One thing that irritates me to no end is to look at a set of geotechnical plans and to realise that there is only one boring for the entire site. In a few cases that’s enough, but very few. The “uniform site conditions” of academic legend are seldom found in real life; soil conditions vary from one place to another on a site and in some stratigraphies in a matter of feet or metres. If it’s worth sending a crew out to a site for one or two borings it’s worth getting more.
The table “Guidelines for minimum number of exploration points and depth of exploration” is shown below, and is taken from the Soils and Foundations Reference Manual, which has much additional information on this and other related topics. It also deals with another issue that bedevils geotechnical exploration: going deep enough to get the information needed, especially with deep foundations. I also spent a great deal of time on this subject in my course Foundation Design and Analysis: Boring Logs and Their Interpretation, evidently more than other undergraduate courses.
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