| A NEW SYSTEM TO INTEGRATE TREES and PAVEMENT 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. At Cornell's Urban Horticulture Institute, we have been testing a series of materials which focuses on characterizing their engineering as well as horticultural properties. The materials we are testing are gap-graded gravels which are made up of crushed stone, clay loam, and a hydrogel 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. There will be a need to ensure moisture recharge and free gas exchange throughout the root zone which is not the entire subbase. The challenge is met by the installation of a three dimensional geo-composite (a geo-grid wrapped in textile one inch thick by eight inches wide) which could be laid above the subbase as spokes radiating from the trunk flair opening. This form of passive irrigation is currently in the testing stage. Other previous surface treatments could also provide additional moisture recharge, as could traditional irrigation. When compared to existing practice, the addition of a moisture recharge sytem, additional drainage sytems, and the redesigned subbase layer represent additional costs. The addition of the proposed subbase necessitates deeper excavation of the site which also may be costly. In some regions this excavation is a matter of standard practice for other construction needs; however, one could envision this material best suited for new construction and infrastructure replacement/repair, since the cost of deep excavation is already incurred. The Urban Horticulture Institute continues to work on refining the specifications for producing a structural soil material to make the system cost effective. 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. |
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