Printer-friendly .pdf file.
Click for larger image.
Click for larger image.
- Nina Bassuk, Director and Professor Urban Horticulture Institute,
Cornell University, lthaca, NY
- Jason Grabosky, Urban Horticulture Institute, Cornell University,
- Peter Trowbridge, FASLA, Professor Landscape Architecture, Cornell
University, lthaca, NY
- James Urban, FASLA, James Urban and Associates, Annapolis, MD
The major impediment to establishing trees in paved urban areas is the
lack of an adequate volume of soil for tree root growth. Soils under pavements
are highly compacted to meet load-bearing requirements and engineering
standards. This often stops roots from growing, causing them to be contained
within a very small useable volume of soil without adequate water, nutrients
or oxygen. Subsequently, urban trees with most of their roots under pavement
grow poorly and die prematurely. It is estimated that an urban tree in
this type of setting lives for an average of only 7-10 years, where we
could expect 50 or more years with better soil conditions. Those trees
that do survive within such pavement designs often interfere with pavement
integrity. Older established trees may cause pavement failure when roots
grow directly below the pavement and expand with age. Displacement of
pavement can create a tripping hazard. As a result, the potential for
legal liability compounds expenses associated with pavement structural
repairs. Moreover, pavement repairs which can significantly damage tree
roots often result in tree decline and death.
The problems as outlined above do not necessarily lie with the tree
installation but with the material below the pavement in which the tree
is expected to grow. New techniques for meeting the often opposing needs
of the tree and engineering standards are needed. One new tool for urban
tree establishment is the redesign of the entire pavement profile to
meet the load-bearing requirement for structurally sound pavement installation
while encouraging deep root growth away from the pavement surface. The
new pavement substrate, called ‘structural soil’, has been developed
and tested so that it can be compacted to meet engineering requirements
for paved surfaces, yet possess qualities that allow roots to grow freely,
under and away from the pavement, thereby reducing sidewalk heaving
from tree roots.
Convential Tree Pits are Designed for Failure Looking at a typical
street tree pit detail, it is evident that it disrupts the layered pavement
system. In a sidewalk pavement profile, a properly compacted subgrade
of existing material often is largely impermeable to root growth and
water infiltration and significantly reduces drainage if large percentages
of sand are not present. Above the subgrade there is usually a structural
granular base material. To maintain a stable pavement surface the base
material is well compacted and possesses high bearing strength. This
is why a gravel or sand material containing little silt or clay is usually
specified and compacted to 95% Proctor density (AASHTO T-99). The base
layer is granular material with no appreciable plant available moisture
or nutrient holding capacity. Subsequently, the pavement surrounding
the tree pit is designed to repel or move water away, not hold it, since
water just below the pavement can cause pavement failure. Acknowledging
that; the above generalizations do not account for all of the challenges
below the pavement for trees, it is no mystery why trees are often doomed
to failure before they are even planted.
The subgrade and granular base course materials are usually compacted
to levels associated with root impedance. Given the poor drainage below
the base course, the tree often experiences a largely saturated planting
soil. Designed tree pit drainage can relieve soil saturation, but does
nothing to relieve the physical impedance of the material below the
pavement which physically stops root growth.
A New System to Integrate Trees and Pavement
‘Structural soil’ is a designed medium which can meet or exceed pavement
design and installation requirements while remaining root penetrable and
supportive of tree growth. Cornell’s Urban Horticulture Institute, has
been testing a series of materials over the past five years focused on
characterizing their engineering as well as horticultural properties.
The materials tested 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 sustainable root growth. The new system essentially forms a rigid,
load-bearing stone lattice and partially fills the lattice voids with
soil (Figure 1). Structural soil provides a continuous base course under
pavements while providing a material for tree root growth. This shifts
designing away from individual tree pits to an integrated, root penetrable,
high strength pavement system.
This system consists of a four to six inch rigid pavement surface,
with a pavement opening large enough to accommodate a forty year or
older tree (Figure 2) .
The opening could also consist of concentric rings of interlocking
pavers designed for removal as the buttress roots meet them. Below that,
a conventional 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 course would act as a root
exclusion zone from the pavement surface. Although field tests show
that tree roots naturally tend to grow away from the pavement surface
in structural soil. A geotextile could segregate the base course of
the pavement from the structural soil. The gap-graded, structural soil
material has been shown to allow root penetration when compacted. 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 structural soil thickness would depend on
the designed depth to subgrade or to a preferred depth of 36 inches.
This depth of excavation is negotiable, but a 24 inch minimum is encouraged
for the rooting zone. The subgrade should be excavated to parallel the
finished grade. Under-drainage conforming to approved engineering standards
for a given region must be provided beneath the structural soil material.
The structural soil material is designed as follows. The three components
of the structural soil are mixed in the following proportions by weight,
crushed stone: 100; clay loam: 20; hydrogel: 0.03. Total moisture at
mixing should be 10% (AASHTO T-99 optimum moisture).
Crushed stone (granite or limestone) should be narrowly graded from
3/4 -1 1/2 inch, highly angular with no fines. The clay loam should
conform to the USDA soil classification system (gravel <5%, sand
25-30%, silt 20-40%, clay 25-40%). Organic matter should range between
2% and 5%. The hydrogel, a potassium propenoate-propenamide copolymer
is added in a small amount to act as a tackifier, preventing separation
of the stone and soil during mixing and installation. Mixing can be
done on a paved surface using front end loaders. Typically the stone
is spread in a layer, the dry hydrogel is spread evenly on top and the
screened moist loam is the top layer. The entire pile is turned and
mixed until a uniform blend is produced. The structural soil is then
installed and compacted in 6 inch lifts.
In a street tree installation of such a structural soil, the potential
rooting zone could extend from 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 continuous and parallel to the curb,
eight feet wide and three feet deep would be minimally adequate for
continuous street tree planting.
There will be a need to ensure moisture recharge and free gas exchange
throughout the root zone. The challenge may be 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 structural
soil as spokes radiating from the trunk flair opening. This is currently
in the testing stage. Other pervious surface treatments could also provide
additional moisture recharge, as could traditional irrigation.
When compared to existing practice, additional drainage systems, and
the redesigned structural soil layer represent additional costs to a
project. The addition of the proposed structural soil necessitates deeper
excavation of the site which also may be costly. In some regions this
excavation is a matter of standard practice. However, this process might
best be suited for new construction and infrastructure replacement or
repair, since the cost of deep excavation is already incurred.
The Urban Horticulture Institute continues to work on refining the
specification for producing a structural soil material to make the system
cost effective. It is patent pending and will be sold with the trademark
‘CU-Soil’ to insure quality control. Testing over five years has demonstrated
that stabilized, gap-graded structural soil materials can meet this
need while allowing rapid root penetration. Several working installations
have been completed in lthaca, NY, New York City, NY, Cincinnati, OH,
Cambridge, MA and elsewhere. To date, the focus has been on the use
of these mixes to greatly expand the potential rooting volume under
pavement. It appears that an added advantage of using a structural soil
is its ability to allow roots to grow away from the pavement surface,
thus reducing the potential for sidewalk heaving as well as providing
for healthier, long-lived trees.
Grabosky, J. and Bassuk, N. “A New Urban Tree Soil to Safely Increase
Rooting Volumes Under Sidewalks”, 1995, Journal of Arboriculture 21(4),
197-201. Grabosky, J. and Bassuk, N. “Testing of Structural Urban Tree
Soil Materials for Use Under Pavement to Increase Street Tree Rooting
Volumes”, 1996, Journal of Arboriculture 22(6), 255-263.