Cone Penetration Testing (CPT) for ground investigation and verification
Cone Penetration Testing (CPT) pushes an instrumented cone into the ground at a constant rate, recording tip resistance, sleeve friction and, with a piezocone (CPTu), pore water pressure. It gives a continuous, repeatable profile used to classify soils, estimate design parameters and verify ground improvement by comparing results before and after treatment.
When the test is used
Typically used for…
- Site investigation in sands, silts and clays.
- Verifying densification by comparing profiles before and after treatment.
- Liquefaction assessment in sandy soils.
- Locating soft layers and checking layer thickness across a site.
Limitations
- Penetration stops in very dense layers, cemented ground, gravel or rock.
- No soil sample is recovered, so boreholes may still be needed.
- Correlations to design parameters depend on soil type and should be calibrated to the site.
The test type, number and locations are agreed with the consultant for each project.
How the test works
A rig pushes rods with a cone at the tip into the ground at a standard rate, typically 20 mm/s. Sensors in the cone measure the resistance at the tip (qc), the friction on a sleeve behind it (fs) and, in a piezocone, the pore water pressure (u2).
Readings are logged continuously with depth. The profile identifies soil layers and their state and, for ground improvement, shows cone resistance before and after treatment against the target.
Test planning inputs
- Purpose: investigation, design parameters or verification.
- Test locations and depths, including positions between treatment points.
- Target cone resistance or acceptance criteria.
- Expected ground conditions and risk of refusal.
- Applicable standard and reporting format.
Calibration and equipment checks
- Cone calibration certificates checked before mobilisation.
- Zero readings recorded before and after each test.
- Baseline tests in trial areas before treatment.
Test execution controls
- Push rate kept within the standard tolerance.
- Verticality and depth monitored.
- Zero drift checked after each test.
- Refusal and termination recorded.
- Test location surveyed.
Interpretation and reporting
The test plan, the raw record and the interpretation are kept separate, so it is always clear what was specified, what was measured and what it shows.
What is specified
Test locations, depths and acceptance values agreed with the consultant.
What was measured
Calibrated qc, fs and u2 profiles with depth for each test.
What it shows
Soil classification, derived parameters and comparison with the targets.
Deliverables
- Test location plan
- CPT logs (qc, fs, u2, friction ratio)
- Calibration and zero-reading records
- Interpretation report
- Before/after comparison for verification
Common questions
What does a CPT measure?
The resistance at the cone tip, the friction on a sleeve behind it and, with a piezocone, the pore water pressure, all recorded continuously with depth.
Why is CPT done before and after ground improvement?
Comparing the two profiles shows how much the cone resistance has increased at each depth, against the target.
When can CPT not be used?
In very dense or cemented layers, gravel or rock, where the cone meets refusal. Pressuremeter tests or boreholes are used instead.
Discuss the testing your project needs.
Share the project stage, the ground conditions and the specification or acceptance criteria.