What is CU-Structural Soil® ?
CU-Structural Soil® (U.S. Patent # 5,849,069) is a medium for tree planting that can be compacted to pavement design and installation requirements while permitting vigorous 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.
Using CU-Structural Soil® for Street Trees
CU-Structural Soil® is intended for paved sites to provide adequate soil volumes for tree roots under pavements (Fig. 1). It can and should be used under pedestrian mall paving, sidewalks, parking lots, and low-use asphalt to provide more porous parking areas. Research has shown that tree roots in CU-Structural Soil® profiles grow deep into the base course material, away from the fluctuating temperatures at the pavement surface. One benefit of this is that roots are less likely to heave and crack pavement than with conventional paving systems.
Planting a tree into CU-Structural Soil® is much like conventional planting. If possible, the pavement opening should be expandable (via removable pavers or using a mulched area) for the sake of the anticipated buttress roots of maturing trees (Fig. 2). CU-Structural Soil® should be at a depth of at least 24” but preferably 36”. CU-Structural Soil® can be used right up to the surface grade where there is a pavement opening that is large enough to allow for tree installation.
CU-Structural Soil® Technical Information
CU-Structural Soil® (U.S. Patent # 5,849,069) is a two-part system comprised of a rigid stone “lattice” to meet engineering requirements for a load-bearing soil, and a quantity of soil, to meet tree requirements for root growth. The lattice of load-bearing stones provides stability as well as interconnected voids for root penetration, air and water movement (Fig 3). The uniformly graded 3/4’ -1 1/2'’ angular crushed stone specified for CU-Structural Soil® is designed to ensure the greatest porosity. Crushed or angular stone provides more compaction and structural interface of stone to stone than round stone. Because stone is the load bearing component of structural soil, the aggregates used should meet regional or state department of transportation standards for pavement base courses.
Since among soil textures, clay has the most water and nutrient-holding capacity, a heavy clay loam with a minimum of 20% clay, is selected for the CU-Structural Soil® system. Cu-Structural Soil ™ should also have organic matter content ranging from 2-5% to ensure nutrient and water holding while encouraging beneficial microbial activity. A minimum of 20% clay is also essential for an adequate cation exchange capacity.
With carefully chosen uniformly-graded stone and the proper stone to soil ration, a medium for healthy root growth is created that also can be compacted to meet engineers’ load-bearing specifications. (Fig. 1.8). The intentions are to “suspend” the clay soil between the stones without over-filling the voids, which would compromise aeration and bearing capacity. CU-Structural Soil® utilizes Gelscape® hydrogel as a non-toxic, non-phytotoxic tackifier, in addition to stone and soil components.