During new sidewalk installation, why not take the opportunity to create a soil volume under pavement that can support root growth? Making your own structural soil with site soil is easy! Site soil should first be tested for contaminants, and works best if it has a clayey texture (more nutrient and water holding capacity). Newly planted trees will have their best chance to thrive and live a long life. Best to install 6" of crushed rock between sidewalk and structural soil. There is no water or nutrient holding capacity in the crushed rock to attract roots and cause future root related hardscape displacement.