Structural soil design
Structural soil is designed to support the weight of walks, roads, pedestrians and vehicles as well as provide a well-aerated soil substrate for tree root growth. Weight is transferred from aggregate to aggregate then to the soil under the aggregate.
No weight is borne by the soil between aggregates. This allows roots to grow well in the soil between the aggregates. In normal design, the soil and sub-base under a walk or pavement is compacted to 95% proctor density. This reduces air exchange below the critical level needed for root growth. As a result, roots are unable to grow into this compacted soil. (Illustration credit: Dr. Jason Grabosky, Rutgers University)