The major impediment to establishing trees in paved urban areas is the lack of an adequate volume of soil for tree root growth. Soils under pavements are highly compacted to meet loadbearing requirements and engineering standards. This often stops roots from growing, causing them to be contained within a very small useable volume of soil, without adequate water, nutrients, or oxygen. Hence, urban trees with most of their roots under pavement grow poorly and die prematurely. It is estimated that an urban tree in this type of setting lives for an average of only 7-10 years, where we could expect 50 or more years with better soil conditions. In addition, those trees that do survive within such pavement designs often become problematic relative to pavement integrity. Older established trees may cause pavement failure when roots grow directly below the pavement and expand with age. Displacement of pavement becomes a tripping hazard. As a result, legal liability compounds expenses associated with pavement structural repairs. Moreover, pavement repairs, which significantly damage tree roots, often result in tree decline and death.
The problem does not necessarily lie with the tree installation but with the pavement cross-section in which the tree is expected to exist. New techniques for meeting the opposing needs of the tree and engineering standards are needed. One new tool for urban tree establishment is the redesign of the entire pavement profile to meet the load-bearing requirement for safe pavement installation while encouraging deep root growth away from the pavement surface. Our new soil, called "structural soil" has been developed and tested so that it can be compacted to meet engineer's requirements for paved surfaces and yet possesses qualities that will allow roots to grow freely underneath pavements. Moreover, our soil allows roots to grow away from the pavement, thereby reducing sidewalk heaving from tree roots.