The roots of most trees are shallow (i.e., not more than 1' to 1.5' deep), but spread out very wide (see the picture below from here) . Given the height of your trees, they're not very young and you can be sure that the roots spread at least as far out as the tree is tall. Add a factor of half to that for every 10 years of the tree's life.
How far (on average) do tree roots extend out from the base of the tree?
roots - What should I do about a tree that is growing over my retaining ...
That tree is worth far more than a few bricks and a mason's time! I would not chance messing with roots, as Kevinsky said, support is a factor and the tree would be stressed.
So this tall tree better not have its roots expand horizontally too aggressively as it will damage the walls and powerlines. Horizontal expansion of roots must be roughly limited within 2 by 2 meters. Question. Which trees can grow up to 5 meters tall (at least) such that their roots expand vertically mainly, and not much horizontally?
The expansion of the out-of-control holly tree roots puts all of your pipes, overall foundation, plumbing, etc. at risk of getting damaged. The best way to prevent this damage from happening is by making sure your holly plants are properly taken care of and considering planting them away from anything the roots could damage should they get out ...
Roots of trees planted near or close to each other do not exactly support or necessarily connect with each other - each tree still has its own, individual root system, though the roots from each tree, if close together, may cross over into each other's space. Probably what you're thinking of is mycorrhizae or mycelium connections; these mycorrhizae have a beneficial, symbiotic, interconnected ...