What Is KPT?

The Kunzelstab Penetration Test (KPT) is a light dynamic penetration soil test in which a hammer is dropped freely from a fixed height onto an anvil, repeatedly, to drive a steel cone tip into the ground, and the number of blows (blow count) for every 20 centimeters of penetration is recorded. The result is called N/20 cm.

Because the cone tip is larger than the drive rod, it reduces friction along the rod, so the measured value mainly reflects the resistance at the cone tip. This N value is used to assess the soil's bearing capacity and the thickness of soft and stiff layers without a drilling rig, which is why it is popular in Thailand for shallow foundations of houses and small buildings and for preliminary soil surveys.

Kunzelstab Penetration Test (KPT) light dynamic probing rig being set up for a field test
KPT rig (light penetrometer) with a 10 kg hammer, drive rods and anvil — lightweight, easy to set up, run by a crew of 2–3, and able to test in tight spaces
💡 Rule of thumb

Stiff soil → cone penetrates with difficulty → many blows → high N value → high bearing capacity  |  Soft soil → cone penetrates easily → few blows → low N value → low bearing capacity

Why Run a KPT, and What Are the Benefits?

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BenefitDetails
Assess bearing capacityCalculate a preliminary allowable bearing capacity (Qa) for shallow foundation design
Find soft-layer thicknessIdentify the depth where soil changes from a soft to a stiff layer from the N–depth plot
Survey across the whole plotTest many points on a limited budget and map soil uniformity before deciding on actual boreholes
Reach difficult sitesPortable equipment with no drilling rig, suited to tight spaces, narrow alleys or areas vehicles cannot reach

KPT's main strength is that it can probe many points across a plot on a limited budget, showing that the stiff layer lies at different depths at different locations before the foundation level is decided, as in the model below.

3D model of a house plot probed with KPT at several points around the footing locations, showing the stiff layer at different depths within a single plot Conceptual 3D view of a soil block under a house plot, with the front and right faces cut away to show the soil layers. The upper layer is soft to medium soil, shown transparent so the drive rods inside are visible. The lower layer is dense or stiff, with an uneven top surface that slopes and undulates from about 1.4 meters to about 2.6 meters, so the soft-layer thickness varies within a single plot. On the ground surface, a dashed line marks the house outline and small squares mark nine footing locations. Six KPT probe points are shown as orange rings near the footing locations. Each drive rod penetrates down to the stiff layer, and a red dot at the rod tip marks refusal, which occurs at different depths. At one point a KPT rig — guide rod, 10 kilogram hammer and anvil — is shown with two crew members. A box on the right summarizes four points: one, probe KPT at several points around the footing locations; two, the stiff layer lies at different depths, which can be seen from the N versus depth plot; three, KPT reaches about 5 meters, gives no soil samples and does not show the groundwater level; four, multi-story buildings or piling work require soil boring with SPT, with a note that the number and location of points are set by the engineer. This is a conceptual image. 3D model: KPT probes across the plot before footing design Not to scale ≈ 1.4 m ≈ 2.6 m House (dashed) = footing location KPT rig 10 kg hammer · crew of 2–3 Orange ring = KPT point near footing location Soft/medium layer shown clear to see rods Dense/stiff uneven top, varying depth Red dot = refusal rod tip at stiff layer Soft-layer thickness varies within one plot Orange line = soft-layer thickness · conceptual KPT on a house plot 1 Probe KPT at many points around footing locations 2 Stiff layer depth varies → seen on N–depth plot 3 Depth ≈ 5 m No soil samples No groundwater level 4 Multi-story/pile work → boring + SPT No. & location of points set by the engineer Conceptual · not an actual layout
Conceptual 3D model: KPT probes at several points near the footing locations of a house. Each rod tip reaches refusal at a different depth because the top of the stiff layer is uneven, so the soft-layer thickness varies within a single plot (Diagram: SPN Soil Engineering)

What Does a KPT Rig Consist Of?

A standard KPT rig consists of the following main parts:

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ComponentTypical specification
Hammer10 kg
Drop height50 cm (500 mm)
Cone tipSteel cone with a base larger than the rod, to reduce friction along the rod
Drive rodSectional steel rods that can be extended for greater depth
Recorded valueNumber of blows (blow count) per 20 cm of penetration
Energy per blow (theoretical)≈ 49 J (10 kg × 9.81 m/s² × 0.5 m) — close to a DCP with an 8 kg hammer (≈ 45 J), but about 10 times lower than SPT
🔧 Part of the light penetrometer family

KPT belongs to the light dynamic probing group, originally covered by the German standard DIN 4094 and now superseded by DIN EN ISO 22476-2 (Dynamic Probing). The light DPL rig in that standard uses a 10 kg hammer dropped 50 cm, the same as KPT, but counts blows per 10 cm, whereas the KPT used in Thailand counts per 20 cm and its cone size follows the manufacturer. The numbers therefore should not be compared directly.

How Is a KPT Performed?

  1. Set up plumb — place the cone tip at the test point, align the rod perpendicular to the ground surface, and check that the rod joints and anvil are tightened
  2. Lift and drop the hammer freely — lift the 10 kg hammer to the 50 cm drop height and let it fall freely onto the anvil, repeatedly (do not push down)
  3. Count blows per 20 cm — record the number of blows needed to drive the cone each 20 centimeters, as N/20 cm, over the full depth
  4. Add rods as it goes deeper — when the rod has sunk close to the ground surface, attach the next rod section and continue testing
  5. Stop and summarize — stop at the required depth or when the refusal criterion is reached, then plot the results and calculate bearing capacity
❌ Stopping criterion (refusal)

Stop the test when the cone barely penetrates under driving (e.g. more than 50–100 blows per 20 cm), which indicates a stiff layer or a dense sand/gravel layer — in the real example below, the test stopped at N = 100 blows/20 cm at a depth of 2.4 meters.

The figure below sums up the whole test in one image — a 10 kg hammer dropped freely through 50 cm drives the cone tip into the ground, blows are counted every 20 cm until a stiff layer is reached and the refusal criterion is met, and the N values are then plotted against depth to assess bearing capacity.

Section of a KPT light dynamic probing rig during testing, with a plot of N per 20 centimeters versus depth at the same levels Two side-by-side panels sharing the same depth scale. The left panel is a section of a KPT rig, or light penetrometer, on the ground surface: a hammer guide rod, a 10 kilogram hammer lifted and dropped freely onto the anvil through a 50 centimeter drop, and sectional drive rods with two joints transmitting the force down to the cone tip at the end of the rod. Beside the rod is a depth scale marked every 20 centimeters, because blows are counted for every 20 centimeters of penetration. The upper soil to about 1.3 meters is medium-strength soil; below about 1.4 meters is a dense or stiff layer. The cone tip stops at refusal at a depth of 2.4 meters. A detail box shows that the cone tip is larger than the rod, so the rod does not rub against the soil, reducing friction along the rod. The right panel is a horizontal bar chart of N values in blows per 20 centimeters from 0.2 to 2.4 meters, illustrative values following the same pattern as the actual results at point KZ-1. In the upper part N is about 13 to 22 blows; the stiff layer starts at about 1.4 meters with N equal to 48 and increases steadily to refusal, N equal to 100, at 2.4 meters. A formula box shows the empirical formulas: for sand, Qa equals 0.64 times N minus 3.57, and for clay, Qu equals 0.34 times N plus 0.954, in tonnes per square meter. Example: N equal to 48 in sand gives Qa of about 28.4 tonnes per square meter. For preliminary assessment only; a geotechnical engineer must review the results. 2D section: KPT rig and N per 20 cm Not to scale KPT rig (light penetrometer) Free-fall hammer drives cone Driving result: N per 20 cm Illustrative (same pattern as KZ-1) Drop 50 cm Guide rod (Guide Rod) 10 kg hammer Lift, then free fall Anvil Takes the impact 1.0 2.0 Blows per 20 cm = N (blows/20 cm) Medium soil Upper ~1.3 m Drive rods Added as it goes deeper Dense/stiff From ≈ 1.4 m Refusal at 2.4 m Cone barely moves → stop Cone wider than rod → less rod friction Rod clear End bearing Sand: Qa = 0.64 × (N − 3.57) t/m² Clay: Qu = 0.34 × (N + 0.954) t/m² Example: N = 48 (sand) → Qa ≈ 28.4 t/m² Empirical, preliminary use only — geotech engineer to review 0 20 40 60 80 100 N (blows/20 cm) → 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 ← Depth (m) 13 15 18 22 20 17 48 55 63 72 86 Refusal N = 100 Stiff from ≈ 1.4 m Upper N ≈ 13–22 Medium soil
Section of a KPT rig during driving: a 10 kg hammer dropped freely 50 cm, with blows counted every 20 cm, paired with an N–depth plot on the same depth scale (N values are illustrative, following the same pattern as the actual results at point KZ-1), plus formulas for preliminary Qa/Qu assessment (Diagram: SPN Soil Engineering)

How Is Bearing Capacity Calculated from the N Value?

The N value (blows per 20 cm) can be used to estimate the allowable bearing capacity (Qa) with empirical formulas, chosen by soil type, in tonnes per square meter (t/m²):

Sand:   Qa = 0.64 × ( N − 3.57 ) Allowable bearing capacity for sand
Clay:   Qu = 0.34 × ( N + 0.954 ) Bearing capacity for clay
✅ Worked example

Sand with a measured N = 21 blows/20 cm → Qa = 0.64 × (21 − 3.57) = 11.2 t/m²  |  For sand with N = 48 → Qa = 0.64 × (48 − 3.57) = 28.4 t/m²

N Value vs Bearing Capacity Table (Approximate)

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N (blows/20 cm)Qa, sand (t/m²)Qu, clay (t/m²)Strength level
< 5< 1~2Very soft
104.13.7Soft
157.35.4Medium
2010.57.1Medium
3016.910.5Dense/stiff
5029.717.3Very dense
≥ 10061.734.3Very stiff (refusal)

Calculated from the formulas above — preliminary estimates only. Actual design must take into account the soil type, groundwater level and the judgment of a geotechnical engineer.

Example of Actual KPT Test Results

The example below is a KPT test report from an actual SPN project (test point KZ-1), tested from the ground surface to a depth of 2.4 meters. The report shows the blow count (N/20 cm) and the allowable bearing capacity (Qa) at each depth, with a blow count versus depth plot on the right.

Example Kunzelstab Penetration Test (KPT) report with a table of N values and a blow count versus depth plot
Example of an actual KPT test report (point KZ-1) — table of N/20 cm and Qa by depth interval, with a blow count–depth plot, prepared by the SPN Soil Engineering engineering team

Note that from 0.2–1.2 meters N is about 13–22 blows (medium-strength soil). At 1.4 meters N jumps to 48 and keeps increasing to 100 blows at 2.4 meters, indicating a dense/stiff soil layer suitable as a foundation bearing stratum. Plotting the results this way lets the engineer choose a safe foundation level right away.

What Are the Limitations of KPT?

⚠️ No soil samples

KPT measures only "driving resistance" and recovers no soil samples for classification, so the correct formula (sand/clay) must be chosen using nearby soil profile data or engineering judgment; otherwise the Qa value will be off.

⚠️ No groundwater level

KPT cannot identify the groundwater level, which is an important factor in foundation design. If it is needed, soil boring is required.

❌ Limited depth — not a substitute for boring + SPT

KPT reaches about 5 meters (7–10 meters in soft soil, but with reduced reliability), which is not enough for multi-story buildings or piles, where you need to know the pile-bearing sand layer at much greater depth. Such work requires soil boring + SPT.

Which Standards Does KPT Follow?

  • DIN EN ISO 22476-2 — Geotechnical investigation and testing: Dynamic Probing, the current standard for dynamic probing, which covers the light rig (DPL) in the same family as KPT (it supersedes the former DIN 4094, a name some Thai project documents still cite)
  • The bearing capacity (Qa) formulas are empirical equations used in soil surveys for shallow foundations and should be adapted to local soil conditions

Because KPT is a relative test, the resulting N and Qa values should be used together with local experience and a geotechnical engineer's interpretation.

KPT is often combined with other soil tests to give complete data for design:

📌 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.