Quality and confidence

The cone penetration test data available via DINOloket vary markedly in quality. The quality of a cone penetration test and the confidence in the accuracy of the associated data depend on several factors. Firstly, it is important whether or not the cone penetration test was performed according to a published standard and, if so, which: the old NEN3680, the more recent NEN5140, or the current ISO 22476-12:2009. All guarantee the standardised performance of a cone penetration test with equipment of known dimensions, so it is problematic when some cone penetration tests in the database lack information on the standard used. Cone penetration tests dating from the 1980s and earlier predate the introduction of standards. As a result, the procedures implemented and equipment used may have differed considerably from those in the standardised cone penetration tests that we are familiar with today.

A second factor is the method of cone penetration. NEN3680 stipulates that a cone penetration test must be performed both mechanically and electrically. In contrast, NEN5140 requires only the electrical method. The difference between the two methods is in how the force is recorded. In the mechanical cone penetration tests, it is transmitted to the surface (where it is measured)  via rods inside tubes. In electrical cone penetration tests the sensors in the cone measure the cone pressure directly.

The date of the test is also a factor affecting the quality of a cone penetration test. Old cone penetration tests, especially those performed before the introduction of standards in 1982,  tend to be less accurate than recent ones.

Another important aspect is whether the cone penetration test data have been delivered digitally to DINO, although of course, the database predates the advent of digital data. Many of the cone penetration tests that TNO has were  originally on paper and so have had to be digitised so that the data could be input into the database and made available via DINOloket. Paper records are vulnerable and their legibility can diminish over time, for instance as a result of multiple copying, but digitisation may introduce inaccuracies. Cone penetration test data is therefore currently delivered to TNO from the field in Geotechnical Exchange Format (GEF) for immediate incorporation into the database.

The final factor that affects quality is the number of parameters measured. During the test, various kinds of measurements are performed. The most basic cone penetration test measures only the cone resistance, which provides insight into the strata transitions but no picture of the lithological structure of the subsurface. However, cone penetration tests that combine the cone resistance with the sleeve friction and friction ratio do provide this picture. And if we also have data from pore water pressure measurements, then we can even distinguish thin, poorly permeable layers at centimetre scale.