Soil Classification based on CPT test results

The Robertson CPT interpretation methods (1986, 1990, and 2010) provide a widely adopted framework for classifying soil behavior types (SBT) using cone penetration test (CPT and CPTU) data. These methods relate measured cone parameters to empirical soil zones, with classification focused on soil behavior (e.g., stiffness, drainage, compressibility) rather than strictly particle size or plasticity.

At the core of the method are the following measured or derived parameters:

• q_c: cone tip resistance (MPa or kPa)

• f_s: sleeve friction (kPa)

• R_f = (f_s / q_c) × 100%: friction ratio (%)

• u: pore water pressure (CPTU only)

• σ_v0: total vertical stress

• σ′_v0: effective vertical stress

Non-normalized SBT index (pre-1990):

In the 1986 and 1990 charts, Robertson plotted q_c vs. R_f (or log-scaled versions) to define empirical zones corresponding to soil types such as:

• Sensitive fine-grained soils

• Clays

• Silty clays to clayey silts

• Sandy silts to silty sands

• Sand mixtures

• Sands to dense sands

• Gravelly sands

This approach works well in uniform, normally consolidated soils but can be affected by depth or overburden stress.

Normalized parameters and SBT Index (2010):

To address depth effects and stress dependency, Robertson (2010) introduced normalized CPT parameters, most notably:

• Qt = (qc - σv0) / σ′v0 – normalized cone resistance

• Rf = (fs / qc) × 100% – friction ratio

• Ic: Soil Behavior Type Index

The normalized SBT index (Ic) is calculated as:

This formula allows continuous soil classification with depth and accommodates varying overburden pressures.

1. Robertson, P.K., & Campanella, R.G. (1986). Interpretation of cone penetration tests — Part I: Sand. Canadian Geotechnical Journal, 23(1), 62–78.

2. Robertson, P.K. (1990). Soil classification using the cone penetration test. Canadian Geotechnical Journal, 27(1), 151–158.

3. Robertson, P.K. (2010). Evaluation of cone penetration tests (CPT) for geotechnical design. Canadian Geotechnical Journal, 47(8), 845–875.