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It seems that use of the Robertson charts (Robertson, P.K. [1990] " Soil Classification using the cone penetration test" Canadian Geotechnical Journal 27(1), 141-8) to classify soil type based on CPT results is widely accepted practice in the Netherlands. I cannot, however find any prescriptions in the current Eurocode which explicitly allows this. I have checked all relevant standards:

  • EN 1997.1
  • EN 1997.2
  • NEN-EN ISO 22476-1
  • ISO 14688-1 and ISO 14688-2

Some of these standards are perhaps not the most recent and others do not contain the national annexes, so perhaps that is my problem. However, I cannot find a clear stipulation which allows or prescribes use of the above charts to actually determine soil type based on CPT results.

Could anyone indicate why this is allowed? Am I missing something in the standard? If it is common practice how does it not contradict the standard?

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The place to look is in EN1997-2 and as far as I can tell it does not specify how to determine soil type from CPT results. It also does not specify that such a method is a nationally determined parameter.

That means your are outside the codes and it is your own responsibility to ensure that the method you use is sufficiently reliable. And there is no well-defined criteria for how to do that, but a method that 1) is written by and expert in the field, 2) is generally considered to be best practise and 3) has been tested on a large number of cases, should make the cut. In short, you are allowed to use Robertson's charts because everyone else is using Robertson's charts.

I haven't checked the Dutch national annex to EN1997-2, as I don't already have a copy and I'm not going to buy one for the purposes of this question, but it is not possible to specify a method for something that is not a nationally determined parameter in the national annex. The only thing, they can do, is provide "supplementary information" that a given method is sufficiently accurate or similar. They cannot require you to use a specific method or outrule a method. Therefore, the charts are allowed, regardless of what the national annex says on the topic. (But they could potentially make it a lot easier to answer the question of whether the charts are okay to use.)

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Eurocode 7 and 8 document soil classification which may be found here and here respectively.

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