The context of ‘experienced contractor’ here is of a Contractor making a judgement with regard to the likely occurrence of certain physical conditions. It follows, therefore, that the ‘experience’ relates to a degree of understanding gained from having previously organised, managed, constructed and delivered works of a similar nature, whether they actually have or not, because that is what the CE clause states.
The reference to ‘small chance of occurring’ is not specifically defined, although the paragraph continues with the inclusion of a test of reasonableness, based on a ‘considered opinion’ from an ‘experienced contractor’.
Consequently, it would seem reasonable to include an allowance where a risk is more likely than not to occur, possibly based on a sliding scale assessment rather than an ‘all or nothing’ principle.
The fact that the Works Information included a statement that 'unchartered utilities are to be expected' does not make this any easier in some ways. Was it referenced as part of the 'scope'? If so then this provides a baseline reference level of work. However, how do you define this as the insertion itself is quite vague about type and location so it is unclear how you would include this, not least in the programme.
What it does do, however, is suggest that services are expected to be encountered so the argument should come down to the last paragraph in clause 60.1 (12) which relates to the 'difference'.
Never an easy CE to deal with and in my experience one of the most subjective, well apart from the weather of course.