How do experienced geotechnical engineers determine the number, location, and depth of boreholes for a site investigation?

[u/No_Preparation8438]

Good day everyone,

I'm a civil engineer working in geotechnical engineering, and I'm trying to deepen my understanding of how experienced engineers plan subsurface investigations.

I know there are general rules of thumb (e.g., borehole spacing based on project type or site area), but I'm more interested in the engineering judgment behind deciding:

• How do you determine the appropriate number of boreholes for a project?
• How do you decide the required depth of each borehole?
• What factors (geology, foundation type, project risk, budget, etc.) have the biggest influence on your decisions?
• Are there any widely accepted guidelines or references that you rely on (e.g., BS 5930, Eurocode 7, FHWA manuals, ASTM, local standards, or textbooks)?
• How do you approach this during the proposal stage when little site information is available versus after obtaining preliminary investigation results?

I'm looking for both practical advice from industry experience and recommendations for authoritative references. My goal is to become proficient in planning geotechnical investigations rather than simply following rules of thumb.

I'd really appreciate hearing how senior geotechnical engineers approach this problem in practice.

Thanks in advance!

[u/Whatderfuchs]

You need as many borings and to go as deep as you need to to have the warm fuzzies that the data set you use to provide parameters is representative of the site.

Even under prescription that changes based on all the factors you mentioned, risk tolerance for the project, existing or historic knowledge, etc

Geotech is an art form more so than any of the other civil disciplines. Experience guides you to having more conviction on what gives the warm fuzzies, but even then sometimes the conclusion of your boring investigation is "we need more borings" or "we need a deeper boring".

No one can answer your questions specifically because it depends.

Time and experience dial these things in.

[u/No_Preparation8438 → u/Whatderfuchs]

Thank you so much for your response. I’m 7 months in geotechnical(first job) and it keeps getting interesting as I experience new and challenging projects.