What is the friction ratio in cone penetration testing?
The friction ratio (Rf) in a cone penetration test (CPT) is the ratio between the local friction on the friction sleeve and the cone resistance, expressed as a percentage. Together with the cone resistance, it indicates the soil type at each depth. The Dutch term is wrijvingsgetal.
Formula
Rf = (fs / qc) x 100%
- fs = local sleeve friction in MPa;
- qc = cone (tip) resistance in MPa.
Indicative values
Sand and gravel: high cone resistance, low Rf. Clay and peat: low cone resistance, high Rf.
| Soil | qc (MPa) | Rf (%) |
|---|---|---|
| Gravel | 15-30 | 0.2-0.5 |
| Sand | > 5 | 0.6-1.0 |
| Clayey sand | > 2 | 1.0-2.0 |
| Loam | 1-3 | 2.0-4.0 |
| Soft clay | 0-2 | 4.0-6.0 |
| Peat | 0-4 | 5.0-10.0 |
Values apply below the water table and vary by location.
Reading a CPT log
- Find the bearing sand layer: a jump to high qc with low Rf.
- Rf peaks with low qc point to peat or soft clay: compressible layers prone to settlement.
- Above the water table, interpret with care.
Rf is an indication, not a soil classification. Foundation design belongs with a geotechnical engineer.
Related terms
- Shaft friction
- Foundations
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