Unexpected ground conditions is a legal term and many claims and disputes hang on whether or not the conditions should have been foreseen or were foreseeable in some way. Following the checklist approach advocated by Sir John Knill and others of the "three verbal equations" should mean that most potential hazards are considered during site investigation and certainly when planning the scope and requirements of any ground onvestigation.
These ideas have been used and discussedin various papers and a presentation that can be downloaded here. A fuller discussion on the philosophy of ground investigation and site investigation more generally is given in Practical Engineering Geology (2012)
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