The old adage is that most roots are in the top three feet of soil. This cutaway image, showing roots exposed by the removal of a wall, seems to bear this out. It also shows what happens when roots don’t have as much room to roam as they would like. Still, there are few roots at the base of the wall, yet an overabundance of them in the uppermost soil layer.
The irrigation line (white object) is just below the original soil level.
Deep watering – soaking the soil well yet infrequently is supposed to promote deeper root growth. This of course depends on the nature of the soil in which the roots are growing and the species of tree. It’s impossible to tell what watering practices were used here, so there is no way to test this theory from this site.
Another way to force roots downward so they don’t lift pavement is by using a root barrier. The effectiveness of this device is somewhat debatable, and current thinking tends to favor making the soil all around the tree suitable for healthy root growth so that the roots don’t need to run to the soil surface seeking air and water. Roots cannot survive in waterlogged, oxygen-free soil, unless they’re from species specifically adapted to these conditions – not typical for most landscape trees.
The best way to imagine how much space a tree’s roots might need is to imagine the tree upside down, with its branches buried. A tree’s roots typically extend up to or beyond the limit of its branches. So, putting a thirty foot diameter tree in a ten foot square planting pocket is not likely to yield a happy, healthy tree that lives long and prospers.