14 October 2026 · 1 min read · CivilWorks Atelier

Watching a CPT happen: what the trace is telling you as the cone goes down

A companion to the CPT reading guide: push a virtual cone through a layered profile and see how sand, soft clay and dense layers show up in qc and sleeve friction, then how SBT zones and Ic classify them.

CPTSite investigationIn-situ testing

The cone is pushed at a steady 20 mm/s. Every 10–20 mm the rig logs tip resistance qc, sleeve friction fs and, on a piezocone, pore pressure u2. Sands give high qc and a low friction ratio. Soft clays give very low qc, a relatively higher friction ratio and large excess pore pressures.

Interactive: push the cone, pause it, or drag the depth slider.

Robertson's soil behaviour type index combines normalised tip resistance Qtn and friction ratio Fr into one number, Ic. Values below about 2.05 behave like clean sand, values above 2.6 like clay, and the range in between is transitional. Ic is the same quantity the CPT liquefaction method uses to decide whether a layer is sand-like and how much to correct for fines.

Interactive: drill a borehole through a similar profile. Boreholes and CPTs complement each other: the borehole gives samples and descriptions, and the CPT gives a continuous strength profile.

Try it: CPT interpretation (SBT, Ic, su, φ′)Liquefaction: CPT method SPT corrections