Structural Soil — Part 2

image: Nina Bassuk
image: Nina Bassuk

In Structural Soil—Part 1, the many problems confronting urban trees were outlined and the need for a specially designed growing medium for such trees was amply demonstrated. In Part 2, the authors describe their development of CU-Structural Soil™ and share lessons learned after using this load-bearing soil for 15 years.

Why was CU-Structural Soil™ developed?

Soils under pavement need to be compacted to meet load-bearing requirements so that sidewalks and other pavements won’t subside and fail. Soils are often compacted to 95% peak (Proctor or modified Proctor) density before pavements are laid. When trees are planted into these soils, root growth is severely reduced or eliminated beyond the tree-planting hole. When root growth is restricted, tree growth suffers as water, nutrients and oxygen are limited. The need for a load-bearing soil under pavement gave rise to the development of CU-Structural Soil™, a blended soil that can be compacted to 100% peak density to bear the load of a pavement while allowing tree roots to grow through it.

The concept behind it

CU-Structural Soil™ is a mixture of crushed gravel and soil with a small amount of hydrogel to prevent the soil and stone from separating during the mixing and installation process. The keys to its success are the following: the gravel should consist of crushed stone approximately one inch in diameter, with no fine particles, to provide the greatest large void space. The soil needed to make structural soil should be loam to clay loam containing at least 20% clay to maximize water and nutrient holding capacity. The proportion of soil to stone is approximately 80% stone to 20% soil by dry weight, with a small amount of hydrogel aiding in the uniform blending of the two materials. This proportion ensures that each stone touches another stone, creating a rigid lattice or skeleton, while the soil almost fills the large pore spaces that are created by the stone. This way, when compacted, any compactive load is being borne from stone to stone, and the soil in between the stones remains uncompacted.

Conceptual diagram showing loading or compaction effort on stone and soil particles image: Nina Bassuk
Conceptual diagram showing loading or compaction effort on stone and soil particles
image: Nina Bassuk

How is it used?

Structural soils in the context of this discussion have specific uses. The material must support pavement designed to withstand pedestrian and vehicular traffic, and can be designed for use under pedestrian malls, sidewalks, parking lots, and possibly some low-use access roads. The material is intended as a tool to be used when there are no other design solutions to provide adequate soil volumes for trees in close proximity to pavement.

Structural soils can be used as base material under pavement. There is no need to put an additional base course on top of it. Our research has shown that tree roots in structural soil profiles grow deep into the full depth of the structural soil below the pavement. It is anticipated that when roots grow to this depth any upward pressure caused by radial root growth will be distributed over a wider section of pavement, reducing or eliminating sidewalk heaving. Research is underway to test this possibility.

By design, structural soils are fully compacted with conventional equipment to standard relative compactness (100% Proctor density). However, we do recommend normal topsoil be used around the tree root ball in the tree pit opening where the tree is planted. This will maximize soil water holding capacity for the tree.

CU-Structural Soil™ requires an adequate volume of soil under pavement, approximately 2 cubic feet of soil for every square foot of envisioned crown spread. We recommend a 36-inch soil depth, although several projects have been successful using soil as shallow as 24 inches. We would not recommend any less than 24 inches. CU-Structural Soil™ has an available water holding capacity between 7% and 12% depending on the level of compaction. When we calculate soil volumes for CU-Structural Soil™, we use a water holding capacity of 8% to be on the conservative side (Grabosky et al, 2009). This water holding capacity is equivalent to loamy sand. Based on water holding capacity, you would need approximately 1.3-1.6 times the amount of CU-Structural Soil™ as you would need for an equivalent sized tree growing in loam. See the table below for soil volume equivalents recommendations. Because of its well-drained nature, trees that prefer well-drained soils do best in CU-Structural Soil™. Depending on the stone type used to make CU-Structural Soil™, the pH of the soil may be affected (e.g. limestone vs. granite). Good tree selection practices and establishment procedures should be used with CU-Structural Soil™ as would be done with any tree installation.

Soil volumes of CU-Structural Soil™, and loam necessary to support a large tree in the Midwest or Mid-Atlantic US without irrigation after 3 years of establishment. It would take between 1.3-1.6 times the amount of structural soil to equal the water holding capacity of loam with 15% water holding capacity. image: Nina Bassuk
Soil volumes of CU-Structural Soil™ and loam necessary to support a large tree in the Midwest or Mid-Atlantic US without irrigation after 3 years of establishment. It would take between 1.3-1.6 times the amount of structural soil to equal the water holding capacity of loam with 15% water holding capacity.
image: Nina Bassuk

Provision for an irrigation-aeration system between the pavement and structural soil materials may be considered. Given the large volume of structural soil for tree roots to explore, this need for irrigation is a long-term view of future management needs or where rainfall is scarce. While there is less moisture in a structural soil on a per volume basis, the total root system occupies a much larger volume than does a tree growing in compacted ordinary soil. Fertilization can be dissolved into the irrigation water for nutritional management, although over 10 years we have not seen nutritional deficits as long as trees are chose which tolerate the pH of the soil. There must also be positive drainage provided to prevent water-logging.

The sub-grade below the structural soil will still be compacted and rendered essentially impermeable to moisture. A perforated drain connected to the storm drainage system should be placed between the structural soil material and the compacted sub-grade to insure that excess water will not back up into the structural soil profile.

Plant selection for structural soils should include alkaline-tolerant and drought-tolerant plant species. The stone used, whether limestone or granite, will heavily influence pH. However, if the system is paved with concrete products, the pH will continue to climb as the concrete slowly breaks down.

Planting a tree into structural soil is fairly simple. The pavement opening should allow for buttress root formation and water infiltration. We recommend at least a 50 square feet opening although a functionally wider opening could be paved in removable pavers or other porous pavement. The tree is simply planted into the structural soil as it would be in a normal, albeit very stony, field situation. Normal good topsoil should be placed around the tree ball. Structural soil under the root ball will prevent tree subsidence. It is presumed that supplemental irrigation will be provided during the first growing season as would be consistent with any new planting. In areas where there is little rainfall low-volume, trickle irrigation may be used at the tree opening and under the pavement.

Although CU-Structural Soil™ is made of readily available local crushed stone and soils, it is essential to make it correctly. To insure quality control, CU-Structural Soil™ is made by licensed producers who make it according to its specification all over the country (there are approximately 75 producers currently). Samples from the licensed producers are tested at an independent soils lab for compliance. Over 1,700 CU-Structural Soil™ projects have been installed successfully all over the US, Canada, and Puerto Rico during the past 15 years. Costs range from $40-$75 per cubic yard.

Cross-section of typical tree installation into CU-Structural Soil™. Note where the tree pit is open, topsoil should be placed around the tree ball, but CU-Structural Soil™ should be placed under the ball to prevent tree ball subsidence. image: Nina Bassuk
Cross-section of typical tree installation into CU-Structural Soil™. Note where the tree pit is open, topsoil should be placed around the tree ball, but CU-Structural Soil™ should be placed under the ball to prevent tree ball subsidence.
image: Nina Bassuk

CU-Structural Soil™ for stormwater capture

CU-Structural Soil™ has a rapid infiltration rate (>24 inches per hour) and has 26% porosity after it has been compacted to 100% peak density. Ordinary loam soil compacted to 100% peak density has an infiltration rate of 0.5 inch/hour. This allows CU-Structural Soil™ to be used for stormwater capture under porous pavements. Twenty four inches of CU-Structural Soil™ can hold the 100-year storm in Ithaca, NY of 6 inches of rain in 24 hours.

Things we’ve learning after 15 years of experience with CU-Structural Soil™

Tree selection matters

CU-Structural Soil™ is very well drained and may have an altered pH depending on the stone type that is used. Limestone gravel will result in a higher pH (about 8.0) while granite and other stone types will have less effect. There are numerous trees that grow very well at a variety of soil pH’s. Also, as CU-Structural Soil™ is well drained, trees should be chosen that prefer these soils conditions.

See Recommended Urban Trees for recommendations on trees that tolerate a range of pH and soil moisture conditions.

Initial maintenance matters

As with any tree, initial watering is important to get it off to a good start. This may not matter in a lawn or park situation; however, it is critical when pavement limits water from entering the soil. This happens more frequently when trees are planted in pavement. Twenty gallons of water every 5-7 days is generally sufficient for newly planted trees in areas where there is adequate rainfall. In areas where trees are normally irrigated, low-volume trickle irrigation works well for trees planted in CU-Structural Soil™.

Soil volume matters

Structural soil less than 24 inches deep is inadequate for tree growth. Earlier trials with 12-inch or 15-inch depth of structural soil did not result in good tree growth after 12 years. Soil volume should be sized to be at least 2 cubic feet per square foot of envisioned crown projection (the area under the tree’s drip line). CU-Structural Soil™ depth should be at least 24 inches, but preferably 30 or 36 inches. The deeper the soil, the greater the soil’s water holding capacity. It will also be less likely that tree roots will heave sidewalks. Research has shown the roots grow to the full depth of structural soil so radial growth of tree roots and their resulting upward force would be spread over a larger area as compared to roots that grow right under the sidewalk.

Production of CU-Structural Soil™ according to research-based specification matters

Many years of research and testing went into the development of CU-Structural Soil™. When there is at least 20% clay in the soil, the soil coats the stone and there is greater surface area for roots to gain the water and nutrients they need. This clay content is critical to achieve adequate nutrient and water storage. There is also 3-5% organic matter in the soil helping it to achieve a good cation exchange capacity and to feed soil microorganisms.

Literature Cited

Bassuk, N. L. and T. H. Whitlow. “Evaluating street tree microclimates in New York City.” Proc. 5th METRIA Conference (May 1985): 18-27.

Craul, P. J. Urban Soil in Landscape Design. New York: John Wiley & Sons, Inc., 1992.

Grabosky, J. Identification and testing of load bearing media to accommodate sustained root growth in urban street tree plantings. M.S. Thesis, Cornell University, 1995.

Grabosky, Jason, Edward Haffner, and Nina Bassuk. “Plant Available Moisture in Stone-soil Media for Use Under Pavement While Allowing Urban Tree Root Growth.” Arboriculture & Urban Forestry 35, no. 5 (2009): 271-278.

Lindsey, P. and N. Bassuk. “Redesigning the urban forest from the ground below: A new approach to specifying adequate soil volumes for street trees.” Arboricultural Journal 16 (1992): 25-39.

Moll, G. “The State of our Urban Forests.” American Forests, November/December 1989.

Morris, L.A., and R.F. Lowry. “Influence of Site Preparation on Soil Conditions Affecting Stand Establishment and Tree Growth.” Southern Journal of Applied Forestry 12, no. 3 (1988): 170-178.

Patterson, J. C., J. J. Murray, and J. R. Short. “The impact of urban soils on vegetation.” Proc. 3rd METRIA Conference (1980): 33-56.

Perry, T. O. “The ecology of tree roots and the practical significance thereof.” Arboricultural Journal 8 (1982): 197-211.

by Nina Bassuk, Cornell University; Peter Trowbridge, FASLA, Cornell University; and Jason Grabosky, PhD, Rutgers University

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