Without digging too deep (as time won't allow right now), there are all kinds of underground closures from JUF (Jointbox Underground Footway), to JRC, (Jointbox Reinforced Carriageway), to MH (Manholes).
The depths are various and will differ dependant on requirements and accessibility. We bog-standard engineers can only work in them if the depth is 3 feet or less (This is from memory so may be slightly more or less ?), any deeper and 'Blowers' are required to pump in fresh air. Of course, common-sense used in conjunction with our gas detectors probably also plays a part.
If for example a UG box lets say 5 feet deep, is being worked in as part of a cable-changeover, then initially 'Blowers' could be despatched but if left venting overnight and our testers show no dangers present, then I would imagine the SSR engineers wouldn't deploy the blowers again ?? This is not my forte though, as I don't get involved in SSR work.