What Is a CPT? The Cone Penetration Test Explained

Anyone involved in construction on soft ground will sooner or later come across CPTs. The cone penetration test — in the Netherlands universally called a "sondering" — is by far the most widely used method there to map the subsurface. In this article we explain what a CPT is, what is measured, how to read the plot and what the results are used for.

What is a cone penetration test?

In a CPT, a steel cone with a standardized tip is pushed vertically into the ground at a constant rate of 2 cm per second. This is done with a heavy CPT truck or, in hard-to-reach places, with a lighter tracked or portable rig. As the cone advances, sensors continuously record how much resistance the soil offers. The result is a virtually continuous measurement profile from ground level down to the final depth — often tens of metres — without a single scoop of soil being brought to the surface.

What is measured?

• Cone resistance q_c (MPa): the force required to push the cone tip through the soil, divided by the area of the tip. This is the key parameter: it says a lot about the strength and bearing capacity of the soil.
• Local sleeve friction f_s: the shear resistance along a friction sleeve just above the cone tip.
• Friction ratio R_f = f_s/q_c × 100%: the ratio between the two. This number is the key to identifying soil types.
• Pore pressure u₂: in a piezocone test (CPTU), a filter behind the cone tip also measures the pore water pressure. That helps distinguish low-permeability layers and determine piezometric heads.

How do you read a CPT plot?

A CPT is almost always presented as a chart: depth on the vertical axis (in metres relative to a datum such as NAP, or to ground level) and cone resistance on the horizontal axis. The friction ratio and, where measured, the pore pressure are usually plotted as additional traces. Two rules of thumb help when reading it:

• High q_c (roughly above 5–10 MPa) indicates sand or gravel: strong, load-bearing material.
• Low q_c combined with a high friction ratio indicates clay or peat: soft, compressible material.

This lets you recognize the typical Dutch profile at a glance: soft clay and peat layers at the top, underlain by the load-bearing Pleistocene sand on which piles are founded. For a more rigorous interpretation, engineers use a classification method such as Robertson's — here's how to identify soil types in a CPT.

What are CPTs used for?

• Foundation design: from the cone resistance, a geotechnical engineer calculates the bearing capacity of foundation piles and the required pile tip levels.
• Settlement calculations: the thickness and compressibility of soft layers determine how much an embankment or structure will settle.
• Determining the soil profile: layer boundaries, sand channels and weak intermediate layers become visible without drilling.
• Verifying existing data: a new CPT next to an old one confirms whether the archived picture still holds.

Standard and file format

The execution of CPTs is specified in EN ISO 22476-1, which sets requirements for, among other things, the cone geometry, the penetration rate and the accuracy class of the measurement. In the Netherlands, results have traditionally been exchanged as GEF files, and since the introduction of the BRO (the Dutch Key Register of the Subsurface) increasingly also as IMBRO XML. Virtually all geotechnical design software reads GEF directly.

What does a CPT cost, and how fast is it?

Compared with a borehole, a CPT is fast and economical. On an accessible site, a CPT truck completes several soundings per day, and the data is available digitally right away — no samples need to go to a lab. A borehole does yield actual soil material for laboratory testing, but easily costs several times more per metre and takes far longer. In practice, CPTs are therefore carried out first, with boreholes added only where samples are genuinely needed.

Reusing old CPTs

Because CPTs have been performed in the Netherlands since the 1930s, there is a vast archive of old CPT charts on paper or as scans. They often contain exactly the information you need for a new project. With the CPT Converter you digitize such a scanned sounding into a valid GEF file in a few minutes — here's how that works step by step.