The idea is to make a drainable, mud-free surface that can be adjusted easily with time. Though I don’t think these big trees are going to grow much more. Certainly not while we owned the house.
@wion How about large format pavers laid with wide gaps in-between – large enough for the roots to pass between? You may need some kind of concealed spacers to stop them shifting about laterally but you'd get a drivable surface without hindering water soakaway plus ‘free passage' for the roots that then seek the gaps as the path of least resistance. Not as loud underfoot/wheel as gravel either.
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