DCP to CBR: converting dynamic cone penetrometer readings

To convert a DCP reading to CBR, divide the penetration in millimeters by the number of hammer blows to get the DCP index in millimeters per blow. Then apply the U.S. Army Corps of Engineers correlation in ASTM D6951: CBR = 292 ÷ DCP^1.12. A DCP index of 10 mm per blow works out to a CBR of about 22.

Step by step

Record penetration against blows. Read the scale after each blow, or after a set number of blows in harder material.
Work out the DCP index for each layer. Divide the penetration by the blows that produced it. 50 mm over 5 blows is 10 mm per blow.
Apply the correlation. CBR = 292 ÷ 10^1.12 = 292 ÷ 13.18 = 22.

If your scale reads in inches, multiply by 25.4 first. 0.4 inches per blow is 10.2 mm per blow.

DCP index to CBR table

Each line gives the DCP index in millimeters per blow, then the CBR for all soils except lean clay (CL) below CBR 10 and fat clay (CH). Where a second figure appears, it is the CBR for lean clay below CBR 10.

2 mm per blow: CBR over 100
3 mm per blow: CBR 85
5 mm per blow: CBR 48
7.5 mm per blow: CBR 31
10 mm per blow: CBR 22
15 mm per blow: CBR 14
20 mm per blow: CBR 10 (lean clay: 8.6)
25 mm per blow: CBR 7.9 (lean clay: 5.5)
30 mm per blow: CBR 6.5 (lean clay: 3.8)
40 mm per blow: CBR 4.7 (lean clay: 2.2)
50 mm per blow: CBR 3.7 (lean clay: 1.4)
75 mm per blow: CBR 2.3 (lean clay: 0.6)
100 mm per blow: CBR 1.7 (lean clay: 0.3)

Which equation for which soil

All soils except the two below: CBR = 292 ÷ DCP^1.12
Lean clay (CL) with CBR below 10: CBR = 1 ÷ (0.017019 × DCP)²
Fat clay (CH): ASTM D6951 gives a separate correlation; use the one in the standard.

DCP is in millimeters per blow. These are the U.S. Army Corps of Engineers correlations (Webster, Grau and Williams, 1992) that ASTM D6951 adopts. Many state DOTs use them.

A DCP gives in-place CBR, not lab CBR

ASTM D6951 says the DCP estimates the in situ CBR of the material, and that this estimate will not normally correlate with the laboratory or soaked CBR of the same material. Compare a DCP result against a field CBR, not a lab one.

Some agencies use their own correlation. North Carolina DOT, for example, uses CBR = 435 ÷ DCP^1.08 (Wu and Sargand, Ohio DOT and FHWA). If your specification names a correlation, use that one.

Where the conversion stops working

It is an estimate. DCP to CBR is an empirical correlation, not a CBR test.
Not for highly stabilized or cemented materials, or granular material with a large percentage of aggregate over 50 mm (2 in.), per ASTM D6951.
Correlations belong to the instrument. ASTM D6951 notes they are only appropriate for specific DCP configurations. These equations assume the standard instrument: 8 kg hammer, 575 mm drop, 60° cone with a 20 mm base.

Which kit reads the range you need

Single mass DCP kits: 8 kg hammer, CBR 10 to 100
Dual mass DCP kits: 8 kg and 4.6 kg hammers, CBR 0.5 to 100

Every Salem Tool kit is built in Northville, Michigan, to ASTM D6951 and U.S. Army Corps of Engineers standards. Compare the kits, or request a quote for a purchase order.

Common questions

What is the DCP index?

The DCP index is the penetration per hammer blow, in millimeters. A lower number means stiffer material.

Is a DCP CBR the same as a lab CBR?

No. ASTM D6951 says the DCP estimates in situ CBR, which will not normally match the laboratory or soaked CBR of the same material.

Which standard covers converting DCP readings to CBR?

ASTM D6951, Standard Test Method for Use of the Dynamic Cone Penetrometer in Shallow Pavement Applications, includes the U.S. Army Corps of Engineers correlations.