Classics
The following article has been selected because it is deemed very popular or very important to the arboricultural profession and deserves special recognition. It has appeared in a previous Seminar and because of this it is not eligible for earning certification credits; there is no test at the end of this article.
Tree Planting Soils Under Pavement
Edited by Len Phillips
Trees that are growing in areas surrounded by pavement are often expected to grow in very poor quality urban soils, which shorten their useful life expectancy. This article describes a comparison of the growth of trees in five different soil treatments that have been tried under a pavement.
Two species of trees were selected to grow in each of the five different soils. The trees were Snowgoose Cherry (Prunus serrulata) and Bosque Lacebark Elm (Ulmus parvifolia). These species were selected because they are medium-sized trees at maturity and their roots grow aggressively. The trees were planted in each of the following different soils according to the detailed descriptions below.
1. Stone/Soil Mixture
Researchers at Cornell University experimented with and developed a stone and soil mix, consisting of 1 to 1½ in (2.5 to 3½ cm) diameter stone, sandy clay loam soil, and a small amount of hydrogel sprayed on the stone to glue the soil to the stone during mixing. Cornell's research suggests that this mixture, known as “CU Soil” is more beneficial for urban tree growth than standard compacted urban soil. Since the CU Soil is not available near the experiment site, the Stone/Soil Mixture is very similar, but not exactly CU-Soil. It is comprised of 80% stone (by weight) and 20% soil. The soil was placed in the 10 x10 feet (3 × 3 m) planting pit in lifts. The lifts were 8 in (20 cm) deep and were compacted with an impact compactor to 95% Proctor. See Photo
2. Expanded Slate
Expanded Slate is a porous expanded slate rock that is created by heating slate to over 1,000° F. It can hold water and nutrients and carry the load of concrete and vehicles. In addition, it can be installed and compacted more easily than the Stone/Soil Mixture. The Expanded Slate was installed in 12 in. (30 cm) lifts and compacted with a vibratory plate compactor according to the manufacturer’s specifications. This option was the fastest and simplest treatment to install. It required only pouring the stone and using a vibratory compactor in fairly large lifts. See Photo
3. Expanded Slate/Soil Mixture
Expanded Slate and Stone/Soil mixtures are both capable of meeting engineering and the load bearing requirements in urban areas by forming a stone matrix under the pavement. The soil between the stones is not compacted, leaving room for air exchange, holding water, and permitting root growth. This research showed that the Expanded Slate/Soil Mixture encourages roots to penetrate deeper into the ground rather than growing upward and causing pavement failures. This mixture is comprised of 80% Expanded Slate – ¾ to 1 in (1.5 to 2.5 cm) in diameter mixed with 20% sandy clay loam. The Expanded Slate was wetted before mixing with soil. Lifts were 12 in (30 cm) thick and compacted with a vibratory plate compactor according to the manufacturer’s specifications.
4. Compacted Soil
A sandy clay loam was installed in 8 inch (20 cm) lifts and compacted with an impact compactor to 95% Proctor. No amendments were added to this treatment. See Photo
5. Rehabilitated Soil
The existing soil at this site was rehabilitated using a backhoe excavator. This construction technique allowed the use of a loose, root friendly soil under the pavement. A suspended pavement was installed over the rehabilitated soil. The pavement may be either precast concrete lowered onto footers or concrete poured in place. Although the soils in the first three options contain approximately 20% soil by volume, which may affect water and nutrient availability, the suspended pavement can have nearly 95% of the soil volume in non-compacted soil. This option was second in speed and ease of installation. However, there was the additional step of drilling and pouring concrete footer pilings, which added to the time and cost of this treatment.
See Photo
Materials and Methods
The research was overseen by Dr. Tom Smiley at the Bartlett Tree Research Laboratory in Charlotte, North Carolina beginning in the spring of 2004. Each of the plots were lined with a thick Biobarrier non-woven fabric that was trenched 2 ft (60 cm) deep to surround the 10 × 10 ft (3 × 3 m) plots. The fabric was intended to confine the root growth. One tree was planted in the center of each plot giving the tree approximately 190 cu ft (5½ cu m) of medium to grow in. Trees were set in the middle of each plot and soil, stone, or mix was added and compacted as required in lifts around the trees.
As a result of the different construction techniques used in the non-compacted treatment, all of the non-compacted treatments were confined to a single row. All the other treatments were randomly assigned within a row, creating a randomized block design.
The final step was to install a concrete “urban plaza” or "sidewalk" over the plots leaving a 32 in (80 cm) round hole centered on each tree trunk. The concrete was 2 in (5 cm) thick near the center hole.
In the non-compacted soil plot, 6 in (15 cm) diameter footer holes were augured 24 in (60 cm) deep to act as footer pilings, and concrete was poured into the holes. A layer of gravel was applied to the soil surface before pouring the concrete slab.
In each treatment plot, two trees of the two different species were planted. Tree caliper was 2 in (5 cm) dbh when installed. Wire baskets and burlap were removed from the top of the root balls at planting.
Results
Fourteen months after planting, there were distinct differences in tree growth, color, root growth, and crown dieback.
Conclusion
The trees growing in the non-compacted soil suspended pavement treatment are visually healthier in appearance and provide more shade more quickly than any of the other treatments. When designing for planting in spaces that require a solid surface for vehicles and pedestrians, the option of using suspended pavement over non-compacted soil has not received much attention over the past few years. If suspended pavement is to be used, the pavement will need to be engineered to take expected loads without fracturing. This may require greater reinforcement and installation of footers than pavement installed over structural soil.
Source
The following article has been selected because it is deemed very popular or very important to the arboricultural profession and deserves special recognition. It has appeared in a previous Seminar and because of this it is not eligible for earning certification credits; there is no test at the end of this article.
Tree Planting Soils Under Pavement
Edited by Len Phillips
Trees that are growing in areas surrounded by pavement are often expected to grow in very poor quality urban soils, which shorten their useful life expectancy. This article describes a comparison of the growth of trees in five different soil treatments that have been tried under a pavement.
Two species of trees were selected to grow in each of the five different soils. The trees were Snowgoose Cherry (Prunus serrulata) and Bosque Lacebark Elm (Ulmus parvifolia). These species were selected because they are medium-sized trees at maturity and their roots grow aggressively. The trees were planted in each of the following different soils according to the detailed descriptions below.
1. Stone/Soil Mixture
Researchers at Cornell University experimented with and developed a stone and soil mix, consisting of 1 to 1½ in (2.5 to 3½ cm) diameter stone, sandy clay loam soil, and a small amount of hydrogel sprayed on the stone to glue the soil to the stone during mixing. Cornell's research suggests that this mixture, known as “CU Soil” is more beneficial for urban tree growth than standard compacted urban soil. Since the CU Soil is not available near the experiment site, the Stone/Soil Mixture is very similar, but not exactly CU-Soil. It is comprised of 80% stone (by weight) and 20% soil. The soil was placed in the 10 x10 feet (3 × 3 m) planting pit in lifts. The lifts were 8 in (20 cm) deep and were compacted with an impact compactor to 95% Proctor. See Photo
2. Expanded Slate
Expanded Slate is a porous expanded slate rock that is created by heating slate to over 1,000° F. It can hold water and nutrients and carry the load of concrete and vehicles. In addition, it can be installed and compacted more easily than the Stone/Soil Mixture. The Expanded Slate was installed in 12 in. (30 cm) lifts and compacted with a vibratory plate compactor according to the manufacturer’s specifications. This option was the fastest and simplest treatment to install. It required only pouring the stone and using a vibratory compactor in fairly large lifts. See Photo
3. Expanded Slate/Soil Mixture
Expanded Slate and Stone/Soil mixtures are both capable of meeting engineering and the load bearing requirements in urban areas by forming a stone matrix under the pavement. The soil between the stones is not compacted, leaving room for air exchange, holding water, and permitting root growth. This research showed that the Expanded Slate/Soil Mixture encourages roots to penetrate deeper into the ground rather than growing upward and causing pavement failures. This mixture is comprised of 80% Expanded Slate – ¾ to 1 in (1.5 to 2.5 cm) in diameter mixed with 20% sandy clay loam. The Expanded Slate was wetted before mixing with soil. Lifts were 12 in (30 cm) thick and compacted with a vibratory plate compactor according to the manufacturer’s specifications.
4. Compacted Soil
A sandy clay loam was installed in 8 inch (20 cm) lifts and compacted with an impact compactor to 95% Proctor. No amendments were added to this treatment. See Photo
5. Rehabilitated Soil
The existing soil at this site was rehabilitated using a backhoe excavator. This construction technique allowed the use of a loose, root friendly soil under the pavement. A suspended pavement was installed over the rehabilitated soil. The pavement may be either precast concrete lowered onto footers or concrete poured in place. Although the soils in the first three options contain approximately 20% soil by volume, which may affect water and nutrient availability, the suspended pavement can have nearly 95% of the soil volume in non-compacted soil. This option was second in speed and ease of installation. However, there was the additional step of drilling and pouring concrete footer pilings, which added to the time and cost of this treatment.
See Photo
Materials and Methods
The research was overseen by Dr. Tom Smiley at the Bartlett Tree Research Laboratory in Charlotte, North Carolina beginning in the spring of 2004. Each of the plots were lined with a thick Biobarrier non-woven fabric that was trenched 2 ft (60 cm) deep to surround the 10 × 10 ft (3 × 3 m) plots. The fabric was intended to confine the root growth. One tree was planted in the center of each plot giving the tree approximately 190 cu ft (5½ cu m) of medium to grow in. Trees were set in the middle of each plot and soil, stone, or mix was added and compacted as required in lifts around the trees.
As a result of the different construction techniques used in the non-compacted treatment, all of the non-compacted treatments were confined to a single row. All the other treatments were randomly assigned within a row, creating a randomized block design.
The final step was to install a concrete “urban plaza” or "sidewalk" over the plots leaving a 32 in (80 cm) round hole centered on each tree trunk. The concrete was 2 in (5 cm) thick near the center hole.
In the non-compacted soil plot, 6 in (15 cm) diameter footer holes were augured 24 in (60 cm) deep to act as footer pilings, and concrete was poured into the holes. A layer of gravel was applied to the soil surface before pouring the concrete slab.
In each treatment plot, two trees of the two different species were planted. Tree caliper was 2 in (5 cm) dbh when installed. Wire baskets and burlap were removed from the top of the root balls at planting.
Results
Fourteen months after planting, there were distinct differences in tree growth, color, root growth, and crown dieback.
- The trees in the non-compacted/suspended pavement treatment ranked the highest in trunk diameter growth, twig growth rates, foliar color, and root length.
- The second best treatment in terms of tree quality was the Stone/Soil Mix treatment.
- Expanded Slate/soil and rehabilitated soil treatments were overall slightly worse than the Stone/Soil Mix treatment.
- There was significantly more dieback in the Expanded Slate treatment and rehabilitated treatment than the other treatments.
- Trees in the Expanded Slate and Expanded Slate/soil treatments exhibited a severe chlorosis within a month after planting. Foliar nutrient analysis found deficiencies in manganese and iron induced by the high pH of the Expanded Slate. The high pH factor of this product has since been corrected by the manufacturer.
- The Expanded Slate treatment was the easiest to install, However, it did not provide a favorable environment for tree growth.
- Mixing and compacting the Stone/Soil Mix was the most time-consuming portion of all these tree planting projects.
Conclusion
The trees growing in the non-compacted soil suspended pavement treatment are visually healthier in appearance and provide more shade more quickly than any of the other treatments. When designing for planting in spaces that require a solid surface for vehicles and pedestrians, the option of using suspended pavement over non-compacted soil has not received much attention over the past few years. If suspended pavement is to be used, the pavement will need to be engineered to take expected loads without fracturing. This may require greater reinforcement and installation of footers than pavement installed over structural soil.
Source
- Thomas Smiley, Lisa Calfee, Bruce R. Fraedrich, and Emma J. Smiley. “Structural and Non-compacted Soils for Trees”, Arboriculture & Urban Forestry, July 2006.