With regard to level of detail, generally I put in as much as is required at tender. depending on client and to an extent Project Manager our side, this is variable, but given the fact that programme is usually a markable item (and an important one) for most tenders it needs a fair bit.
I tend to break down works into areas, and programme each. Makes it a lot easier to know what you are referring to and where. If you have diversions or whatever, I would suggest breaking down the work into more summary activities- it gives clarity of intent, and indicates where the work is placed.

As far as digging a hole goes, no task has constant outputs. Weather, amount of labour, plant, time of year, how people feel today, will all affect outputs. you are always looking to find the output that applies to the item you have- in your case, I would be looking at the steps involved, as much as anything. Topsoil strip, high level strip, place sheet piles, excavate to depth, time allowance for water extraction- all items thart would need to be considered.
Sometimes trying to be too high level can make you lose sight of the fact that each task has a place- not a criticism by any means, but even if you dont want detail in th eprogramme, consider it so that you can make a good estimate.