Identifying Soil Types from a CPT: The Robertson Classification

A CPT does not measure soil types — it measures resistances. Yet every geotechnical engineer instantly sketches a soil profile alongside it in their mind: peat, clay, sand. That works because the combination of cone resistance and friction gives a recognizable pattern per soil type. The best-known method to formalize that is the Robertson classification.

What does a CPT actually measure?

During a cone penetration test (CPT), a cone is pushed into the ground at a constant rate. At least two quantities are recorded:

• the cone resistance qc (MPa): the pressure on the tip of the cone;
• the sleeve friction fs (MPa): the shear resistance along the friction sleeve just above the tip.

From these two follows the friction ratio Rf = fs / qc × 100%. That number is the key to the soil type: sand gives a high qc with little friction (Rf ≈ 0.5–1%), clay a low qc with relatively high friction (Rf ≈ 3–5%) and peat a very low qc with high friction (Rf > 5%).

The Robertson classification (SBT)

Robertson mapped this in the well-known Soil Behaviour Type (SBT) chart: qc plotted against Rf, divided into zones that each represent a soil behavior type. The non-normalized variant (Robertson 2010) uses the SBT index Isbt and distinguishes, among others:

• Zone 2 – peat and organic material;
• Zone 3 – clay to silty clay;
• Zone 4 – silt mixtures: clayey silt to silty clay;
• Zone 5 – sand mixtures: silty sand to sandy silt;
• Zone 6 – clean to silty sand;
• Zone 7 – gravelly to dense sand.

Important to realize: the method classifies the behavior of the soil, not the grain size distribution. A heavily overconsolidated clay can "behave" like silt. For a foundation design, that behavior is usually exactly what you want to know.

What if there is no friction data?

Older CPTs — especially digitized scans — often contain only the cone resistance. Robertson cannot be applied then. However, rules of thumb for Dutch soils do give an indication:

• qc < 0.5 MPa: peat or organic material;
• qc 0.5 – 1.5 MPa: clay;
• qc 1.5 – 5 MPa: silty clay or clayey sand;
• qc 5 – 15 MPa: loose to medium dense sand;
• qc > 15 MPa: dense sand.

Note: without a friction ratio, the distinction between stiff clay, silt and loose sand cannot be made reliably. Use such a qc profile as a first indication, not as a definitive soil profile.

Extracting a soil profile from your own CPT

The CPT Converter performs this classification automatically. Load a GEF file and, alongside the qc and Rf charts, you get a soil profile according to Robertson (or, without friction data, the qc rules of thumb) including a legend. A digitized scan also gets the same view after digitizing, via the Chart & soil profile tab.

From soil profile to calculation

The soil profile determines which layers are load-bearing and where negative skin friction occurs. Combined with the cone resistance, it forms the basis for pile bearing capacity under NEN 9997-1, the Dutch national annex to Eurocode 7. Why qc is so decisive in that calculation is covered in the article on cone resistance and pile design.