Probably the most critical factor in tree health and longevity is the provision of enough quality soil for the tree roots. When planning trees into urban projects, it is crucial that the target soil volume is established in the early design stages. This will maximise return on your green investments and ensure they grow quickly into large healthy specimens.
In a natural environment root systems can extend to 3 times the radius of the tree canopy. Probably the simplest way if calculating a minimum required soil volume is to take the projected canopy area of the mature tree, multiplied by a depth of 0.6. The shape of this area can be configured to suit the particular site.
Please note, this formula does not apply to fastigiated and columnar habit trees. The soil volume requirement can be calculated for narrow form trees by basing the canopy diameter on the natural growth form, i.e. the widest spread of a particular species.