Free Online CPT Interpretation Tool: Robertson Soil Behavior Type

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Each row is normalized against vertical stress, then placed on the Robertson chart through the behavior index Ic. Clays additionally get undrained strength from qt through your cone factor Nkt; sands get relative density. Ic between 2.60 and 2.95 is the transitional silty zone where both estimates are rough.

The cone that logs soil without sampling

A cone penetration test pushes an instrumented cone through the ground at two centimeters per second, reading tip resistance and sleeve friction continuously. The tool above turns those readings into geology: each row is normalized against vertical effective stress into Qt and Fr, placed on the Robertson chart through the behavior index Ic, and labeled from gravelly sand to organic soil. Clay-like points get undrained strength through your cone factor Nkt; sand-like points get relative density. Paste your own sounding or run the demo profile, which shows a clean sand over a soft clay, the classic settlement trap.

Cone penetration test diagram showing the cone tip and friction sleeve pushed through sand over clay, with the Robertson Ic behavior zones listed alongside
Tip resistance and sleeve friction combine into the Robertson behavior index, a continuous soil log without a single sample.

This opens the Geotechnical Site Investigation Suite in the bar above, alongside the SPT corrections and Atterberg limits calculators that handle the drilling and lab sides of the same campaign.

Reading the numbers

  • The friction ratio is the first instinct: sands push hard with little friction, clays push soft and drag; Rf below 1 percent usually means sand, above 4 percent clay.
  • Ic makes the instinct quantitative: the zones between 1.31 and 3.60 walk from clean sand through silty mixtures to clay, and the transitional 2.60 to 2.95 band is where both strength estimates deserve suspicion.
  • Nkt between 12 and 18 covers most clays; calibrate it against lab or vane strength when you have it, because su scales directly with it.
  • Relative density from Qt uses the Kulhawy and Mayne form; it assumes clean, unaged quartz sand and drifts for anything else.

What this tool is for

Turning raw CPT rows into a first stratigraphy in seconds, teaching the Robertson framework, and cross-checking commercial CPT software. Production interpretation adds pore pressure from the u2 sensor and site-specific calibrations, all layered onto exactly this normalization.

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