What is a cone penetration test?
A cone penetration test (CPT) is a ground investigation method in which a rod string with a cone-shaped tip is pushed into the soil at a constant rate while the resistance is measured continuously. The readings reveal which soil types occur at which depth, how thick the layers are and how strong they are. In Dutch it is called sonderen, and in the soft soils of the Netherlands it is the standard site investigation tool. This article covers machine-pushed CPT from a truck or crawler rig; for the lightweight manual version, see hand cone penetration test.
How it works
- Equipment. A heavy CPT truck or crawler hydraulically pushes rods with an instrumented cone into the ground. The standard cone has a 10 cm² base area and a 60 degree apex, pushed at 20 mm per second. The method is set out in EN ISO 22476-1.
- Cone resistance (qc). The force on the tip; high in dense sand, low in soft clay or peat.
- Sleeve friction (fs). Friction measured on a sleeve just above the cone.
- Friction ratio (Rf = fs/qc). High in clay and peat, low in sand. Together with qc it is used to classify the soil.
- Pore pressure. A piezocone test (CPTU) also measures pore water pressure.
| Dense sand | Cone resistance (qc) high; friction ratio (Rf) low |
|---|---|
| Soft clay or peat | Cone resistance (qc) low; friction ratio (Rf) high |
The output is a CPT log plotting these values against depth.
Application
- Commission it early. Have the CPTs done before the foundation design is fixed; the structural engineer works directly from the results.
- Test enough locations. One CPT only describes one spot. Let the geotechnical engineer decide the number and positions.
- Set pile toe levels. For piled foundations, the log shows the depth of the bearing stratum and therefore the required pile length.
- Combine with boreholes. A CPT recovers no samples. Boreholes and groundwater monitoring wells are needed for samples, groundwater levels or contamination testing.
- Plan access. CPT trucks are heavy, so provide a firm route. On tight or indoor sites, a lighter crawler or hand cone test may be the answer.
Common mistake: relying on a CPT from the neighbouring plot. Ground conditions can change markedly over short distances.
Related terms
- Hand cone penetration test
- Site investigation
- Shaft friction
- Soil type
- Pile foundation
- Groundwater monitoring well
- Geotechnical engineering
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