Cone penetration tests (CPTs) and boreholes are the two workhorses of geotechnical site investigation. They are not rivals: each gives information the other cannot.
Cone penetration tests
A CPT pushes an instrumented cone into the ground at a steady rate and records the resistance at the tip, the friction on the sleeve and, usually, the pore water pressure. It gives a continuous profile, is quick to run and repeats well, so it is good for covering many locations and for spotting thin layers.
Boreholes and sampling
A borehole recovers the ground itself. Samples can be described, photographed and tested in the laboratory for strength, stiffness, grading and chemistry. Boreholes also reach depths and materials, such as rock or dense gravel, where a cone may stop.
Using both
- Use geophysics first to see how the layers change across the site.
- Place CPTs to cover the variation and to check each foundation or route section.
- Place boreholes where samples are needed to calibrate the CPT results and to test the key layers.
- Agree the test schedule with the designers before the field work starts.
The right mix depends on the structure, the ground and the design method. A short conversation with your geotechnical engineer at the scoping stage is worth more than a standard number of holes.