CPT soil behavior type classification standard

Equations

Q_t = \frac{q_t - \sigma_{v0}}{\sigma_{v0}^'} $$F_r = 100 \cdot \frac{f_s}{q_t - \sigma_{v0}}$$

If the selected chart or workflow uses a derived index, document that equation and the chart edition explicitly in the revision notes.

Variables and Units

• $q_t$ = corrected cone resistance in consistent stress units.
• $f_s$ = sleeve friction in the same stress units used for $q_t$.
• $\sigma_{v0}$ = total overburden stress and \sigma_{v0}^' = effective overburden stress.
• Keep normalization inputs, pore-pressure corrections, and unit conversions aligned with the cited chart basis.

Required Assumptions

• CPT measurements are quality-checked and normalized before interpretation.
• The soil behavior chart used for classification matches the chosen normalization scheme.
• Thin interbeds and transitional zones are reviewed with engineering judgement.

Validity Ranges

• Intended for conventional CPT soundings with enough quality control to trust the normalized measurements.
• The method should be cross-checked when calibration chamber behaviour or local geology differs materially from the reference chart basis.

Implementation Checks

• Confirm zero shifts, depth alignment, saturation assumptions, and any pore-pressure corrections before normalization.
• Cross-check interpreted soil behavior zones against nearby logs, samples, and geologic context instead of relying on the chart alone.
• Record the exact SBT chart edition and any project-specific normalization choices used in the classification.

Known Limitations

• Soil behavior type charts do not replace laboratory identification or site-specific geological interpretation.
• Transitional or cemented soils can be misclassified when the CPT signal is noisy or incomplete.

References

• Robertson, P. K. (2010). Soil behaviour type from the CPT: an update.
• Jefferies, M. G., and Davies, M. P. (1993). Use of CPTu to estimate equivalent SPT N60.

Evidence to Attach

• Raw CPT/CPTu file, normalization worksheet, and any pore-pressure correction notes.
• The referenced SBT chart or paper edition used for interpretation.
• Nearby borehole or laboratory evidence used to confirm or challenge the interpreted zones.

Worked Example

Normalize cone resistance and friction ratio, confirm the chart basis, and then review the interpreted zone boundaries against nearby logs before assigning a soil behavior type for design screening.