Convert a dynamic cone penetrometer (DCP or Scala) reading in mm per blow to CBR by soil type, or find the penetration rate that meets a target CBR.
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Calculator Functions
The calculator converts a dynamic cone penetrometer (DCP) reading, in millimeters of penetration per hammer blow, into an estimated in-place California Bearing Ratio (CBR). Select a function to open it in the calculator.
| Function | Use it when | You enter |
|---|---|---|
| CBR from a DCP reading | You have a penetrometer reading and want the CBR it corresponds to | Penetration per blow, or the number of blows and the total penetration |
| DCP reading for a target CBR | A specification asks for a minimum CBR and you want the reading to look for on site | Target CBR |
Both functions ask for the soil type, because clays follow their own equations. Choose the first option if the soil is gravel, sand or silt, or if you do not know. Readings can be entered in millimeters or inches. To work out a CBR from the loads of an actual CBR test, use the CBR Ratio Calculator.
DCP to CBR Formulas
| CBR | California Bearing Ratio (%) |
| D | DCP index: penetration per blow (mm/blow) |
This is the U.S. Army Corps of Engineers correlation for gravel, sand and silt [1], with the DCP index in millimeters per blow [2]. A reading of 15 mm per blow gives 292 ÷ 151.12 = 14.1%. The DCP index is the penetration divided by the number of blows that produced it, leaving out the first seating blow [2].
Low-plasticity clay
This equation is for low-plasticity clay (CL) and applies where the CBR is below 10 [1] [2]. At 40 mm per blow it gives 1 ÷ (0.017019 × 40)² = 2.2%. When it returns 10 or more, the calculator uses the general equation instead.
High-plasticity clay
For high-plasticity clay (CH), 30 mm per blow gives 1 ÷ (0.002871 × 30) = 11.6%.
DCP reading for a target CBR
This rearranges the general equation. A target CBR of 10% needs (292 ÷ 10)0.893 = 20.3 mm per blow or less. The clay equations are rearranged the same way.
DCP to CBR conversion chart
| mm per blow | Gravel, sand, silt | CL clay | CH clay |
|---|---|---|---|
| 2 | 134 | 134 | 174 |
| 3 | 85 | 85 | 116 |
| 4 | 62 | 62 | 87 |
| 5 | 48 | 48 | 70 |
| 6 | 39 | 39 | 58 |
| 8 | 28 | 28 | 44 |
| 10 | 22 | 22 | 35 |
| 12 | 18 | 18 | 29 |
| 15 | 14 | 14 | 23 |
| 20 | 10 | 8.6 | 17 |
| 25 | 7.9 | 5.5 | 14 |
| 30 | 6.5 | 3.8 | 12 |
| 40 | 4.7 | 2.2 | 8.7 |
| 50 | 3.7 | 1.4 | 7.0 |
| 75 | 2.3 | 0.6 | 4.6 |
| 100 | 1.7 | 0.3 | 3.5 |
The chart gives CBR in percent from the three equations above.
Limits of This Estimate
- The result is an estimate from an empirical correlation with field CBR tests, not a measured CBR. Other correlations exist and give different values [1].
- The equations are for a penetrometer with an 8 kg hammer falling 575 mm onto a 60-degree cone 20 mm across [2]. If your penetrometer, including one sold as a Scala penetrometer, has a different hammer, drop or cone, it will not follow them exactly. One with an automatic release hammer reads about 15% higher in CBR [1].
- The DCP is highly variable in gravelly soils and tests only to about 1 m deep [1].
- The reading reflects the moisture in the ground on the day of the test, so the result is not the same thing as a soaked laboratory CBR.
Sources
- Federal Highway Administration, Geotechnical Aspects of Pavements Reference Manual (NHI-05-037), Chapter 4, Table 4-9: the three DCP to CBR correlations by soil type, the automatic hammer difference and the limits of the test
- Siekmeier, Young and Beberg (Minnesota Department of Transportation), Comparison of the Dynamic Cone Penetrometer with Other Tests During Subgrade and Granular Base Characterization in Minnesota, ASTM STP 1375: the units of the DCP index, the full coefficient of the low-CBR equation, the penetrometer dimensions and the seating blow
