HomeMy WebLinkAboutLandscape Plan~®
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BUCKINGHAM COMPANIES, AMO®
~~
April 10, 2001
Mr. Scott Brewer
City of Carmel
One Civic Squ e
Carmel, IN 46 2
Dear.Mr.
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(~'0~ /
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pPR 11
ppGS
RE: PROVIDENCE AT OLD MERIDIAN
Enclosed, please find the landscape plan for Providence at Old Meridian, specifically
outlining the areas in which the Amsterdam Soil Recipe will be used. Unfortunately I do not
have an exact date of when that work will begin, however it will most likely commence in the
next two to four weeks. Once I have an exact date I will let you know.
In the meantime, if you have atry questions or corntnents please feel free to contact the at
317-974-1234, ext. 241.
F
Respectfully Submitted,
PROVIDENCE HOUSING PARTNERS,
LLC.
V'"' _' U
Lynnette Deogracias, AICP
enclosures
cc: Bradley Chambers
Bill Bauer
Dan Laycock
Dale Rea
Rick Riddle
®~;
333 N. Pennsylvania 5[reet, 10[h floor ^ Indianapolis, IN 46204.317 974.1234 • Fax 317 974.1238 ^ www.buckingham-co.com
Journal of Arboriculture 24(3): May 1998
DESIGNING URBAN PAVEMENT SUB-BASES TO
SUPPORT TREES
by Palle Kristoffersen
Abstract. In Denmark, poor growth conditions for urban
trees have been perceived as a problem since the 1980s.
Restricted planting-pit sizes are mainly responsible for this
problem. In 1996, a survey found that the average size of
municipal planting pits had increased from 0.7 m' (3.5 fN) in
the late 1960s, to 3.4 m' (t20 fi') in 7996. Tc increase the
volume of the planting pit, several materials have been
introduced to mix with soil to allow root growth under
pavements. Three methods are available for installing these
materials under pavements. During the last 5 years, more
than 800 trees have keen planted on more than 30 sites
using these materials and installation methods. Every
method has advantages and disadvantages. No serious
difficulties due to load-bearing capacity or frost heaves have
been recorded.
Planting pits for urban trees are surrounded with
soil that has been either unintentionally com-
pacted orreplaced with compacted road-building
materials (Kopinga 1985; Lichter and Lindsey
1994; Randrup 1997). The restricted planting pits
allow only limited root growth, and insufficient
rooting volumes in urban situations often result
in destructive roots. Roots outgrow the planting
pits and often spread immediately below the
pavement surface, lifting and eventually destroy-
ing the pavement. This is a serious and expen-
sive problem that stresses trees (Kopinga 1991,
1992). Sycamore maples (Acerpseudop/atanus)
grown under stressed conditions exhibit limited
height growth, more capital axes, fewer cardinal
axes, and twisted and crooked stems (Bonsen
1996). This irregularity of growth causes aesthetic
as well as maintenance problems in system
plantings in which all trees are important in form-
ing aunified appearance. If system plantings are
established in paved, partly paved, and unpaved
areas, the chances of obtaining a homogeneous
appearance will be reduced further.
The usual method of establishing urban trees
in Denmark has been to plant trees in individual
planting pits. The size of the planting pit has in-
creased since the 1960s, when it was often less
121
than O.i ma (3.5 ft3). Since then, the average vol-
ume of planting pits, as found in 11 Danish mu-
nicipalities, has increased to 3.4 m3 (120 ft3),
ranging from 0.8 to 8.1 m3 (28 to 286 ft3) (Teilmann
and Kristoffersen 1996).
Applying one of the predeveloped models to
estimate soil volumes for urban trees based on
the soil's water-holding capacity shows that a tree
with a canopy diameter of 10 m (33 ft) requires a
35 to 40 mz (377 to 431 ft2) planting pit (Lindsey
and eassuk 1992). In urban areas, ii is almost
impossible to plan unpaved planting beds of this
size. Therefore, several methods for integrating
the growing media under paved surfaces have
been described. These methods consist of a mix
of medium-course sand (Couenberg 1994), a
load-bearing matrix mixed of stones and soil
(Grabosky and Bassuk 1995), and pavements
constructed with a span above a growing media,
such as top soil (Urban 1989).
In Denmark, pavements are defined to con-
sist of a sub-base and a base, both usually com-
pacted to densities that impede root growth, and
of a top layer of bricks, pavers, or asphalt
(Patterson 1977; Vejdirektoratet 1984). The Sub-
base material usually consists of sand or other
materials that promote drainage. The base is of-
ten made of screened and graded gravel but al-
mostnever concrete, as it is in the United States.
Design and Installation Methods
1n Denmark, more than 800 trees were integrated
into pavements at 30 different sites using a
load-bearing matrix or different sand mixes. A
number of matrix materials were used, and 3 mix-
ing methods have been developed.
Materials. Stones, ranging from 32 to 150
mm (1 to 6 in.) diameter gravel, granite, lava
slags, crushed bricks, and blocks of
Leca-concrete have been used asload-bearing
matrixes (Table 1). Leca-concrete is made of
i22
Kristofferson:. Integrating Trees and Pavement
Table 1. Materials and mixing methods used as sub-base to support trees
' Materials
Stones/
crushed stones Leca-concrete Crushed Crushed
Stones (32-80 mm) blocks' lava bricks Sand mix
(80-150 mm) (32-45 mm) (100 x 100 mm) (32-45 mm) (25-50 mm) (0.2-0.5 mm)
Installation methods water mix premix premix premix premix premix
dry mix water mix water mix water mix
dry mix
Growing medium topsoil topsoil topsoil topsoil topsail humus/
compost
Density of material 2.7 2.7 1.0 t.8 1.8 2.6
(9/cm')
Porosity of material D 0 approx. 15 approx. 55 approx. 30 0
(vol. - %)
Voids in compacted 45 45/40 56 38 35 45
material (vol. - %)
Leca nuts mixed with cement and formed to
blocks. Leca is a commercial name and an ac-
ronym for Light Expanded Clay Aggregates. The
Leca nuts consist of an inner clinkerized cellu-
lar structure with a hard, resistant outside coat-
ing, The material porosity is presumed to
influence the air and water exchange in the
soil mix.
Methods of installation. Soil is used to fill
the voids in the load-bearing matrix. Because
compacted soil impedes root growth, it is neces-
sary to prevent compaction during the installa-
tion process. Three mixing and installation
methods have been developed and tested:
premixing before installation, water mixing dur-
inginstallation, and dry mixing during installation.
Premixing before Insfallafion. Stones and
soil are mixed at a predetermined ratio. Mixing
can be done with a front loader, a power shovel,
a concrete mixer, or similar equipment. Separa-
tion of soil from stones may occur after mixing if
the mixture is moved or transported; therefore,
remixing before installation maybe necessary to
restore uniformity. The matrix of stone and soil is
compacted after spreading to establish contact
between the individual stones, thereby transfer-
ring the load of pedestrian and vehicles to the
subgrade. When the soil and stone mix is com-
pacted, the stones will be rearranged as the soil
fills the voids (Harris 1971). To obtain the proper
degree of compaction, the stone-soil mix should
be installed in layers of no more than 15 to 20 cm
(6 to 8 in.).
The correct mixture of stones and soil (i.e.,
the point at which the soil is not compacted dur-
ing the installation) can be determined by mea-
suringthe stone weight per cubic unit or by filling
a known volume of stones with water and then
estimating the volume of the voids. The volume
of soil added should be less than this volume. If
the applied stone material has 20% voids after
compacting, the correct mixing ratio of stone to
soil will be 5:1. An incorrect mixing ratio of 4:1
will lead to a filling level of 125%, which means
that the soil will be compacted in the voids and
the stones will not be touching each other. At a
mixing ratio of 6:1, only 80% of the voids would
be filled but, considering root growth, this seems
to be sufficient and better than overfilling the voids
and compacting the soil (Kristoffersen 1998).
Water mixing during fnstallatlon. Layers of
stones are installed and compacted. Then a layer
of screened soil (e.g., sandy loam) is spread on
top of the stones and watered into the voids. The
stone layer can be up to 25 cm (10 in.) thick when
stones of 80 to 150 mm (3 to 6 in.) in diameter
are used. When using smaller stones and bro-
ken stones, the recommended thickness is 10 to
15 cm (4 to 6 in.).
Dry mixing during installation. Another
method of mixing during installation is installing
stones in layers of 15 to 25 cm (li to 10 in.) and
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Journal of Arboriculture 24(3): May 1998
123
Technical
Construction of
Planting Site
When topsoil and other
materials containing hu-
mus are installed under
pavements, there is a
risk that the obstruction
of aeration could cause
anaerobic conditions.
This risk is increased by
insufficiencies in drain-
age (e.g., if the soil is
subsequently filling the voids with dry soil by
sweeping and vibration. Both the stones and the
soil must be completely dry, and installation must
be carried out in dry weather. The dry-mixing
method has been used only with stones of 80 to
150 mm (3 to 6 in.) in diameter and is not suitable
for use with smaller stones. This method can cause
difficulties because most construction work iscar-
riedout when rainy weather can occur.
Sand mix. The Dutch experience has shown
that uniform graded sand can be used to expand
the rooting volume beneath pavements. The sand
is mixed with up to 4% or 5% of humus in the
form of compost or sphagnum. In Denmark, the
sand mix has been used and installed as de-
scribed by Couenberg (1994) (Figure 1). The in-
tention of using sand mix is to achieve a
compaction level of 70% to 80% proctor density
(Couenberg 1994). Therefore, penetration resis-
tance is measured to ensure that the degree of
compaction does not exceed the critical limit for
root growth. This quality check is not practiced in
Denmark.
compacted) (Harris
1992). Sufficient air
movement in the soil or
sand mix is achieved by
installing a layer of
stones or broken stones
between the root growth
area and the pavement
above. The aeration
layer receives air via
pipes or the planting pit (Urban 1989) and should
be covered with a suitable geotextile to prevent
filling from the layers above (Figure 2). The same
type of aeration layer is recommended when rais-
ing the grade around existing trees (Harris 1992).
Although the effect of an aeration layer has not
been documented in the Danish examples, it is
considered to have a beneficial effect on the soli
and the root growth (Smith 1995). The aeration
layer may also be used to irrigate the trees (Ur-
ban 1989).
Applications in Denmark
More than 800 trees have been successfully
planted since 1991 on more than 30 construc-
tionsites in Denmark using these alternative ma-
terials and methods.
Table 2 shows that stones or broken stones,
in various size classes, are the most commonly
used materials for load-bearing matrixes and are
used with more than 75% of planted trees.
Premixing before installation is the most fre-
quentlyused installation method. Dry mixing dur-
Figure 1. Use of sand mix at The Christians Havns Square, Copenhagen. The
material is used Ina 3-m wide, 57-m long pit, in which 7 1lnden trees (Tilia
tomentosa) were planted. The soil in the planting pits is separated from the
sand mix by a wire basket during the Installation process.
124
Kristofferson: Integrating Trees and Pavement
I
Tap layer Separator
aaatl ng layer
Watering and airing layer
j g ` ; .
Planting site
u
Root-penetrable base material
c~ss"[~
eubgratle
Figure 2. Pavement construction with load-bearing
aeration layer.
ing installation is only occasionally used. Each
installation method has advantages and disad-
vantages (Table 3).
The major disadvantages of using the premix
of stone and soil is that separation might occur
during transportation and installation. A tackifier
can help the soil adhere to the stones (Grabosky
and Bassuk 1995). The calculation of mixing ra-
tio is a4so problematic. In a stone and soil mix, a
calculated void percentage (e.g., 25%) has often
led to a recommended mixing ratio of 3:1. In one
case, this ratio led to compaction of the soil in
the voids, which caused subsequent ettling. The
correct mixing ratio should have been 4:1 to se-
cure 25%voids in a stone and soil mix. Dry mix-
ing has been the most effective and practical
method. However, the dry mixing method requires
dry weather, dry stones, and dry soil.
Measurements of soil densities
are included in the original Dutch
method, in which sand mix was used
(Couenberg 1994). Although the de-
gree of compaction is intended to be
optimized for plant growth, there is a
risk of subsequent soil compaction of
soil and settling of the pavement.
Conclusion
During the last 5 years, soil mixes,
sand mixes, and load-bearing stone
have been introduced for use in
Denmark. Most landscape projects
involving trees in paved areas use
matrix and these new methods. No difficulties
with load-bearing capacities, irregu-
larities, or frost-heaving of pave-
mentshave been recorded. Compared to normal
Danish conditions, the winter of 1995-1996 was
long and cold, with continuous frost from the start
of November to the middle of March. A top layer
of at least 0.5 m (20 in.) was frozen, which influ-
enced all layers of load-bearing mixes installed
below pavements. None of the methods applied
appear to have higher susceptibility to frost heave
than does traditional pavement construction.
To reduce the visual effects of possible irregu-
larities caused by heaves and settlements, land-
scapearchitects often changed the pavement on
the load-bearing matrix materials so that it dif-
fered from the rest of the pavement. As a result,
the possible height differences are less conspicu-
ous than would be the case with uniform pave-
ments. Inaddition, the change in pavement shows
where the tree has its roots.
Table 2. Examples of load-bearing matrixes used in Denmark
Stones Lava Bricks Leca Sand Totals
Premixing 8locations 2locations 6locaiions Vocation ~ 6locations 25 locations
256 trees 3 trees 46 trees 40 trees 714 trees 459 trees
Water mixing during 5locations 2locations 1 location - - 8 locations
Installation 181 trees 12 trees 3 trees - - 196 trees
Dry mixing during 4 locations 1 location - - - 5locations
installation 141 Vees 22 trees - - - 163 trees
Totals 171ocations 5locations 7locations Vocation S locations 38 locations
578 trees 37 trees 49 trees 40 trees 114 trees 818 trees
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Journal of Arboriculture 24(3): May 1998
Table 3. Comparison of advantages and disadvantages of installation methods
125
Mixing method ~ Advantages Disadvantages
Premixing Can be done by machine Separation during transportation and Installation
Risk of soil compaction in voids
Can require special mixing equipment
Water mixing during Contact between stones is ensured Can require large volumes of water
installation Independent of weather Most suitable at large aggregate sizes
Prevents soil compaction
Dry mixing during Contact between stones is ensured Dry weather required
installation Prevents soil compaction Requires dry soil
Sand mix Rational mixing with right equipment Risk of soil compaction to a degree that impedes root growth
Inexpensive ingredients ~ The compaction level must be controlled
Easy to install Poor load-bearing capacity
All examples presented in this article were
established between 1991 and 1996. None of
these trees have been measured for growth rates.
Fast and accurate results were obtained by con-
ductinggrowth experiments with 8 materials and
3 tree species beginning in spring 1994. These
results illustrate the growth capacities of the dif-
ferent materials (Kristoffersen 1998).
Acknowledgements. The author wishes to thank
the municipalities involved in the planning and the
construction of the cases studied. Thanks are also due
to Dr. Thomas B. Randrup for valuable discussions in
connection with this paper.
Literature Cited
Bonsen, K.J.M. 1996. Architecture, growth dynamics
and autoeco/ogy of the sycamore (Ater
i pseudopatanus L.). Arboric. J. 3:339-354.
Couenherg, E.A. M.1994. Amsterdam tree soil, pp 24-
33. In Watson, G.W., and D. Neely (Eds.). The
Landscape Below Ground: Proceedings of an
International Workshop on Tree Rcot Developmen!
in Urban Soils. International Society of
Arboriculture, Champaign, IL.
Grabosky, J., and N. Bassuk. 1995. Anewurban tree
soil to safely increase rooting volumes under
sidewalks. J. Arboric. 21(4):187-201.
Harris, Richard W. 1992 (2nd ed.). Arboriculture:
Integrated Management of Landscape Trees, Shrubs,
and Vines. Prentice Hall, Englewood Cliffs, NJ.
Harris, W.L. 1971. The soil compaction process, pp
9-44. In Barnes, K.K. (Ed.). Compaction of
Agricultural Soils. The American Society of
Agricultural Engineers, St. Joseph, MI.
Kopinga, J. 1985. Site preparation practice in the
Netherlands. Metria 5:72-84.
Kopinga, J. 1991. T(~e effect of restricted volumes of
soil on the growth and development of street trees.
J. Arboric. 17(3):57-63.
Kopinga, J. 1992. Some Aspects of the Damage to
Asphalt Road Pavings Caused by Tree Root,
Including Some Preventive Control Methods.
Proceedings from the 10th OsnabrGcker
Baumpflegetage, pp.10.1-10.23.
Kristoffersen, P. 1998. Growing trees in road base
materials. Arboric. J. In print.
Eichler, J.M., and P.A. Lindsey. 1994. Soil Compaction
and Site Construction: Assessment and Case
Studies, pp 126-130. In Watson, G.W., and D.
Neely (Eds.). The Landscape Below Ground:
Proceedings of an International Workshdp on Tree
Root Development in Urban Soils. International
Society of Arboriculture, Champaign, IL.
Lindsey, P., and N. Bassuk. 1992. Redesigning the
urban forest from the ground 6e/ow: Anew
approach to specifying adequate soil volumes for
street trees. J. Arboric. 16:25-39.
Patterson, J.C. 1977. Soil compaction-effects on
urban vegetation. J. Arboric. 3(9):161-167,
Randrup, T.B. 1997. Soil compaction on construction
sites. J. Arboric. 23(5):207-210.
Smith, K. 1995. Soil Aeration Systems: Do They Work?,
pp 17-21. In Watson G.W., and D. Neely (Eds.).
Trees and Building Sites: Proceedings of an
International Workshop on Trees and Buildings.
International Society of Arboriculture,Chompaign, IL.
Teilmann, S., and P. Kristoffersen. 1996.
Anla=.gsmetoder for bytraeer i 11 kommuner.
Videnblad nr. 4.6-19. Forskningscentretfnr Skov &
Landskab.ln Danish.
Brewer, Scott I
From: Nina Bassuk [nlb2@cornell.edu]
Sent: Friday, October 27, 2000 9:49 AM
To: Brewer, Scott I
Subject: Re: Constructed soils for urban tree plantings
I=! L~l
CUSOILSPECAPRIL99 STRUCiURAL_SOIL. Structural_SOils=Web_ Vitleo_info.doc detail=lree.jpg ATTW66ttxt
.dot tloc article_.. SCOtt,
Thanks for your interest in structural soils. I'll be happy to answer questions as they may come up.
I'm going to send you the regular electronic'packet I send to all who ask for information. Cost,
however ranges from $30-50/ cubic yard delivered. A rough estimate would allow for about 20-25 cu
yds per tree.
Feel free to get back in touch if you have further questions.
Nina
Here is the information that you requested.
There are 5 documents attached. These include a specification
(CUSOILSPECAPRIL99),a brief article(Structural Soil.doc) , a longer article recently
published by ASLA on their web site (Structural Soils/ASLA Web site) ,
information on ordering our video (video info) and a graphic detail of a tree in 'CU
-Structural Soil'(detail/tree)
There is also a slide presentation on my Web site that you are welcome to access:
http://www.cals.cornell.edu/dept/flori/uhi/ssoils/index.htm
The structural soil has recently been patented and trademarked under the name
'CU-Structural Soil '.
The purpose of this is to insure quality control.
To find out more about where there are licensed distributors in your area or to
become asub- licensee, please contact
Mr. Fernando Erazo, at Amereq Inc., 19 Squadron Blvd., New City, New York
10956.
1(800) 832-8788 or Fax (845) 634-8143 or E-mail FE@amereq.com
Feel free to contact me again if you have further questions. Nina Bassuk (607)
255-4586,
e-mail nlb2Cc~cornell.edu
http://www.hort.cornell.edu/uhi/
STRUCTURAL SOIL: AN INNOVATIVE MEDIUM UNDER PAVEMENT
THAT IMPROVES STREET TREE VIGOR
Nina Bassuk, Dnrector and Professor Urban Horticulture Institute, Cornell University, Ithaca, NY Jason Grabosky,
Urban Horticulture Institute, Corneil University, Ithaca, NY
Peter Trowbridge, FASLA, Professor Landscape Architecture, Comeil University, Ithaca, NY James Urban,
FASLA, lames Urban and Associates, Annapolis, MD
INTRODUCTION
The major impediment to establishing trees in paved urban azeas is the lack of an adequate volume of soil fox 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 yeazs with better soil conditions. Those trees that
do survive within such pavement designs often interfere with pavement integrity. OIder established trees may
cause pavement failure when roots grow duectly 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 repanrs. 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 aze needed. One new tool for urban tree establishment is the redesign of the entire
pavement profile to meet the load-bearing requirement for stmcturally 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.
CONVENTIONAL TREE PTTS 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 lazgely impermeable to root growth and water infiltration and significantly
reduces drainage if large percentages of sand aze 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 granulaz 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 sil of the challenges below the pavement for trees,
it is no mystery why trees aze often doomed to failure before they are even planted.
The subgrade and granulaz 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.
ANEW 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. Corneli's Urban Horticulture Institute, has been testing a series of materials over the past five yeazs focused
on chazacterizing their engineering as well as horticultural properties. The materials tested aze gap-graded gravels
which are made up of croshed stone, clay loam, and a hydrogel stabilizing agent. The materials can be compacted to
meet ail 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 I). Stmctural 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 yeaz 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 aze
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 stmctural soil. A geotextile
could segregate the base course of the pavement from the stmctural soil. The gap-graded, stmctural soil material has
been shown to allow root penetration when compacted. This material would be compacted to not less than 96%
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 pazallel the finished grade. Under-drainage confornvng to approved engineering
standazds for a given region must be provided beneath the structural soil material.
The stmctural soil material is designed as follows. The three components of the stmctural soil are mixed is the
following proportions by weight, croshed stone: 100; clay loam: 20; hydrogei: 0.03. Total moisture at mixing
should be 10% (AASHTO T-99 optimum moisture).
Cmshed stone (granite or limestone) should be narrowly graded from 3/4 -1 1/2 inch, highly angulaz with no £mes.
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 2i 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 fumed and mixed until a
uniform blend is produced. The stmctural soil is then installed and compacted in 6 inch lifts.
In a street tree installation of such a stmctural 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, mrming continuous and pazallel 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 moishue rechazge 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 stmcttual 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 rechazge, as could traditional irrigation.
When compazed to existing practice, additional drainage systems, and the redesigned stmctural soil layer
represent additional costs to a project. The addition of the proposed structural soil necessitates deeper excavation
of the site which also maybe 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 refming the specification for producing a structural soil
material to make the system cost effective. It is patent pending and will be sold with the trademazk `CU-Soil' to
insure quality control. Testing over five yeazs has demonstrated that stabilized, gap-graded stmctural soil
materials can meet this need while allowing rapid root penetration. Several working installa5ons have been
completed in Ithaca, 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
appeazs 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, 7. 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.
ATT03661.txt
Nina Bassuk
Urban Horticulture Institute
Dept.of Horticulture
20 Plant Science
Cornell University
Ithaca, NY 14653
(607)255-4586
(607)255-9998 fax
http://www.hort.cornell.edu/uhi/
Page 1
Journal of Arboriculture 21(4): July 1995
A NEW URBAN TREE SOIL TO SAFELY INCREASE
ROOTING VOLUMES UNDER SIDEWALKS
by Jason Grabosky and Nina Bassuk
Abstract. Soil compaction, which is necessary tc safely
support sidewalks and pavement, conflicts with urban trees'
need for usable rooting space to support healthy tree growth.
We have defined a rigid soil medium that will safely bear loads
required by engineering standards yet still allow for rapid root
exploration and growth. This was accomplished by forming a
stone matrix and suspending soil within the matdx pores with
the assistance of a hydrogel gluing agent. Initial studies using
three stone types and various stone to soil ratios showed that
the compacted stone-soil test medium (dry densfties > 1700
kg/ms) increased root growth by a minimum of 320% over the
compacted clay loam control (dry density of 1378 kg/ms). The
proposed system can safely hear load demonstrated by
California Bearing Ratios consistently exceeding 40. Discus-
sion of a critical mining ratio is presented as an approach for
developing a specification for field installation.
Because lack of rooting space is arguably the
most limiting factor affecting a street tree's water
and nutrient demands over time, urban trees need
to have access to larger volumes of soil if they are
to achieve the size, function, and benefits for
whichwe plantthem [13,17]. Urban soilcompaction
generally occurs in what would be the tree's
preferential rooting zone: the shallow lens of soil
no more than three feet deep extending well
beyond the tree's canopy [18]. Compaction con-
tributestoinsufficient rooting volumes by increasing
the soil's bulk density and soil strength to levels
which impede root growth [3,8,10,25].
While several reasons for densification and
compaction of urban soils exist, the most ubiqui-
tous problem we face is the purposeful compac-
tion of the soil surrounding a street tree to support
pavement or nearby structures. Compaction is
necessary as acost-effective wayto increase the
strength and stability of existing soil materials to
preventtheir settlement under oraround designed
structures [7,11,14,23]. It increases the bearing
capacity of the materials below the pavement
system and reduces the shrinking and swelling of
187
soils that occur with water movement or frost
action (11,26]. Thus any effort to increase the
rooting area for street trees under pavement must
accept the necessity of compaction and under-
stand the levels of compaction needed to safely
design pavement structures.
Proctor density. A standard measure of
compactive effortwhich is often specified is termed
Proctor density... It is important to understand
exactly what this term means and how it is iden-
tified. Originally developed for evaluating and
controlling compaction of fine textured soils when
building earthen dams, the Proctor testing pro-
cedure describes the relationship between soil
moisture, a standard compactive effort, and soil
porosity (void space)[19,20,21,22]. As the soil
moisture increases, a standard compactive effort
yields progressively greater bulk densities (and
fewer voids) up to an optimum. After the optimum
is reached, densities decrease with increasing
soil moisture because the soil is held apart by the
incompressible excess water in the test sample
[19,21 ]. ProctorOptimum Density is the high point
on a curve plotting the dry density of the soil
against increasing moisture content as a result of
a standard compactive effort arbitrarily set to
simulate a compaction effort used in the field
(Figure 1)[2,12j. The standard effort consists of
the near equivalent of 25 blows from a 5.5 Ib.
hammer falling 1 foot onto each of 3 equal layers
of material, These layers fill a 4 inch diameter,
4.584 inch depth mold in the usual test [2]. By
adjusting the moisture content to match that as•
sociated with the observed peak of the moisture
density curve, a contractor can get the most
efficient compaction per unit effort. Compaction at
other levels of moisture content would require
more labor to reach the density achieved by
____ __. ---~--V-- .
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188
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Figure 1. Curve showing the moisture density re-
lationshipfound for a clay loam soil as the result of
a standard Proctor compaction effort. The peak of
this curve would be defined as 100% standard
Proctor density. The effects of increased density on
the porosity of the soil is also shown via the void
ratio. Porosity =void ratio / (1 + void ratio).
compacting at the optimum moisture content and
could affect the compacted strength of the mate-
rial despite an acceptable dry density [11]. A
lesser density will usually have an associated
lower strength and bearing capacity [i i].
The density generated by the previously de-
scribed laboratory test at optimum moisture con-
tent is defined as 100%standard Proctor Density
and serves as a benchmark setto maintain quality
control over the compaction process during con-
struction. In practice, construction specifications
require a percentage of Proctor Density and the
term `Proctor Density" could be derived from any
of the compactive efforts described by the ASTM
moisture density relationship specifications [2].
Since the test is standardized, it is often used to
generate a dry density from which to test a
material's shear strength, bearing capacity, and/
or deflection resistance. This information can be
used to evaluate and define a material for safe
engineering design practices, in a sidewalk or
parking situation, a failure could translate into
Grabosky & Bassuk: Soil Under Sidewalks
large financial liabilities such as vehicle damage,
personal injury, increased maintenance, or pre-
mature replacement costs.
California Bearing Ratio. The Proctor mois-
ture/density relationship is also used to identify a
standardized testing point for evaluating a
material's load bearing capacity via the California
Bearing Ratio (CBR) [1,26]. This ratio compares
materials used under, pavements to a standard
material which has been empirically determined
to be a satisfactory pavement base [2,11]. This
value isdependent on frictional strength, therefore
moisture content and bulk density are major fac-
tors inthis testing procedure. A CBR value of 100
would be interpreted to mean. that the tested
material had the same bearing capacity as the
reference standard (100%).
With the CBR value, the necessary pavement
thickness can be determined by evaluating com-
ponents of the soil profile materials for shear
strength under pavement [1,5,27]. A typical soil
profile under pavement would include the
subgrade, i.e. a native or otherwise preexisting
soil typically with a large amount of fine particles.
'The subbase and/or base courses are usually
well-graded gravels, and the wearing surface is
whatwe often think of as pavement (Figure 2) [11].
Acceptable CBR values are assigned for each
layer used in. pavement systems with minimum
acceptable bearing capacfties increasing for each
consecutive layer from the bottom toward the top
surface grade. The subgrade, which is the deep-
est level, often has a comparatively low CBR in
the range of 5 to 10 [1,11]. Base materials are
normally much stronger than the subgrade with
acceptable CBR values ranging from 40 to 80 [11].
These values could be considered acceptable for
matenalsusedunde a\vedsurfacesinlighttraffic
situations which would inc\ de maintained mu-
nicipal sidewalks.
The CBR test places a compacted cylinder of
material onto a loading press that forces a piston
into the soil to a depth of 0.5 inches at a uniform
rate. The strength of the material is found by
measuring the load required to continue the
penetration. A curve plotting load against pen-
etration isshown inFigure 3.This curve is corrected
far surface irregularities as shown by the seg-
0 5 10 15 20 25 30 35
Moisture Content Yo
Journal of Arboriculture 21(4): July 1995
;e>'.
c'. z>
~y ,~..
:: .. ....
Wearing Surface: Often concrete or asphalt, depth
is dependent on the material used.
Base: A very stable layer which is asand-gravel
material or a material stabilized with a binding agent.
This layer can be from 5 - 30 cm in depth.
Subbase: An optional layer of stable material, often
15 - 30 cm of a sandy or gravelly material.
Subgrade: The preexistent soil at the site. The top
layer is compacted before any of the base or pavement
layers are installed.
Figure 2. Definition and locations of the layers
which can make up a sidewalk. Adapted from Holtz
and Kovaks (11).
mented line. The point where the segmented line
intersects the X axis is defined as the corrected
starting point and the segmented line is used to
describe the load/penetration relationship [2]. The
load at 100 mills (0.01 inch) on the corrected
curve is divided by the load needed for the same
penetration into the standard reference material
(6.89 MPa) [2, 26]. The resultantvalue expressed
as a percentage is the CBR value.
A marginally acceptable or unacceptable base
could have an acceptable CBR at field capacity
and lower moisture levels normally found outside
of the laboratory, but could fail in a saturated
condition, which often occurs in the spring. Forthis
reason, CBR tests are often subjected to a 96 hour
saturation period to accommodate the worst case
scenario.
Soil classification systems. Horticulturists and
soil scientists often use the USDA soil classifica-
tion system for characterizing agricultural soil
189
Cali fornia Bearing Ratio te st 4 I
lJmeslone 5.026:1 2066kg/m 3 i
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M14 mm pu MP. ptl MPa pel NPe C0R CBR '.
xs oe dm ].ee x6z 1.a
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1x5 az 991 6es uer e.w
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]W i.5 t6ay 11 t9 1x1] 11.& t9W 13.1 6]YJ w.35
<CO 10 ZOW tl<3 X93 1aA3 2YV 15.69 Bl.w 91.W
Sw 125 2ax0 16.69 ZaYp 16.69 x&V 1]0] 9].06 9106
Figure 3. A typical CBR test from the tested lime-
stonestone/soil mlxreported in graphlcand numeric
form. The CBR value is calculated by dividing the
tested load by the standard load and multiplying the
result by 100.
systems and describe their behavior in terms of
their porosity, nutrient holding capacity, and
.drainage [30]. The geotechnical engineering
community uses the Unified Classification System
to characterize materials. This system could be
called a classification by behavior during engi-
neering uses with divisions in classification coin-
ciding with shifts in engineering characteristics
[7,14,28]. We felt that the soil mixes we were
developing should be defined using the Unified
Classification, System to beater communicate our
most promising soil mixes to the engineering
community.
Geotechnical engineers use the Unified Clas-
sification System to help predict soil properties
such as frost-heave susceptibility, drainage and
water infiltration, expected compacted densities,
bearing strength, and pavement base efficacy
[5,28,29]. Figure 4 shows how a material's clas-
190
Grabosky & Bassuk: Soil Under Sidewalks
yuarevmtxa~ srnma. '' >+~re ~3or~ °~*+103~aue
~ p~yy ~~ -
.. ~ rorgosreenat rorn~rec~ . crt°ox '~ mrt
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smpesiwc ' ______.
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gAlID
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rid-al(onamn
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~~~ ______
e4~tb
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PM m.P~~Y
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_ _ _ _ _ _
10
g .171
5 C ~~Ys~4smfelry~smec P~ n(midbk m P °*b.t~+OS' 3
mpy33~ -fi
Figure 4. Material behavior as related to the Unified Classification system. Adapted from Holtz
and Kovaks (11) and US Waterway Station (28).
sification can roughly predict performance as a
pavement base. The last column notes typical
CBR ranges for listed materials. Note that for a
marginally acceptable base (CBR = 40), a formi-
dable load of 2.76 MPa (400 psi) is required to
penetrate 2.5 mm (0.1 inches). Since the CBR can
be affected by moisture in fine grained soils, a
CBR of 40 would normally be marginally accept-
able in a saturated condition, and penetration
resistance would increase as the soil dried. Luckily,
root tips' are much smaller than a CBR testing
piston and mayfind small zones of less resistance,
but the example serves to highlight the contra-
dictory demands of root expansion and base
compaction for sidewalks.
Implications for roots. By acknowledging the
need for compaction from a structural viewpoint,
we can understand why roots often have trouble
penetrating the bases and subgrades of many
sidewalks. For sidewalks, a minimal removal of
existing material is often the case, so the subgrade
very often will lie within 8 inches of the final grade.
This is the first zone that experiences a compactive
effort during construction. Base materials are
placed onto the subgrade and compacted as well
[11]. Boththesubgradeandthebasearenormally
compacted to at least 95% of an optimum density,
which often is restrictive to root growth [11,15,18],
It is thus no surprise that when roots "escape"
oroutgrowtheirplantinghnles they usually choose
zones of lesser compaction due to sub-surface
structures such as along utility lines, or the base
course immediatelybeneaththe actual pavement
where the open granular nature of the layer might
contain enough voids to allow root growth
[6,15,17,18]. It is also of little surprise to observe
sidewalk damage from those roots which expand
radiallyastheygrowdirectlybeneaththepavement
since this interface can provide greater opportu-
nityforroot penetration and growth in comparison
with the compacted layers below.
Streettrees prefera less dense rooting medium
Journal of Arboriculture 21(4): July 1995
that allows roots to penetrate to a depth of two to
three feet, but this.is currently unacceptable under
sidewalksfrom astructural safetyviewpoint.Those
trees that do not "break ouY' are sentenced to a
limited future dictated by the limited amount of
designed rooting volume within the planting pit or
island. This volume is not likely to support the tree
for the designer's and the public's expected life
span as borne out by the high tree mortality rate
found in planting areas surrounded by pavement;
often dying in as little as 7 years [13,16].
A New System
To solve this problem, our objective was to
develop an easily produced soil medium that
would meet engineers' specifications for load
bearing capacity, but still allow for vigorous root
growth through the compacted profile, thus in-
creasing overall rooting volume without compro-
mising safety. This could be thought of as an
evolution of the compaction-resistant planting
medium employed by Patterson in Washington,
D.C. but with greater load bearing requirements
[16j. Our system would build agap-graded, load
bearing stone matrix that could meet the engi-
neering requirements while suspending a
noncompacted rooting medium within the voids
that exist between the stones.
Materials in which the full range of particle size
classes are lacking except for one or two widely
varying size classes are often termed "gap-graded:'
Figure 5 shows examples of the gap-graded ma-
terials we have tested. These materials exhibit
good drainage capabilities due to the inability of
the particles to tightly nest into a uniform soil
profile. It was our intent to use this fact to our
advantage in manufacturing such a medium.
In our medium, gravel and soils were mixed so
that loads would be transferred from stone to
stone in the gravel while leaving the soil between
the stones essentially unaffected by compaction.
Theoretically, roots would be encouraged to grow
deeper into the undompacted soil between the
stones which allow for greater water and air
movement. This medium might also reduce
sidewalk failure, another goal of this system, by
encouraging deeper root systems.
A series of studies were initiated to identify a
191
Limestone
~ , ,~
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i !~~ ;
~
i
II I~
1
a l
.,
,,
. ~:
,.
gOfi
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~ ~I~I
I' I I ~I
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I •' '5
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. I 1;,6!
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I .... I.: L.
~
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e
m 100
s 0m1 0.a1 01
+
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High Friction
I
0e
a L•I;
06
~0e _`_1 .1 ~i,_
~e~
0 1
,,,
%
0m1 0.01 e.1 + 10 1aa
p.n~a.t:a alaa,.ur 0nm1
Solite
s1
I
~0e
~ L_ ~ ~Ll
e e -- ?.
~02 III `_o I -~
€e -
+ 0.001 0.a1 e.t 1 1fi 100
Pww m. mamn.rlnunl
scone m
Soli po5o Unified Clessi6m5oq
3.66:1 GMGC
5.26:1 GP GMGC
Sbna to
soil fiaUO wm,d cla>smvwn
5.4]:1 GP GMGC
].B1:1 GP
sl0na w
Sell patio Uniesd Clessllrauon
1.48:1 cMGc
209:1 GMGC
Figure 5. Linden test media particle size distribu-
tions. All curves represent the extremes of the
tested stone to soil ratios. In each graph, the higher
curve represents the highesttested ratio, the lower
curve represents the lowest tested ratio. Note the
gap graded nature of the mixes with material between
25 mm and 12 mm all but lacking entirely.
promising stone to soil ratio that would meet our
objectives. To achieve this end, two important
principles had to be recognized. First, to prevent
soil compaction and facilitate the necessary air-
filled porosity, the volume of soil in the stone and
soil mix must be less than the total porosity of the
compacted stone matrix. At.this point, the bearing
capacity of the system would largely become a
function of the strength of the stone alone. The
determination of this point was a critical step in the
definition of this medium. Second, the soil could
not be allowed to sieve to the bottom of the stone
matrix during the mixing or compaction phases of
its installation. A small amount of a hydrated
192
hydrogel was added to the stone matrix before
blending in the soil to prevent the stone and soil
from separating. This hydrogel acted as a glue,
attaching the soil to the stone much as a tackifier
works in hydroseeding applications.
Grabosky & Bassuk: Soil Under Sidewalks
containing the stone sample. The mean of five
measurements determined the noncompacted
matrix porosity of 44.7% for the crushed lime-
stone, 40.0% for the gravel, and 47.8% for the
Solite®.
For each stone type, four mixes were gener-
Materials and Methods sled byadding enough clay loam to fill 100, 90, 80,
Linden study. Thethreetypesofstonechosen and 70%ofthemeasurednoncompactedporos-
fortheinitialtestsaredescribedinTablei.Crushed ity. The resultant blends are listed using a dry
limestone was chosen for its angularity and con- weight ratio as shown in Table 2. The dry weight
sistency as a manufactured material. A high fric- ratios varied due to the different specific gravities
tional quarried gravel was chosen for its pre- of each stone type. The Unified Soil Classification
dominantlyroundshape.Athirdstonetype, Solite System was used to define each blend and to
(a heat expanded slate), was chosen for its rigid predict their performance in an engineering con-
riature, light weight, and porosity. The crushed text.
and quarried stones conformed to a 0.5 -1.0 inch Each of these blends was also blended with a
gravel size range which was purchased as a #2 poly-acrylimide hydrogel tackifier (Gelscape®
size stone [2]. A clay loam was chosen for the Amereq Corporation) to prevent aggregate
interstitialsoilcomponentofthemixbecauseofits separation during the mixing and compaction of
water and nutrient holding capacity, a critical such gap-graded mixes (Figure 4). The tackifier
factor in a mostlystone root environment. Twelve was used at a rate of 38 grams per 13650 cros of
biendswereusedinthisfirsttestrepresentingfour uncompacted mix (approximately 152 grams
increasing stone: soil volumetric ratios for each of hydrogel per 100 kg stone on a dry weight basis).
the three stone types. Compacted clay loam with and without hydrogel
To determine stone to soil ratios, the percent- served as the controls for a total of 26 treatments
age of voids within a matrix of each stone type with six replications.
were measured. Five random samples from each Each mixture was blended in a small rotary
stone type-were placed into containers of knowri concrete mixer in two batches and then com-
volume and brought to a saturated, surface dry, bined.Foreachblend,six14.2Lnurserycontainers
condition. From this point, a loose pack porosity (#5 short) were filled for a single lift compaction.
was determinedforeachstonetypebymeasuring Excessmaterialwasstoredtofillsettlementsafter
the amount of water needed to fill the container the initial compactive effort. The containers were
Table 1. Materials used in developing test~blends. The Solite® was blended to approximate the
same particle size distribution as the other two stone types.
Material Specific % passing % passing % passing Coefficient Description
used gravity 38.t mm 25.4 mm 12.7 mm of
(Gs) sieve sieve sieve uniformity
(1.5") (1.0") (0.5") (Cu)
#2 Crushed 2.71 100 94.1 6.7 1.4 All angular stone
limestone
#2 High friction 2.66 100 ~ 98.8 3.2 1.34 Round quarried gravel,oversized crushed
aggregate and blended back, % limestone = 16%
Solite® 1.50 > 90 83 < 5 2 Exploded slate, very porous
Soil 2.58 - 26.4 % sand - - Shredded clay loam, pH = 5.25
40% silt -dry bulk density = 1110 kg/m3
33.6% clay -Plastic limit = 20.5, liquid limit = 27.5
(USDA) -Std. Proctor opt. density = 1674 kg/m3
Journal of Arboriculture 21(4): July 1995
193
Table 2. Description of linden study media. Densities. were measured at the end of the study.
Overall standard error ofdensity bytreatment =24.69 kg/m3 excepting where single replicates had
died (X); in which case the standard error = 27,04 kg/ms.
Stone type Stone to Calculated Calculated Observed dry density in kg/m3 % Actual porosity
soil dry Proctor porosity (%) (% Proctor optimum) without with
weight optimum at Proctor without with hydrogel hydrogel
ratio density optimum hydrogel hydrogel
(kg/m3) density
Limestone 368:1 2000 25.5 1789 (89%) 1594 (80%) 33.4 40.6
4.09:1 1987 26.0 1767 (89%) 1571 X (79%) 34.2 41.5
4.60:1 1978 26.4 1748 (88°!0) 1602 (81 %) 35.0 40.4
5.26:1 1965 27.0 1638 X (83%) 1594 X (81%) 39.1 40.8
High friction 5.47:1 2068 21.9 1623 (88%) 1681 (81 %) 31.2 36.5
6.03:1 2038 23.1 1812 (89%) 1692 (83%) 31.6 36.2
6.84:1 2004 24.4 1852 (92%) 1716 (86%) 30.1 35.3
7.81:1 1972 25.6 1784 (90%) 1723 (87%) 32.7 35.0
Solite® 1.46:1 1500 22.8 1269 (85%) 924 (62%) 34.7 52.4
1.70:1 1414 25.7 1216 (86%) 1132 (80%) 36.1 40.5
1.78:1 1391 26.5 1226 (88%) 1154 (83%) 35.2 39.0
2.09:1 1317 28.9 1153 X (88%) 1122 (85%) 37.8 39,4
Clay loam (Ck) - 1674 35.4 1378 X (82%) 1248 (75%) 46.8 51.8
fitted with a 6 X 20 cm PVC tube wrapped in
cheese cloth which served as a 565 cm3 place
holder for the planting hole. The cloth prevented
materials from falling into the tube. The tube was
removed afterthe compaction process. This pre-
ventedundue disturbanceofthe compacted profile
while allowing for a planting hole. The tubes were
placed slightly below the plane of the top of the
container to prevent vibratory effects during
compaction. The containers with the stone-soil
blends were then compacted.
To compact the test blends, all containers were
blocked pot to pot and covered with a geotextile.
The geotextile was covered with a 1.5 inch layer of
#2 stone and then compacted with a vibratory
plate tamper (Wacker VPG 160K). The fabric and
stone deformed into the containers as the media
settled, maintaining media/tamper contact for
uniform compaction. Compaction consisted of
four passes with the plate tamper; care was taken
to pass the center of the tamper over ali edges of
the block for uniformity of compactive effort. The
coverings were removed, and the initial excess
test blend was replaced into the containers where
settlements had occurred. The pots were again
covered and four more passes with the tamper
were performed. Controls were compacted in four
lifts with an impact hammer method instead of a
vibratory plate tamper due to the fine nature of the
clay loam. During the plant harvest, the final
densities and porosities were calculated (Table
2).
On June 9,1993, dormant Ti/ia cordata seed-
lings with swollen buds were standardized to a
single stem of 50 cm. The root systems were
standardized to a single root of 15 cm with all
laterals and the root tip removed. Planting tubes
were slid out of the compacted containers, and
lindens were installed with the same shredded,
noncompacted clay loam as was used to fill the
interstitial voids in the stone-soil mixes. Plants
were watered in after planting and placed into a
completely randomized experimental block. They
were grown on an outdoor gravel pad in Ithaca,
NY, kept weed free, and watered as needed until
the end of August, 1993.
Plants were forced into an early dormancy after
the trees had set terminal bud by placing them into
a 6°C cooler on August 31, 1993. After approxi-
mately three months of chilling, the plants were
194
placed into a greenhouse in a completely random-
ized experimental design on December 8, 1993.
The plants received 16-hour day lengths using
supplemental incandescent lighting. The green-
house temperatures were maintained at 21°C/
15.5°C day/night and plants were watered as
needed.
The trees were harvested beginning on March
28,1994, oncethey had again setterminal bud. At
harvest, the final volume of each test container
was calculated by taking the average of tour
measurements from the top of the container to the
soil surface (one from each quadrant of the con-
tainer), and subtracting the empty volume from
the total pot volume. The final weight and moisture
content was measured and the final dry density
calculated.
'The root harvest consisted of a total root ex-
cavation and collection. The initial standardized
root was removed, and the remaining roots were
washed free of soil. The volume of new root
growth was measured using water displacement
in a graduated cylinder. The roots were viewed as
cylinders with a diameter equal to the average root
diameter which was estimated to be 1.5 mm
yielding an average root radius of 0.75 mm. By
taking the-water displacement of the roots as the
volume of these root cylinders, root lengths were
calculated (Table 3) from the following constant
relationship: Length (cm) =Volume (cm3) + [pi x
(0.075cm)2]. This transformation was done to
more effectively communicate root growth by
length rather by volume. Since the data were
transformed by a constant factor, any treatment
differenceswerenotobscuredordeveloped.Plants
were harvested following the randomized design.
Due to the number of plants and the painstaking
nature of the root excavation, the harvest lasted
from March 28 to April 29, 1994.
Engineering behavior of limestone based
media. Initial determination of the engineering
properties of the blendswas accomplished through
the testing of the limestone based medium, which
was chosen for its manufactured consistency, A
series of limestone mediawere blended in batches
in the same manner as described in the linden
study. Blends were based on a 100 kg stone
component contribution. Based on the linden study
Grabosky & Bassuk: Soil Under Sidewalks
Table 3. Respohse of linden root development by
treatment. Overall standard error by treatment
348.4 cm excepting where single replicates had
died (X); in which case the standard error = 381.6
cm.
Stone type Stone to
soil ratio Avg. root length (cm)
without with
hydrogel hydrogel
Limestone 368:1 1971 3216
4.09:1 2264 1879(X)
4.60:1 2047 2839
5.26:1 1947(X) 2773(X)
High friction 5.47:1 2377 2584
6.03:1 .2509 1999
6.84:1 1981 3103
7.81:1 3169 2462
Salite ® 1.46:1 2528 2726
1.70:1 2811 2D84
1.78:1 2113 2226'
2.09:1 2467(X) 2433
Clay loam - 586(X) 3640
observations, the initial hydrogel tackifier rate was
thoughtto be higherthan neededand was therefore
reduced to 38 g of hydrogel per 100 kg of stone in
the engineering tests.
In the linden study, matrix pore volumes were
calculated for noncompacted stone. When the
matrix was compacted, the resulting matrix pore
volumes were reduced. Therefore, the soil that
was initially measured to volumetrically fi1170, 80,
90, and 100% of the noncompacted matrix voids
would now be found to be compacted at least at
the 90 and 100% levels (the two lowest stone to
soil ratios for each stone type). After looking at the
final compacted stone matrix and the bulk density
of the clay loam used in the study, the soil volumes
used would overfill the interstitial voids at the
stone to soil ratios used unless the soil was
compacted. For this reason, the mixes tested in
the engineering phase of the study represented
stone to soil ratios ranging from 4:1 to 7:1 (Table
4). The mixes also represented a range which
would start to define a critical stone to soil ratio and
maximize the soil component of the system.
Moisture density relationships were determined
foNowing standard Proctortesting methods (ASTM
D 698 method D) [2] with the following modifica-
Journal of Arboriculture 21(4): July 1995 195
Table 4. Observed maximum densities and associated moisture contents of limestone blends
resulting from standard and increased Proctor type compaction efforts. Porosities are calculated
for each density.
j Stone Observed maximum Observed 'Porosity at Observed maximum Observed Porosity at
1 to soil dry density from optimum optimum density dry density from optimum optimum
~ ratio 592.7 kJ/ms moisture from 592.7 1609 kJ/m3 moisture density from
effort contenttl % kJ/m3 effort effort content kJ/m3 effort
4.057:1 1990 12.2 26% 2030 12.1 - 13.0 24%
4.997:1 1970 12.0 27% 2050 9.0 - 12.0 24%
5.026:1 1960 11.8 26% 2040 8.0 - 12.5 23%
i 6.28:1 1920 11.0 29% 2030 8.5 - 11.5 25%
~ 7.085:1_ 1910 11.8 29% 2000 11.5 - 13.0 26%
tions. No sieving of the materials was done since
15%+ of the material would be retained. on the
0.75" sieve and its removal would radically change
the tested blend. The 6" mold was chosen to
accommodate the large aggregate. A metal rod
was thrust 21 times at the edges of the mold in the
initial lift to prevent bridging of the stones against
the base of the mold. This bridging would have
created inordinately large voids at the base of the
compacted profile yielding inaccurate dry density
calculations. Screeding the material level with the
top of the mold for accurate volume calculation
required the removal of stone that extended into
the-mold. Upon removal, smaller stone particles
and soil were replaced into the mold in an ap-
proximation of the stone to soil ratio of the tested
material. This material was packed by hand and
pressure applied with the screeding barto reduce
the potential difference in compactive effort in
those replaced areas.
The standard compactive effort of 12,375 ftlb/
ft3 (592.7 kJ/m3) in three lifts was applied in the
mannerdescribedeaylier. Moisture density curves
were based on seven resultant density test ob-
servations at increasing moisture levels. All test
materials were allowedto sitfor24 hours in closed
containers to allow for equilibration of moisture
content since water was added to generate each
increased moisture content. Specific gravity val-
ues for each blend were calculated from the
specific gravity of each ingredient (listed in Table
1) and the stone to soil ratio for each blend. The
specific gravityforeach blend was used to calculate
porosity and void ratios fdrthat material at various
oven-dry densities.
A second set of compactions using a 101b (4.54
kg) hammer, 18" (457 mm) drop, 3 lifts, and 56
blows per lift (ASTM D1557 method D in onlythree
lifts) were also completed. This resulted in a
33,592 ftlb/fts (1609 kJ/m3) compaction effort.
Determination of the expected standard proctor
optimum density for all stone-soil blends was
calculated by first compacting each stone type
with the standard 592.7 kJ/m3 effort and deter-
mining its density as an average of five tests. By
adding to this the dry weight of noncompacted
clay loam soil for each stone to soil ratio, a
predicted optimum dry density for each mix was
calculated.
Variability of moisture content in each test was
estimated to be±1 % due to the stoniness and the
rapid drainage capacity of these blends. Variation
in dry density was assigned at t7.5 kg/m3 calcu-
latedfrom the specific gravity of the blend and the
size of the mold.
California Bearing Ratios were determined on
test blends with limestone to soil ratios of 4.057:1
and 5.026:1 to see how they would sustain loading
and to judge their efficacy as potential pavement
bases. CBR testing was conducted on soaked
samples following the ASTM 1883 protocol [2].
Materials from limestone stone to soil ratio 4.057:1
were compacted with a 1609 kJ/m3 effort (Table
5). Materials for the limestone stone to soil ratio
5.026:1 were compacted using the standard 592.7
kJ/m3 and 1609 kJ/m3 efforts. Piston seat weight
196
Grabosky & Bassuk: Soil Under Sidewalks
Table 5, CBR testing results from two limestone test media, All tests were subjected to a 96 hour
saturation period by submersion.
Stone to Comparative Moisture Resultant CBR at CBR at Post CBR Surcharge
soil effort content (%) dry 2.5 mm 12.5 mm test moisture used during
ratio (kJ/ms) during density penetration penetration content (%) saturation
compaction (kg/ms) period (kg)
4.057:1 592.7 9.5 1961 48 57.2 11.8 6.936
592.7 9.5 2068 76 73.5 11.2 5.71
592.7 9.5 1946 49 45.9 11,5 5.749
1609 9.9 2044 65 113.3 9.8 5.726
1609 9.9 2081 99 93.9 10.5 5.748
5.026:1 1609 9.2 2042 65.1 64.1 11.2 5.715
1609 9.2 2015 101.5 105.3 9.4 5.748
1609 9.2 2025 125.3 78.6 11.3 5.737
1609 9.2 2055 96.5 93 10.8 5.731
1609 9.2 2005 95.3 82.4 9.9 6.936
1609 9.2 1983 79 80 10.6 6.943
on all specimens was 6.75 lbs. All samples were
soaked by submersion for 96 hours and drained
for 15 minutes priorto testing. During the soaking
period, all samples experienced a metal surcharge
of 5715-6943 g to simulate a pavement layer over
the test material duringthe saturation period (Table
5). The. penetration rate of the piston was slowed
to a uniform 0.025 inches/minute and readings
were taken as each stone breakage registered
and at the ASTM standard recording depths. The
curves were generated using these points and
then corrected as per ASTM 1883 [2,26]. The
resultant curve could consequently lie inflated,
but would more accurately reflect each material's
behavior in comparison to only the predetermined
depth readings and would be a function of the
stone's inherent strength.
Results and piscussion
Linden study. Roots in the compacted
nonhydrogel controls were observed only in the
initial noncompacted planting tube area except in
one replication. In the one replicate where roots
did penetrate the soil, roots followed the interface
between two lift compaction zones and grew to-
ward the side of the container but did not reach it.
In all other stone and soil test media the roots
were observed to reach the bottbms and the sides
of the containers throughout the entire profile.
Occasionally, roots were seen to grow around
zones of poor aeration where uneven mixing left
high concentrations of hydrogel. This problem
was observed in 4 replicates of the mix containing
.the highest proportion of soil (4.09 parts limestone
to 1 part soil) with hydrogel. Mycorrhizae were
observed in nearly all test containers, with ex-
ceptions occurring randomly across the entire
range of test media.
Root growth was impeded in the control without
hydrogel compared to all other blends and the
addition of hydrogel to the control increased root
penetration by621%overthenonhydrogel control
(Table 3). The bulk density of the clay loam with
hydrogel was 1.25 as opposed to 1.38 without
hydrogel (Table 2). This could be attributed to
swelling of the hydrogel in the soil separating the
soil aggregates reducing the dry density of the
soil. This would also create relatively large pores
which would allow for vigorous root growth. There
were no significant differences between the stone
types or the stone to soil ratios. There was no
significant effect on root penetration caused by
the use of the hydrogel, type of stone, or stone to
soil ratio. No interactions were found. All treat-
ments significantly improved root growth when
compared to the control (p<.001). Root length in
the stone-soil blends ranged from 1879 cm to
3216 cm, an improvement of 320-548% over the
Journal of Arboriculture 21(4): July 1995
soil control (Table 3).
The overall low standard error of observed
density indicated that compaction variability be-
tween replicates in the linden study was low.
Standard errors of density ranged from 25 to 27
kg/m3 in systemswith densitiesfrom 1090 to 1852
kg/m3 (Table 2). Since there were differences in
~' specific gravities among stone types, it would not
be appropriate to compare groups by observed
densities alone. More revealing was the effect of
~ hydrogel on density for each treatment and as a
I percentage of optimumdensityforeachtreatment.
As the stone tosoil ratio increased, the difference
in density between each mix with and without
hydrogel decreased (Table 2). This may indicate
that in the lower stone to soil ratios, the water
absorbed by the hydrogel held the matrix stone
apart during compaction. By comparing the dry
densities of the linden study mites and factoring in
the particle density of the solids; we calculated the
porosity of the mixture (Table 6). The hydrogel
rate used in the linden study was approximately
150 ghydrogel/100 kg stone. If the material ab-
sorbed 200 times its weight in water, it would have
been able to hold enough water to cause com-
pactfon of the clay loam if the blends had'been
compacted to Standard Proctor Density. As the
Table 6. Porosity of non-compacted loam used to
create the blends=57.1%. Comparative porosities
of stone/soil mixes if compacted to standard Proctor
optimum density. Porosity of interstitial soil also
shown, from dividing volume of voids by the vol-
ume of soil solids. The stone is treated as inert
space and is ignored in the calculation.
Stone- Stone to Porosity blend
soil ratio at standard
Proctor optimum
density (%) %soil solids Porosity soil
in compacted within the
profile .stone
by volume matrix (%)
Limestone
3.68:1 25.5 19.3 56.9
4.09:1 26 17.7 59.5
4.6:1 26.4 16 62.3
5.26:1 27 14.3 65.5
High friction
5.47:1 21.9 14.4 60.4
6.03:1 23.1 13.1 63.8
6.84:1 24.4 11.6 67.7
7.81:1 25.6 10.3 71.4
197
stone to soil ratio increased, empty pore spaces in
the matrix would have to increase. The increased
empty pore volume would allow space for the
hydrogel to swell without displacing the matrix,
resulting in less of a difference between hydrogel
and nonhydrogel treatments of the same stone/
soil mix.
All of the blends were classified by the Unified
Classification System. The blends were charac-
terized bystone type and particle size distribution
showing their gap-graded nature (Figure 5). The
limestone blends rangedfromgravel-silt mixture/
clayey-gravel (GM-GC) to a poorly graded gravel/
gravel-silt mixture/clayey-gravel (GP-GM-GC).
The high friction aggregate mixes ranged from a
poorly graded gravel/gravel-silt mixture/clayey-
gravel (GP-GM-GC) to a poorly graded gravel
(GP). The Solite blends all fell into the gravel-silt
mixture%layey-gravel (GM-GC) category (Figure
5)
Itwould appearthatthenon-Solite®stone-soil
blends would serve as an excellent subbase, and
a good base at the higher stone to soil ratios
(Figure 4). Also, the non-Solite®blends normally
would exhibit only a slight susceptibility to frost
action [4,5,28]. Although Solite® blends com-
pared poorly with the non-Solite® stone blends,
care should be taken before discounting the Solite®
since the classification is based on the weight of
the particles and Solite®, being a heat expanded
slate, is very light per unit particle size when
compared to the clay loam due to entrapped air
voids within the aggregate. The Unified Classifi-
cationmay not be a valid predictor of performance
in this unusual case. Observations of root growth
indicated that Solite® behaved similarly to the
other stone types (Table 3).
Comparison of the poros ity of the u ncompacted
clay loam (57.1%) with the calculated porosities
within the interstitial spaces of the stone matrices
(56.9%-71.4%), showed that the soil within the
stone matrix had equal to or greater porosity than
the uncompacted soil (Table 6). The porosity of
the uncompacted loam was in fact between 10
and 30% less than the soil within the stone matrix
[9]. This would explain why root growth was
unimpeded in all of the stone/soil blends as
compared to the compacted soil without hydrogel
__,-a
198
Grabosky & Bassuk: Soil Under Sidewalk;
control; despite the higher densities of the stone/
soil blends. This is good evidence for using the
porosity of the soil within the stone matrix spaces
and not the porosity of the total stone and soil
system as the critical measurement. if root growth
was impeded with 45.4% porosity in the compacted
clay without hydrogel, then it would seem surelyto
be impeded with the 22 - 27% overall porosity in
the stone/soil blends, (Table 6) yet root growth
increased a minimum of 320%overthe compacted
soil control (Table 3).
Since there were obvious increases in root
growth overthe control in all treatments, there was
reason to believe that this type of system can be
used to successfully sustain streettree root growth.
This system will allow root penetration and normal
short term growth over a wide range of stone to
soil ratios when compacted to 80% standard
Proctor Optimum Density. Studies at higher den-
sities are underway.
Engineering studies. The soil mix density was
seen to consistentlyincrease asexpected with the
increased 1609 kJ/ms compactive effort and with
one exception, with increased amounts of soil in
the stone matrix (Table 4). The exception involved
limestone 4.057:1-where, at the 1609 kJ/m3 effort,
the density observed actually dropped over 2.5
times beyond the assigned margin of error of 7.5
k9/ma• This was taken to indicate that a critical
stone to soil ratio had been crossed, and the soil
portion of the blend had possibly impacted the
formation of the stone matrix.
A minimum CBR value of 40 was considered
satisfactory, and all tested blends showed an
adequate CBR rating (Table 5). All samples would
have been an acceptable base under saturated
conditions (the worst case scenario) provided that
the pavement was thick enough to withstand the
projected maximum load of the sidewalk.
In the remaining ten samples, the CBR values
covered a range of 60 units (a very wide range)
over densities that varied from 1946 to 2081 kg/ms
(a very narrow range). This difference in range
could be caused by uneven stone breakage dur-
ing the test. The surface area of contact of the I
piston and the depth of the penetration affect the
measured CBR as does the placement of the
piston in relatior to the stones beneath the piston
and the location and timing of stone breakage.
Resistance to load would increase to a, point of
stone failure and plummet to a lower resistance
until the next stone was encountered. This is not
surprising due to the open nature of the matrixand
the ability of shattering stones to quickly nest into
surrounding voids. Forthis reason, results should
focus on an acceptable range of CBR values in
relation to density in this type of mix rather than a
single measurement.
It does appear from the initial tests, that the
materials used in this study would be considered
acceptable for use as a subbase or as a base
under light traffic pavement structures. The linden
study has demonstrated thatthese same materials
have the potentialto allowforvigorous rootgrowth.
Normally, materials in these classes would be
expected to possess a low frost-heave potential
[4l• In the blends developed in this study, frost
heave potential would likely be a function of the .
amount of hydrogel in the blend since it normally ,
absorbs up to 300 times its own weight in water.
However, it would be reasonable to believe that '
the material would be less frost sensitive than
current materials in use if pore space existed in '
tar ge enough voids to allow for the expansion of
ice lenses without disturbing the matrix. The rate
of hydrogel used in the system now becomes an '
influential factor, and testing must be done to _~
furtherdefinethisrate. However, at the rate of 38
grams of hydrogel per 100 kg of stone, fully p
hydrated gel would occupy only 1 % or less of the ;
matrix pores.
The Critical Stone to Soil Ratio
If one accepts the assumption that both the
stone and the water in such systems are incom-
pressible, then there is a critical stone to soil ratio
similar to the threshold proportion of sand dis-
cussed by Spomerforlandscape soils [24j. Below
this critical ratio, the excessive soil in the system
would either be compacted, impact the formation
of the stone matrix, or affect the engineering
properties of the total system. By having more soil
n the system than could be accommodated bythe
pores in the compacted stone matrix, the soil
would be compacted. In this case, the stones
would "float" in the compacted soil and not come
Journal of Arboriculture 21(4): July 1995
into contact with other stones thus preventing the
bridging of the stones which form the load-bear-.
ing stone matrix. In this situation, the engineering.
behavior would be that of the soil and not of the
stone, and the soil would be compacted to the
same problematic levels in order to bear loading.
Critical dryweightstone to soil ratios are different
from that mathematically expected and will be
unique for each stone type and shape and for
each soil used. In practice, as the stone and soil
were mixed and compacted, the soil would be
unavoidably compacted to some extent. This would
happen even when the stone matrix pores were
only partially filled with soil. With the introduction
of hydrogel into the system, additional incom-
pressible water would act as another additional
compactive force on the soil within the system. As
the fine materials in soil were added to the stone
matrix, the matrix would form differently. Since
this is a dry weight ratio, the particle density of the
stone and soil used will have a direct effect on the
critical ratio. Highly angular stone will have a
different compacted matrix porosity when com-
pared to arounded stone, and will accept additional
soil volumes. Since the critical stone to soil ratio
will be affected by the stone type and by soil type,
a generalized critical stone to soil ratio or equation
is yet to be thoroughly identified.
A way to estimate this critical stone to soil ratio
would be to chart the observed optim um density of
a mix in relation to its calculated optimum density.
The calculated optimum density is the compacted
stone matrix density with the ratio weight of soil
added. For this initial calculated density, the as-
sumption has been made that this addition of fine
material will not substantially change the final
compacted stone matrix. Since the ratio of stone
to soil could be considered constant regardless of
the density, a change in the relationship between
the calculated and observed optimal density would
indicate a change in the stone/soil system. A ratio
of calculated to observed optimal densities greater
than 1.0 would indicate soil compaction had oc-
curred, impacting the final stone matrix. Since the
overall difference in the soil component over the
entire range of tested materials is relatively small,
this type of shift in density behavior is likely a
change in the stone matrix. Belowthe 5:1 stone to
199
soil ratio there appears to be a shift in density
which would indicate that soil compaction within
the stone voids had occurred in the crushed
limestone experimental system (Figure 6).
Summary
It is apparent that we can grow plant materials
in a load bearing pavement base. The linden study
showed vigorous and healthy root growth in
compacted profiles in excess of 1700 kg/ms bulk
density while roots in controls of compacted clay
loam to 1377 kg/m3 were severely impeded. Ini-
tialengineering tests of a crushed li mestone media
indicated that the blends would function well as
pavement bases if compacted to a density of 2000
kg/m3. A blends's strength is a function of the
strength of stone if the stone to soil ratio is not
lower than the critical ratio which would occur with
the addition of excessive soil. For the one system
we have tested, the critical ratio was defined as 5
parts crushed#2limestone to 1 part clay loam soil
by dry weight. Extension of this system to various
stone and soil types needs to be studied as well as
rates and types of hydrogel tackifiers.
Currently plant materials are being grown and
studied instone/soil mixes compacted to optimum
densities. Water management, nutrient availabil-
Critical Stone/Soil estimation curve
Expected vs Observed opt. Dry Density
~.~
7.OB - ---- rT--
1.06 --~-- ----------..,._I-_. ._
" ~t.oa -- ------ I - - -----__-
s t.oz - $~- --- -----
$r i
g 0.96 ---
~o.9s
7.065:1 6.26:1 5.026:1 4.997:1 4.057: 1
Dry Weight Stone b Sal Ralio
Figure 6. Graphic estimation of a likely critical ratio
for limestone mixes. A ratio above one would indi-
catethe critical stone to soil ratio had been crossed
'and compaction of the interstitial soil had likely
experienced compaction. Error bars represent the
range of ratio val ues due to the asslg ned acceptable
error of each of the measurements (kg/m3)
200
ity, frost-heave susceptibility, as well as root and
shoot growth studies will be incorporated into
these and future .tests. Field installations and
further laboratory testing are underway to further
quantify and generate a common specification for
stone and soil mixtures for commercial use.
Acknowledgments. The authors wish to thank Amereq
Corporation for their donation of Gelscape® used in all of our
initial studies and the Robert Baker Companies which supplied
the plant matedals. We also thank Lynne Irwin and Peter
Messmer of The Cornell Local Road Program for their exten-
sive assistance and guidance in the engineering phase of this
project. Thanks also to B.Z. Marranca, Thomas Randrup, and
Francesco Ferrlni for their assistance In the linden root exca-
vation. We thank ISA and H RI for helping to support this work.
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Urban Horticulture Institute
Cornell University
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