Conventional Tree Pits Designed to Fail

New System of Integration

CU-Soil™ Installations


CU-Structural Soil® is a designed medium which can meet or exceed pavement design and installation requirements while remaining root penetrable and supportive for tree growth. Cornell's Urban Horticulture Institute tested a series of materials characterizing their engineering as well as horticultural properties. The materials are gap-graded gravels which are made up of crushed stone, clay loam, and a hydrogel tackifier stabilizing agent. The materials can be compacted to meet all relevant pavement design requirements yet allow for substainable root growth. The new system essentially forms a rigid, load-bearing stone lattice and partially fills the lattice voids with soil. CU-Structural Soil® provides a continuous base course under pavements while providing material for tree root growth, shifting design away from individual tree pits, to a designed, root penetrable, high strength, pavement system.

One description of such a system could consist of a four to six inch rigid pavement surface, with a pavement opening large enough to accommodate a 40 year or older tree. The opening could be concentric rings of pavers designed for removal as the buttress roots lift them. Below that a six inch base course could be installed and compacted with the material meeting normal regional pavement specifications for the traffic they are expected to experience. The base would act as a root exclusion zone from the pavements surface, although we have found that tree roots naturally tend to grow away from pavement surface in CU-Structural Soil®. A geotextile (weed barrier) would segregate the base course from the subbase and extend as an apron emerging around the edges of the concrete. A gap-graded, structural soil material demonstrated to allow root penetration when compacted would be the subbase and area for subsequent tree root growth. This material would be compacted to not less than 95% Proctor density (AASHTO T-99) and possess a California Bearing Ratio greater than 40 [Grabosky and Bassuk 1995,1996]. The subbase thickness would depend on the depth of subgrade or to a proposed target of 36 inches. This is negotiable, but a 24 inch minimum would be encouraged for the root zone. The subgrade should be excavated to parallel the final grade. Under-drainage must be provided under the CU-Structural Soil® material conforming to approved engineering standard for that region.

In a typical street tree installation of such a structural soil, the potential rooting zone could extend from the building face to curb, running the entire length of the street. This would ensure an adequate volume of soil to meet the long term needs of the tree. Where this entire excavation is not feasible, a trench, running parallel to the curb, eight feet wide and three feet deep would be minimally adequate. Since this profile has adapted the standard surface and base specifications generally in use, less hesitation for engineering approval may result.

To date we have focused on the use of these mixes to greatly expand the potential rooting volume under pavement. It appears that an added advantage of using this material is its ability to allow roots to grow away from the wearing surface, thus reducing the potential for sidewalk heaving as well as providing for healthier, long-lived trees.


A new system

  • There is a zone of overlap
  • Skeletal soils are a viable tool, but only one part of a complete system
  • Root pressure research, assigning a depth to the base
  • Water delivery system
  • Drainage
  • Nutrient Management
  • Adequate pavement openings (root flare & structural roots)

Why was root growth increased in the skeletal soil?

  • The material provided a viable rooting environment
  • Oxygen was available
  • Moisture was present
  • Root penetration was unrestricted within the soil occupying the stone skeleton voids

Root distribution influences in the skeletal soil profile

  • Moisture availability
  • Oxygen levels in the rhizosphere
  • Penetration resistance (a function of void distribution)
  • Nutritional demand
  • Temperature extremes

PDF: Using CU-Structural Soil® in the Urban Environment

PDF: Using CU-Structural Soil® under Porous Asphalt

CAD details & 3-Part Specifications

Everything You Always Wanted To Know About CU-Structural Soil® - A Comprehensive Guide