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When you're putting up a new building, you have to check the soil underneath for stability. How deep do you typically have to check? What's the extent of your "due diligence" when ensuring that a building won't collapse because of unstable ground? I imagine there might be different rules in different jurisdictions, but is there a typical, ballpark depth?

(Relatedly: Has a building ever collapsed because of unstable conditions below the checked area?)

I'm a total newbie, and I apologize for the many flaws you will be able to spot in this question.

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    $\begingroup$ It did not collapse, but nearly: The leaning tower of Pisa... $\endgroup$
    – Solar Mike
    Aug 23, 2019 at 18:15
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    $\begingroup$ Depending on the locale, the geotechnical investigators will judge how deep is warranted. For example in my area there are locations where bad soil is well known at shallow depths, there are other areas where it is known that bedrock can be found after only a short boring distance. Unfortunately it is a matter of judgement, the case of the leaning millennium tower in SF is a good example of why due diligence is required. $\endgroup$
    – ShadowMan
    Aug 23, 2019 at 18:41

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Local governing agencies have the general geology information of the sites. And building departments usually recommend what kind if soils investigation and how many bores and how deep.

If the soil condition is generally known to be good they even have pre approved sheets about the bearing capacity of the soil and type and depth of foundation required.

If the site needs investigation by a geologist and soils engineer, they are familiar with the local type of soils and will plan a strategy for location and depth of bors and type of tests to be done on the excavated samples.

Also in critical cases they have a deputy soils engineer on the job and they take continuous samples and if they deem they need to bore new holes and or modify the report to encorporate unlickly new emerging data they do it, while they are constantly in touch with the owner and building department.

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Practically, the geologist will do boring at different location at the site, we call it Bore Hole.

Based on their preliminary assessment, they will decide the depth of boring. Or by experience or trial & error. Whatever it is, the main thing we are looking for is hard layer. For it to be hard rock or hard dense sand layer.

Basically we can get a soil profile at each Bore Hole containing the information as follow:

1.) Water Table

2.) Type of soil

3.) Strength of sample taken

4.) SPT: number of blow recorded during boring

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Depends on the structure. You go down until you have capacity to carry the load plus a safety factor. If you don't want the 150percent safety factor be prepared to do a core sample for every casson for the entire depth. If that allows you to go down 150ft instead of 225 on 20 supports it's worth it. LOCAL factors rule, ask around for a good old boy driller or excavator and call him. He'll tell you what's under your site. Plan for that and adjust accordingly. He'll be within 10-20 percent on depth of soil transitions 200 feet down.👻

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