What does NEC4 clause 60.1(12) mean?

Clause 60.1(12) is the unforeseen ground conditions event, though it is not confined to ground. The event is that the Contractor encounters physical conditions which are within the Site, are not weather conditions, and which an experienced contractor would have judged at the Contract Date to have such a small chance of occurring that it would have been unreasonable to allow for them. Every element has to be shown. The conditions were physical, such as ground, obstructions, contamination or the state of an existing structure; they were within the Site as the Contract Data defines it; they were not weather, which has its own event at clause 60.1(13); and they pass the experienced contractor test, which asks not whether this Contractor foresaw them but whether an experienced contractor, looking at the Site Information, the Site and its surroundings and what a visual inspection would show at the Contract Date, would have judged them so unlikely that allowing for them was unreasonable. The Project Manager decides under clause 61.4 whether the event qualifies. The Contractor notifies the event under clause 61.3 within eight weeks of becoming aware of the conditions, and should give an early warning under clause 15.1 as soon as they are suspected, because if the Project Manager decides an experienced contractor could have given an early warning and the Contractor did not, clause 61.5 and clause 63 require the event to be assessed as if it had. The change to the Prices is assessed under clause 63.1 as the effect of the conditions on actual Defined Cost by the date of the notification and forecast Defined Cost afterwards, plus the Fee. What most often goes wrong is that the Contractor notifies the cost rather than the condition, weeks after the excavator found it, or cannot show what allowance it made at tender. Any delay to the Completion Date is assessed separately under clause 63 by the programming expert.