Solutions for Trees in Paved Areas: Structural Soils and Soil Vaults
In the old days, trees in paved areas were limited to tiny, 4×4 cutouts, with a small volume of useable soil surrounded by compacted base. The planting environment was essentially a big pot, meaning that the trees would either die young, or their roots would grow in the interface between pavement and base, cracking and lifting concrete and leading to excessive maintenance costs and potential liability. Arborists and some landscape architects worked hard to expand awareness of these issues within the design and development community, with the goal of improving tree rooting environments. Some jurisdictional codes will always result in some trees planted in very limited spaces – but if tree rooting environments could be improved, the associated problems and costs could be reduced, or eliminated.
Now, after years of research and development, we now have two much better options for trees in paved areas: structural soil, and soil vaults. Both dramatically improve soil rooting volume under paving; each has its advantages and drawbacks.
Structural soil was first developed at Cornell University’s Urban Horticulture Institute in the early 1990’s, and patented as “CU SoilTM”. The goal was to develop a load-bearing base for pavement that could also be used as a rooting volume, satisfying the needs of several otherwise conflicting design criteria at one time. The solution was to use an angular crushed stone matrix with high void space, and add 10% by volume of clay loam soil. The soil is bound to the stone with a specific type of hydrogel as a tackifier, so that the soil does not wash away or settle out. High void space allows roots to penetrate the matrix easily; roots gain nutrients from the soil fraction in the matrix. Roots thus have a huge volume in which to grow down below the paved surface, reducing or eliminating surface rooting. In addition, CU Soil can be compacted to 100% of Proctor density, providing the strength needed to support any paving.
Trials in numerous urban and commercial environments have shown that the trees planted in CU Soil do indeed grow well, and that hardscape damage normally associated such trees is greatly reduced; the product is now regularly used across the country in a variety of environments, sometimes in conjunction with permeable paving systems. The city of Frisco, TX, for example, requires at least 500 sf of adequate rooting volume for all trees in paved areas, which means CU Soil is an idea solution. Some important caveats:
- Working with the material requires some care. If overmixed on the jobsite during installation, allowed to dry out before installation, or left exposed to the elements for more than short periods of time, the soil can separate from the stone, rendering it useless as intended.
- For some species, structural soils may not provide an adequate soil fraction for proper mineral and nutrient uptake. Some tree species that require rich soils may not thrive in this environment.
- For some developers, any extra cost is too much. This is an extra first cost, no way around it – around $45 per cubic yard, just for the material – but the long-term costs of removing/replacing trees and repairing hardscape damage is significantly less.
Soil vaults also aim to create a large rootable volume underneath pavement. However, where CU Soil uses the stone base itself as the rooting medium, soil vaults are an actual open structure on which pavement can be poured. Within the structure is ample space for soil, which is not compacted but tamped in place; roots simply grow into the soil under pavement, as they would in an open landscape. Currently, two systems have seen a widespread increase in use: SilvaCell, made by DeepRoot, and StrataVault, made by Citygreen. Both are very similar in design and function, featuring a modular design that can be expanded horizontally and stacked vertically up to three layers high (a maximum of about 3’ deep).
Soil vaults create something much closer to a natural rooting environment, with all of the tree’s mineral and nutrient needs met. The rooting environment can also be tailored to specific tree needs, unlike structural soil. Also, there is no danger of soil/stone separation, and the vault products themselves require no special handling. Finally, soil vaults are capable of holding large amounts of storm runoff, reducing the need for separate surface of subsurface detention facilities. The trade-off is that the system is substantially more expensive than CU Soil: in addition to the overexcavation, soil vaults have the increased costs from materials and installation time. In addition, many contractors are still unfamiliar with soil vaults, and will charge more due to the learning curve.
Both structural soils and soil vaults have their advantages and disadvantages related to suitability for different tree species, flexibility of soil design, and cost. But where trees are required to be in small spaces adjacent to any hardscape, both systems offer substantial benefits, and reduce long term costs and issues.
Urban Horticulture Institute