What does NEC4 clause 60.1(12) mean?

It is the unforeseen ground conditions event, though it is not limited to ground. The conditions must be physical, such as ground, obstructions, contamination or the state of an existing structure. They must be within the Site as the Contract Data defines it. And they must not be weather, which has its own event at clause 60.1(13). The experienced contractor test does not ask whether you foresaw the conditions. It asks whether an experienced contractor would have judged them so unlikely that allowing for them was unreasonable. That judgement is made at the Contract Date, the date the contract was made. It looks at the Site Information, the Site and its surroundings, and what a visual inspection would show. Whether the event qualifies is decided under clause 61.4, which on a subcontract means by your main contractor. Notify under clause 61.3 within 8 weeks of becoming aware of the conditions. Give an early warning under clause 15.1 as soon as you suspect them. If it is decided an experienced contractor could have warned and you did not, clauses 61.5 and 63 require the event to be assessed as if you had. The change to the Prices, your contract price, is assessed under clause 63.1. It is the effect on actual Defined Cost up to the notification and forecast Defined Cost after it, plus the Fee. Two things go wrong most. You notify the cost rather than the condition, weeks after the excavator found it. Or you cannot show what you allowed for at tender. Any delay to the Completion Date is assessed separately under clause 63, and that is a job for your planner.