What Are DCP, KPT and SPT?

1. DCP — Dynamic Cone Penetrometer (ASTM D6951)

An 8 kg hammer falls freely 575 mm to drive a 60° cone into the soil; the penetration per blow (mm/blow) is measured and converted to an in-situ CBR — a tool designed specifically for pavement works. Read the full details in What Is the DCP Test?

  • ✓ Light and portable; one or two people can run the test
  • ✓ Gives field CBR immediately; used for compaction quality control and finding soft spots
  • ✗ Reaches only ~1 m (rods can be extended to about 2 m, but results start to lose accuracy)
Cone tip and drive rod of a DCP (Dynamic Cone Penetrometer) for testing pavement layer strength
DCP — 60° cone tip, 8 kg hammer, a lightweight portable device used to obtain the in-situ CBR of pavement layers

2. KPT — Kunzelstab Penetration Test (Light Dynamic Probing)

A 10 kg hammer falls 50 cm to drive a cone through the soil, and the number of blows per penetration (blows/20 cm) is counted. The cone is larger than the rod, which reduces rod friction — widely used in Thailand for shallow foundations of houses and small buildings. Read the full details in What Is the Kunzelstab Penetration Test (KPT)?

  • ✓ Fast, economical, fits tight spaces, no drilling rig needed
  • ✓ Estimates bearing capacity and the thickness of soft/stiff layers
  • ✗ Reaches about 5 m (up to 7–10 m in soft soil, but with lower reliability)
  • ✗ No soil sample is recovered — you only know the "resistance", not exactly what type of soil it is
Light penetrometer for the Kunzelstab Penetration Test (KPT) during field testing
KPT — light penetrometer with a 10 kg hammer, run by a 2–3 person team without a drilling rig; suited to shallow foundations

3. SPT — Standard Penetration Test (ASTM D1586)

Performed in a borehole: a 63.5 kg hammer falls 76 cm to drive a split-spoon sampler 45 cm into the soil, and the blows over the last 30 cm give the N-value (SPT-N) — the main standard for foundation and pile design (see how to interpret it in SPT N-Value Interpretation)

  • ✓ Reaches 10–30+ m, as deep as the borehole
  • ✓ Recovers soil samples for classification and laboratory testing
  • ✓ The N-value has the most complete set of correlations with design parameters (shear strength, density, pile capacity)
  • ✗ Requires a drilling rig/boring set; the most expensive and time-consuming option
Soil boring rig performing a Standard Penetration Test (SPT) in a borehole
SPT — performed in a borehole with a drilling rig and crew; 63.5 kg hammer; yields both the N-value and soil samples for deep foundation/pile work

How Do DCP, KPT and SPT Differ?

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TopicDCPKPTSPT
Reference standardASTM D6951/D6951MBased on DIN 4094 (now DIN EN ISO 22476-2 · light DPL)ASTM D1586
Hammer mass / drop height8 kg / 575 mm10 kg / 50 cm63.5 kg / 76 cm
Energy per blow (theoretical)≈ 45 J≈ 49 J≈ 473 J
Measured valueDCP Index (mm/blow)Blows/20 cmSPT-N (blows/30 cm)
Practical depth~1 m~5 m10–30+ m
Used forin-situ CBRApprox. bearing capacityN-value → foundation/pile design
Soil sample?✗✗✓ (Split Spoon)
EquipmentPortable, 1–2 peoplePortable, 2–3 peopleDrilling rig + crew
SpeedFastest (a few minutes per point)FastSlowest (1–2 days per borehole)
Relative cost฿฿฿฿฿฿฿

Energy per blow in the table is calculated from the hammer's potential energy m·g·h (e.g. SPT = 63.5 kg × 9.81 m/s² × 0.76 m ≈ 473 J) — SPT uses about 10 times more energy than DCP and KPT, so it can drive through stiff layers and reach much greater depths. The energy actually transferred to the rods is lower than the theoretical value and depends on the hammer type and operation.

The figure below summarizes the table above in a single section — all 3 methods use a drop hammer, but the hammer mass, drop height and achievable depth differ greatly.

Section comparing DCP, KPT and SPT tests on the same soil profile, showing hammer mass, drop height, energy per blow, achievable test depth and how the results are used A 2D section divided into three panels over the same ground surface and soil profile: pavement or fill, soft clay, stiff clay and dense sand. Left panel: DCP per ASTM D6951, portable equipment, 8 kg hammer, 575 mm drop, 60-degree cone, about 45 J per blow, about 1 m deep, giving the DCP Index in millimeters per blow converted to field CBR. Middle panel: KPT based on DIN 4094, now ISO 22476-2, 10 kg hammer, 50 cm drop, about 49 J per blow, cone larger than the rod, about 5 m deep, up to 7–10 m in soft soil but with lower reliability, giving blows per 20 cm used to estimate Qa. Right panel: SPT per ASTM D1586 in a borehole under a tripod with pulley and winch, 63.5 kg hammer, 76 cm drop, about 473 J per blow, driving a split spoon 45 cm and counting the last 30 cm as the N-value; the borehole goes 10–30 m or more, so a depth break symbol is shown; gives SPT-N and soil samples for foundation and pile design. Energy per blow is mass times gravitational acceleration times drop height, the theoretical potential energy. 2D section: DCP · KPT · SPT — driving energy and achievable depth Not to scale DCP Dynamic Cone Penetrometer ASTM D6951 KPT Kunzelstab Penetration Test DIN 4094 (now ISO 22476-2) SPT Standard Penetration Test ASTM D1586 8 kg Drop 575 mm ≈ 45 J/blow Portable, 1–2 people ≈ 1 m 60° cone 10 kg hammer Drop 50 cm ≈ 49 J/blow 2–3 crew · no rig ≈ 5 m Soft soil: up to 7–10 m but less reliable Cone > rod Split Spoon Count last 30 cm of 45 cm → N Borehole 63.5 kg hammer ≈ 473 J/blow Drop 76 cm Boring rig + crew 10–30+ m per borehole Pavement / fill Soft clay Soft Clay Stiff clay Stiff Clay Dense sand Dense Sand DCP Index (mm/blow) → field CBR N/20 cm → approx. Qa SPT-N + soil samples → foundation/pile design Energy per blow = m·g·h (theoretical potential energy)
DCP, KPT and SPT compared on the same soil profile — hammer/drop height, theoretical energy per blow (≈ 45 / 49 / 473 J), achievable test depth (≈ 1 m / ≈ 5 m / 10–30+ m) and how the results are used (Diagram: SPN Soil Engineering)

Real Test Results from All 3 Methods

To show how the output of each method looks different, below are reports from real SPN projects — note that DCP gives the DCP Index/CBR, KPT gives blows/20 cm with an approximate bearing capacity, while SPT gives the N-value alongside a soil description from actual soil samples.

Sample DCP test report showing DCP Index, CBR and a CBR vs depth plot
DCP — report of DCP Index and in-situ CBR by depth interval, with a CBR–depth plot (real example, point LS-DCP-9A)
Sample Kunzelstab Penetration Test (KPT) report showing blows per 20 cm and bearing capacity
KPT — report of blows/20 cm and approximate allowable bearing capacity (Qa) at each depth (real example, point KZ-1)
Sample boring log from the Standard Penetration Test (SPT) showing N-values and soil descriptions
SPT — boring log showing N-values alongside soil descriptions and groundwater level (real example, borehole BH-1)

Which Method Suits Which Job?

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Type of workRecommended methodReason
Roads, parking areas, pavement layers, slab-on-gradeDCPCBR of subgrade/subbase layers needed, no deeper than 1 m
Compaction quality control of fill, finding soft spotsDCPMany points tested quickly to compare uniformity
Houses, 1–2 storey buildings, spread footingsKPTBearing capacity assessed to ~5 m is enough for shallow foundations; low cost
Preliminary check of soft layer thickness before committing to boreholesKPTMany probes spread across the site on a limited budget
Multi-storey buildings, factories, piled projectsSPT (soil boring)Soil profile needed down to the pile bearing layer + samples for lab testing
Projects where the law/designer requires a boring logSPTKPT/DCP cannot replace a soil investigation report
📌 Rule of thumb

DCP = pavement (very shallow)  |  KPT = shallow foundations (medium)  |  SPT = deep foundations/piles (deep)

Can the 3 Methods Be Combined, and Why Is That More Cost-Effective?

On real projects the three methods do not compete — they complement each other:

  1. KPT probes spread across the site at many points on a modest budget → a rough map of soft layer thickness
  2. SPT borings at representative points chosen from the KPT results → N-values and soil samples for actual design
  3. DCP for acceptance of fill/pavement layers during construction → confirms that compaction achieved the specified strength

This approach covers the whole site without drilling an SPT borehole at every point, saving a lot of the investigation budget (see budget guidance in Soil Boring Prices 2026)

The model below shows where each of the 3 methods sits on the same project and which soil layers each one reaches — KPT spread across the site, SPT only at representative points but down to the sand layer, and DCP only in the pavement layers of roads and parking areas.

3D model of KPT, SPT and DCP used together on one project, on a site cut away to reveal the soil profile A conceptual 3D view of a construction site with the soil block cut away to show sections on two sides. The top layer is soft clay, followed by stiff clay and then a sand layer below. Dashed lines on the ground surface show the outline of a future building or factory. Many blue pins spread across the site are KPT points; the section shows a KPT rod reaching about 5 m and stopping in stiff clay. Three SPT boreholes are at representative locations; the one at the edge of the section has a tripod boring set with pulley and hammer, and the borehole in the section goes through the clay layers to the sand layer, ending in a split-spoon sampler. Orange ticks beside the borehole mark the driving intervals for N-value counts and soil sampling. On the right is a road and parking area with pavement layers below the surface; green points are DCP with rods going only about 1 m into the pavement layers. The right-hand box lists the sequence: KPT probes spread out to map soft layer thickness on a modest budget; SPT borings at representative points give N-values and soil samples for actual design; DCP checks pavement layers and fill during construction. A note says that where the law or designer requires a boring log, SPT borings are needed and KPT or DCP cannot replace them; Bangkok soft clay is about 12–15 m thick, so KPT cannot reach the sand bearing layer for piles; and the number and location of test points are set by the engineer. 3D model: 3 methods combined on one project Not to scale 1 KPT probes Blue pins site-wide 2 SPT points Rig, key boreholes Future building/factory 3 DCP on road/parking Pavement check KPT rod ~5 m Stops in stiff clay SPT borehole to sand Orange = N-count zone + soil sampling DCP ~1 m Pavement/fill only Section: Pavement/fill Soft clay Stiff clay Sand Combined survey plan 1 KPT probes site-wide → soft layer map Low cost 2 SPT at key points → N-value + samples → for actual design 3 DCP checks pavement/fill During construction When SPT is required If law/designer requires a boring log, drill SPT — KPT/DCP can't replace it BKK soft clay ~12–15 m KPT can't reach pile sand Points set by engineer Conceptual Not an actual survey plan
Conceptual example of combining the 3 methods: KPT probes spread across the site, SPT borings at representative points down to the sand layer, and DCP acceptance testing of pavement layers on roads/parking areas — the actual number and location of test points are set by the engineer (Diagram: SPN Soil Engineering)

What to Watch Out for When Interpreting Results

⚠️ Do not compare values directly across methods

DCP Index, KPT-N and SPT-N are different quantities with very different driving energies. Correlations between them exist in research, but they depend on soil type and location — interpretation should be left to a geotechnical engineer.

⚠️ KPT/DCP do not reveal groundwater

Groundwater level strongly affects foundation design; if you need to know it, you need a soil investigation

❌ Bangkok soil and the soft clays of the Central Plains

With a soft clay layer ~12–15 m thick, light probing can only say "soft all the way down", but pile design requires knowing the sand bearing layer, which is deeper than KPT can reach — such projects require soil boring + SPT only (read more in Bangkok Soft Clay and Foundations)

📌 Note

This article is provided as engineering knowledge. The choice of test method, number of points and interpretation of results should follow the project requirements and the judgment of a licensed engineer.