent data.
10. Is local correlation available? Local experience in Kenyan soils may favour one method over the other.

The Best Answer Is Often Both

For many projects, especially larger or more critical ones, the best specification combines methods:

• CPT or CPTu to provide continuous profiles across the site, identify layering and locate critical zones efficiently.
• Boreholes with SPT and sampling at selected locations to confirm soil types, obtain samples for laboratory testing and penetrate layers that stop the cone.
• Trial pits for shallow observation and testing, particularly in expansive soils.
• Rotary coring where rock is encountered.
• Laboratory testing to determine strength, compressibility, swelling and chemical properties.

Using CPT to map the site and boreholes to calibrate it combines speed, continuity and certainty, and often gives better value than relying on one method alone.

Writing a Good Specification

Whichever test you choose, specify it clearly:

• Standard to follow, such as ASTM D1586 or ISO 22476-3 for SPT, and ASTM D5778 or ISO 22476-1 for CPT
• Equipment requirements, including hammer type and energy measurement for SPT, and cone class and calibration for CPT
• Test spacing and depth, including termination criteria such as refusal definitions or target depth
• Reporting requirements, including raw and corrected values, plots and logs
• Additional data, such as pore pressure dissipation tests for CPTu
• Qualifications of personnel and supervision requirements
• Contingency for refusal, such as authority to switch method or continue by drilling

A clear specification avoids disputes and ensures that the data you receive are usable.

Common Mistakes

• Specifying CPT without checking for gravel, cobbles or cemented layers, leading to early refusal.
• Relying on SPT alone in soft or layered ground where thin weak layers matter.
• Ignoring the need for samples and laboratory testing.
• Using uncalibrated equipment or poorly supervised crews.
• Applying correlations from other regions to local soils without checking.
• Choosing the cheapest option without considering data quality.
• Failing to define refusal and termination criteria.
• Not reporting raw and corrected SPT values or full CPT data.

Conclusion

CPT and SPT are complementary tools, not rivals. CPT offers continuous, repeatable and detailed profiles in soft and sandy ground, and is fast and cost-effective where it can penetrate. SPT works in a broader range of ground, recovers samples and is deeply embedded in design practice. Choose according to the expected soils, the design needs, the risk of refusal, local equipment availability and budget. For important or complex projects, specify both, together with sampling and laboratory testing, so that each method fills the gaps of the other. Good ground data, gathered with the right tool, is the foundation of a safe and economical design.