CPT Friction Ratio Soil Classification Tool
Work out the CPT friction ratio Rf from cone tip resistance and sleeve friction, and get a quick soil-behaviour indication (sands, silts or clays). Used by geotechnical engineers and site investigators for a first read of cone penetration test logs.
Enter Values
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How to use this calculator
- Enter the cone (tip) resistance qc in MPa.
- Enter the sleeve friction fs in kPa.
- Read the friction ratio Rf (%) and the simplified soil-behaviour band.
How it works
The friction ratio is the sleeve friction as a percentage of the cone resistance: Rf = (fs / qc) × 100. Because qc is entered in MPa and fs in kPa, qc is first converted to kPa (× 1000), so the calculation reduces to Rf = fs / (10·qc).
Low friction ratios (Rf < 1%) point to clean sands and gravels, intermediate values (1–4%) to silty sands and silts, and high values (> 4%) to clays. This is a screening rule only — a full Robertson soil-behaviour-type chart uses the normalised cone resistance Qt and normalised friction ratio Fr, which correct for overburden stress and pore pressure.
Worked example
Sandy silt at 8 m depth. A cone reads qc = 5 MPa (= 5000 kPa) and fs = 100 kPa. Rf = 100 / 5000 × 100 = 2.0%. An Rf of 2% falls in the 1–4% band, indicating silty sands / silts.
Common mistakes
- Mixing units — qc must be converted to the same unit as fs (MPa × 1000 = kPa) before taking the ratio.
- Treating the Rf band as a definitive soil classification; it is a behaviour indication, not a USCS/AS 1726 class from a sample.
- Using Rf alone for design — normalised Qt and Fr (Robertson SBTn) are needed for a defensible soil-behaviour type.
Frequently asked questions
What is the CPT friction ratio?
It is the sleeve friction divided by the cone tip resistance, as a percentage: Rf = (fs / qc) × 100. It is one of the primary indicators of soil type in a cone penetration test.
Why does the formula become fs / (10·qc)?
Because qc is entered in MPa and fs in kPa. Converting qc to kPa (× 1000) gives Rf = fs / (qc × 1000) × 100 = fs / (10·qc).
What friction ratio means clay?
As a rough guide, Rf greater than about 4% suggests clays or silty clays, because fine-grained soils generate relatively high sleeve friction compared with tip resistance.
Is the friction ratio enough to classify soil?
No. A proper Robertson soil-behaviour-type chart uses the normalised cone resistance Qt and normalised friction ratio Fr, and design work uses dedicated CPT interpretation software.
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