[Cob] Question about foundation and earth's stability

[Cjristos]

Apart from calling an expert, how could somebody determine the stability of
the place he chooses to set the foundation of the cobhouse? How can i say
(with a practical method) if there is, or there is not an underground cave,
or a porous silty earth in a small depth from the end of my foundation? Do i
have to dig more than i need for the foundation (so as to check what is
going on underground)?

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