Structural Soil

Structural Soil is a medium that can be compacted to pavement design and installation requirements while permitting root growth. It is a mixture of gap-graded gravels (made of crushed stone), clay loam, and a hydrogel stabilizing agent to keep the mixture from separating. It provides an integrated, root penetrable, high strength pavement system that shifts design away from individual tree pits.

Previously the main problem facing the establishment of trees in paved areas is the lack of enough volume of soil for tree root growth. Soils under pavements are typically so compacted that it stops roots from growing. Older established trees with their roots under pavement grow poorly and often die. They can also cause pavement failure and displacement. Overall pavement preparation and repairs can shorten the life expectancy of a tree to 7–10 years where we could see them grow for at least 50 more years.

Structural soil was researched and developed in the 90’s by Cornell University’s Urban Horticulture Institute. In 1999, AMEREQ signed a licensing agreement with Cornell University and currently holds the patent rights to Cornell’s CU-Structural Soil Urban Tree Planting Mix. It is marketed as CU-Structural Soil for quality control and is produced by a network of qualified AMEREQ-licensed companies. CU-Structural Soil on average costs $35–$42 per ton. Other companies have formed their own brand of structural soil based on Cornell’s work. For example, STALITE has developed STALITE MATRIX Structural Soil that they claim holds more moisture.

Wallace Laboratories modified work in 1994 which had been done by others to help improve the technology. The concept had been used in Europe in the 1980s. They did not seek a patent on their work but left the technology in the public domain for others to freely use. Their work preceded the filing of the Cornell University patent.

Briefly, the aggregate size was increased to about 2 inches, the soil texture was changed to a clay or clay loam in order to increase the water holding capacity and nutrient capacity, the soil was conditioned with the linear polyacrylamide, and the soil chemical and physical properties were specified. The procedures are available at <http://us.wlabs.com/structualgap-graded-soils/>

Their procedure has been extensively used worldwide. One municipal installation used about 50,000 cubic yards of the Wallace Labs formulation.

...
Wikipedia