Structural Soil

| April 24, 2019

by Horace Aikman

The urban forest thrives upon a foundation of soil, water, sunlight, and atmosphere. Reduce any of these elements and the growth and health of trees will be limited. A polluted atmosphere, heavy shade, droughty or saturated conditions, or small soil volumes can diminish the stature and robustness of urban plantings. In spite of the efforts by urban foresters — landscape architects, urban planners or city arborists — to select the right tree for the right place in city streets, the longevity of individual trees will always be diminished when these critical elements — soil, water, sunlight, and atmosphere — are curtailed.

University of Tennessee, Knoxville, Neyland Stadium: suspended slab supported by plastic structural matrix with horticultural soil infill, 2011 (installation) . . .

. . . and 2018

Often the shade cast by buildings and the narrowness of city sidewalks thwart opportunities to plant trees. Even collectively, the people who plan and manage the urban forest have little sway over urban pollution or how the sun reaches the city street. What we can control are the environments of soil and water that are critical to tree growth and the urban forest. When considering the below-grade infrastructure that supports tree growth, the urban forester must eke out the best tree pit that the available funds will accommodate.

As we renew and reconstruct our urban landscape to promote healthy tree growth, we are continuously rethinking and reevaluating the means of providing adequate soil volume for trees. Thoughtful design of the urban forest must plan for providing enough soil volume below pavements to support tree growth. While there is no consensus about the volume of soil needed to ensure trees grow to maturity, most of us subscribe to the idea that more soil is always better. But the questions that challenge us are these: What kind of soil? What kind of infrastructure?

University of Massachusetts Lowell University Suites: sand-based structural soil, 2013 (installation) . . .

In our practice of landscape architecture, we have used all of the major systems designed to provide soil for tree growth. We have utilized various structural-soils: sand-based; crushed stone/clay loam/polymer wetting agent system; and expanded slate and loam. We have also used reinforced concrete tree vaults and sidewalk slabs supported by plastic boxes holding planting soil. And of them all, we believe an open bed of sandy planting soil that is protected from pedestrian trampling and provided with adequate irrigation will provide the best environment for street trees.

. . . and 2018

In urban settings, we rarely have the square foot area to devote to this preferred landscape. So, in the absence of enough area and funding, we must choose between all of the other options. Weighing all of the demands and limitations on and to the urban forest — politics, money, space, pedestrian safety, rodent control, vehicular traffic, stormwater infiltration — we have reached several conclusions.

First: Even if we do not have adequate soil volume and water, let’s plant trees. If the trees only last a few years then we have provided some succor to the urban landscape. This is good politics at its very basic level. Some trees might actually break out of their confined spaces and find enough soil volume to prosper.

Second: Retrofitting an existing city street with enough soil volume to support street trees is incredibly expensive. Don’t be lulled into thinking otherwise. Even a streetscape renovation project that provides insufficient soil volume will be more expensive than you could ever imagine. Financial planning is worth the exercise.

Third: If we set aside the optimal solution of the large open treeway, then we are left with two broad systems of providing a planting medium below sidewalks: structural soil or suspended slabs. Structural soils utilize either coarse sand or crushed stone to create a matrix that can support sidewalks and accommodate enough topsoil to promote tree root growth. Suspended slab construction refers to the structural system that supports sidewalks and accommodates a below grade bed of topsoil. It is critical to note that whether we utilize one or the other system type, we must provide irrigation, aeration, and underdrainage to keep these soils in a modicum of health.

Fourth: Structural soil systems are better suited for sidewalks next to city streets. Unless the street and its utility infrastructure are going to be entirely redesigned and reconstructed, existing utilities will always restrict the use of a suspended slab system. All those pipes, duct banks, conduits, and wires will inhibit the easy and ready placement of either reinforced concrete walls and slabs, or the modules of the plastic grid systems. It is much more cost effective to excavate around the utilities, install underdrains/aeration/irrigation, and then place the structural soil than to do otherwise. We have also found that the structural soils are better able to withstand the vibrations from passing trucks and automobiles and are less prone to vibratory consolidation than a topsoil in a suspended slab condition.

Fifth: Alternately, in the absence of constant, repetitive vibration of passing vehicles, the topsoil that can be placed in a suspended slab system will promote faster and healthier tree growth than the relatively sterile structural soils. Suspended slab systems are better suited to urban plazas or pedestrian malls because the planting soils have more fine sand, silt, clay, and organics.

Maintaining and expanding upon the urban forest contributes to the health of our cities on many levels — to the public health of the inhabitants, to heat island mitigation and stormwater management, to psychological relief from urban hardscape, to creating wildlife habitat, and to increased property value. It is worth the upfront investment in the variables under our control, such as soil, water, and aeration. Because, ultimately, the cost of not planting trees is higher.

Horace Aikman, RLA, is a senior associate and the director of construction technologies at CRJA-IBI Group.




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Category: All, contributor, Landscape/Civil, Trends and Hot Topics

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