CPT Friction Ratio (Rf): What It Tells You About Soil
The friction ratio Rf is the ratio of local sleeve friction fs to cone resistance qc, expressed as a percentage. After qc, it is the most important parameter for deriving soil type from a CPT (cone penetration test). A low friction ratio roughly indicates sandy layers, while a high friction ratio points to clayey or organic layers. In this article you will read how the friction ratio of a CPT is built up, how to interpret it and where to find it in a GEF file.
CPT friction ratio: how Rf is determined
During a CPT, the cone measures two things: the cone resistance qc at the tip and the local sleeve friction fs along the friction sleeve directly above the cone. The friction ratio follows from these two:
Rf = fs / qc × 100%
Because both values are in MPa, the unit cancels out and a percentage remains. The friction ratio is therefore not a separate measurement but a derived quantity. On a CPT plot it usually appears as the third curve next to qc and fs, plotted against penetration length or against depth relative to NAP (the Dutch ordnance datum). If you want to revisit the basics first, read what a CPT is.
What do high and low Rf values tell you?
In sand, the friction on the sleeve is small relative to the high cone resistance, so Rf is low. In clay, qc is low while the sleeve experiences relatively high friction, so Rf is higher. In peat, qc is very low and Rf is often at its highest. The general pattern:
| Soil type | qc | Rf |
|---|---|---|
| Sand | High | Low |
| Clay | Low to moderate | Higher |
| Peat | Very low | High |
This is an indication only. A silty sand or a sandy clay falls between these extremes, and the same Rf can belong to different soil types. A CPT therefore does not replace a borehole with a sample description.
Rf and qc together: the Robertson classification
The friction ratio only becomes truly useful in combination with qc. The Robertson classification plots both in a chart and divides the data points into soil behaviour zones. You can read how this works in identifying soil types with the Robertson classification. For a Robertson indication you therefore need Rf (or fs to calculate Rf) in addition to qc. If the friction is missing from a file, only an indication based on qc alone remains.
What should you watch for when interpreting Rf?
- Low qc, erratic Rf. You divide by qc. At very low cone resistance, a small measurement inaccuracy in fs already causes large spikes in Rf.
- Thin layers. The friction sleeve sits above the cone tip, so fs relates to a slightly different depth range than qc. In thin sand or clay layers, Rf is therefore less reliable.
- Transitions. At the boundary between layers, Rf can show a value that occurs in neither of the two layers.
- Missing or truncated data. Check that fs has been recorded over the entire penetration length and that there are no missing values in the column.
Use Rf as a trend over multiple measuring points, not as a single value at one depth.
Finding the friction ratio in a GEF file
In a GEF file, the measured columns are described in the #COLUMNINFO lines. There you can see which column is penetration length, cone resistance, local sleeve friction or friction ratio, and in which unit. Not every file contains all columns: some have only qc and fs, others also Rf and pore pressure u2. The structure of these headers is covered in the GEF file explained.
In CPT Converter you open a GEF file directly in your browser and see the plot with an indicative soil profile (Robertson, or based on qc alone). This is useful for quickly checking that the columns are what you expect. If you want the values in a spreadsheet, opening GEF and converting it to Excel, CSV or AGS4 describes the options.
Friction ratio in old, paper CPTs
On a paper CPT, qc, fs and Rf appear as separate curves side by side. If you digitise such a CPT, check afterwards which columns are in the resulting file and whether they match the original. How to get from scan to file is described in digitising an old CPT to GEF. To find out whether an old CPT is usable for a renovation, read reusing old CPTs.
Frequently asked questions
What is the friction ratio in a CPT?
The friction ratio Rf is the local sleeve friction fs divided by the cone resistance qc, times 100%. It gives an indication of soil type.
Is a high friction ratio always clay?
No. A high Rf points to clayey or organic soil, but measurement inaccuracy at low qc and layer transitions can also produce high values. Always assess Rf together with qc and its variation with depth.
Do I need Rf for a pile calculation?
The pile calculation relies mainly on qc, as described in cone resistance qc and pile calculation. Rf mainly helps to identify the soil layers and to support the profile.
Can I calculate Rf myself if it is missing from my GEF?
If fs and qc are in the file in the same unit, you can calculate Rf with the formula above. If needed, export the file to Excel or CSV and calculate the column there.