What is a cone penetration test report?

A cone penetration test (CPT) report presents the results of pushing an instrumented cone into the ground: for each test location, a plot shows how much resistance the soil offers at every depth. A geotechnical engineer uses it to interpret the soil profile, and above all where a firm bearing layer begins. Electrical CPTs are carried out to EN ISO 22476-1. In the Netherlands, where soft soils make CPTs routine, the report is called a sonderingsrapport.

What does the plot show?

• Depth. Plotted vertically, usually against a site or national datum. Check the ground level at the test point.
• Cone resistance (qc). Force on the tip divided by the tip area, in MPa. High values suggest dense sand or gravel; low values suggest clay or peat.
• Sleeve friction (fs). Friction on the sleeve just above the tip, also in MPa.
• Friction ratio (Rf). fs divided by qc, times 100%. The main clue to the soil type.
• Pore pressure and inclination, if measured. A piezocone (CPTu) also records pore water pressure; the inclination shows whether the rods have drifted.

Application

A practical way to read a CPT plot:

1. Use the friction ratio to identify the soil. Indicative values: sand about 0.6-1.0%, clay about 2-6%, peat about 5-10%. Above the water table, values can deviate considerably.
2. Combine it with cone resistance. Low qc with a high friction ratio points to soft clay or peat; high qc with a low ratio points to sand or gravel.
3. Look for the bearing layer. For piles, what counts is a thick layer with consistently high cone resistance, not a single spike.
4. Compare several tests. Layer depths vary across a site; one CPT does not describe the whole footprint.
5. Leave the design to the engineer. Pile toe level and capacity are determined by the structural or geotechnical engineer under Eurocode 7.

Common mistake: taking a thin sand band with high peaks for the bearing layer. If soft soil lies beneath it, a pile can still settle.

Related terms

• Cone penetration test
• Hand cone penetration test
• Soil investigation
• Shaft friction
• Bearing capacity of a pile
• Soil type
• Pile foundation
• Soil mechanics

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