Structural Soil for Urban Trees

Structural Soil for Urban Trees

Currently most urban trees are planted directly into existing compacted urban soil or tree pits with limited root space. Trees that are planted in areas surrounded by paving tend to struggle for air space and usually decline well before they should. Where soil volume is limited by pavement, tree roots suffer and tend to take the path of least resistance searching for air, usually in and around pipes, foundations, or to the surface. Healthy trees need a large volume of non-compacted soil with adequate drainage and aeration and reasonable fertility.structural-soil-expanded-view

While the need and desire for large trees in the urban landscape still is the desired intent, the trees do not survive long enough to fill the need. Not planning for root growth is ignoring the biological requirements of trees and is not economical or environmentally prudent. The failure to provide adequate soil for both drainage and root growth is critical to the life of the tree and without an engineered soil specific to this application, trees have a shortened life span and may die. Ensuring a good supply of air to the tree roots is essential for satisfactory tree growth, however in urban situations, the movement of air into the soil is often restricted. By providing additional root space below the pavement in what otherwise was compacted urban soil, Structural Soil can allow most newly planted trees to have a chance for healthy growth. This mix consists of 80% 75 mm angular clear aggregate  and 20%  approved sandy clay loam. The aggregates bear the load, providing the structural stability for the pavement above. The angularity of the rock create for the non-compacted soil, providing space for air, water and nutrients the roots, as well as provide for future root expansion. Engineered structural soil provides a resource for root growth beyond the traditional tree pit, allowing for much stronger root growth and ongoing tree health. Bitter_orange_-_Citrus_aurantium_08

Denbow has been providing structural soil to many municipalities within the Greater Vancouver and Fraser Valley area. These departments use Denbow’s manufactured soil to allow city planners and engineering departments to add trees for the health and beauty of urban communities.

Contact Denbow’s soil experts today to find out more about how structural soil can work in your city or municipality.



What is Structural Soil and Why is it Good for Trees

What is Structural Soil Why is it good for Trees?

Why is structural soil effective?

Structural soil is a type of soil mixed with a specific type of gap-upgraded rock (typically 75mm clear). This is a very important part of the specification; the reason structural soil works well is that the gaps created by the rock are then filled with soil, enabling penetration by tree roots. This penetration allows the tree access to a larger space underground, and has the double function of preventing the tree roots from lifting up the hard surface, i.e. the sidewalk that is on top.

How does the structural soil get tested?

The testing of structural soil is essentially a three-part process. The soil must be tested separately. The rock must be tested and sourced with a specific sieve size attached to it to determine that it is in fact a clear rock and not a minus rock. With a minus rock all of which would be filled and therefore the soil would be effective.

There is also a stabilizing compound that is used with structural support to help the soil adhere to the rock. The specifications for this material would come directly from the stabilizer supplier; there are more than one of these.

Who can make structural soil?

Like any engineered soil, structured soil is a technical process which is best done by people who understand all the specific components and have all the components readily available with testing and prior knowledge. Making good structural soil to specification is an extremely important endeavour. Structural swell that is not up to specification is a very difficult situation to remediate once it’s already in the ground.

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BC – A River Delta: Understanding Soil. Ask Tom Series

The River Delta Soil

There’s a perception in the lower mainland of British Columbia that because we live in a river delta, all of our soil is ideal for growing. This puts to question the need to purchase manufactured soil.  

We decided to “Ask Tom” his opinion on the matter.


Tom, we live in a river delta so why do people need to purchase engineered soil? Shouldn’t our native soil be ideal for growing?


Yes, we do live in a river delta and much of our soil is good for growing. However, in the land development process, the top layer of organic soil is usually removed.  Some of this soil is actually really good. The problem is you can’t guarantee it’s composition. To ensure that your soil is the right composition for growing, it is best to purchase engineered soil.  

Soil structure an important aspect of a grow medium.  There is a difference between good soil and the right soil structure for a grow medium.


What is right soil structure?


Good Question.  Let me tell you about our engineered soil and why, in my professional opinion, it provides the best soil structure for your grow medium.  

First, We ensure that our engineered soil has the right amount of Sand, Silt and Clay.  We also test that the ideal amount of water and air are able to move throughout the soil.  

The sand used in our soil composition is a medium-fraction river sand. Since we have such a large river running through the southern portion of our province we have amazing access to high quality of sand. The Canadian Government of Fisheries does an amazing job of conserving the river while still allowing proper access to the sand.

So there are three main components, sand, silt and clay, to an engineered soil.  At Denbow, we regularly test our soil to make sure it is in the right amount of these parts.  Our soils adhere to strict laboratory-tested guidelines and meet BC Landscape Standards. When you just mix the excavated dirt on sight, you are unable to achieve this same standard.

When you have the right structure then the right amount of air and water are able to move naturally through the soil. Air is just as important as water as it allows the right drainage for plant growth. Residents of British Columbia know we get a significant amount of rainfall. This amount of water requires our soil structure in BC to have a specific amount of sand to allow the water to drain.

To conclude, there you have it. Although the mighty Fraser Runs runs through B.C.’s entire lower mainland, we still need engineered soil so we can be successful with our landscaping.

In our next post we’ll be discussing the importance of pH balance in addition to soil structure.  

Stay tuned for more Ask Tom Articles.

Single Most Important Aspect Of Soil – Ask Tom series

An interview with Tom on soil

We decided to ask Tom the question, “What the single most important aspect of soil?”

We were surprised by his response.

Denbow: Ok Tom, what is the single most important aspect of soil?

Tom: Ah that’s a simple question!

Denbow: Really? Tell us more.

Tom:  Of course. Now my first response was a little tongue-in-cheek. You see, there isn’t a single most important aspect of soil.  Soil is so much more dynamic than most people understand.  As I will hopefully help illuminate in this series, soil has multiple components, which can be engineered to best support what needs to grow.  These components are sand, silt, and clay, together with organic matter.  The proportions may vary, as does the PH level.  So, in fact, your question is not simple at all. A better question might be “What is the best soil composition to support what needs to grow?”

The soil is the grow medium for what needs to grow. When sourcing soil for your project, it is important to ask the right questions. In what geographic region is the soil being used? What kinds of natural environments surround the soil? What are you planting in the soil? What is the slope of the land? There are literally hundreds of questions you could ask when considering what type of soil to use on your specific project.

I could also argue that the single most important aspect of soil is having standardized soil regulations that govern soil usage.

It’s ironic that when constructing a building there are stringent rules regarding the building code, yet when it comes external landscape components of the building, there are mixed standards and styles. However, when you consider the impact of the landscape and the potential the external part of the building has on the environment, as well as any surrounding water sources, you start to see that the exterior of a building is just as important the interior.

Soil manufacturers are not equal and soils are not equal. Many different components like yard waste, compost, and, in some situations even bio-waste, can potentially contain metallic elements in the soil. This is why we need to educate the public as well as lobby for more consistent and informative guidelines for the uses of soil.

So, in constructing a building you have some clear guidelines. Attempting to create similar standards for soil has been challenging to say the least. Over the last 20 years it’s been difficult because we haven’t passed along the right information.

I can explain this way, Tom says. Soil in British Columbia and the regulations of soils in British Columbia has been like a telephone game (you know, the game you played as a kid). One person starts the conversation, whispering in the next person’s ear. “Soil needs to have these three components,” they say. The next person passes it on: “There are three things we should discuss when it comes to soil”. The next person says, “There are three important ways to install soil”. This continues until the last person says, “Soil is great for growing.” Yes, this is true but it has very little—if any—of the information that is actually useful for soil regulations.

Conversations managed this way can get lost in translation. And that’s what’s happening with BC soil. We have several people trying to establish rules regarding the soil views in different cities and municipalities around the lower mainland of BC. Although these rules are somewhat informative, they also need to be consistent and useful, and therefore easily followed.

So, to sum it all up (and sorry for the tangent at the end!), there is no single aspect of soil that is most important. Soil is a dynamic grow medium that needs to be properly engineered to meet the requirements of each specific project. From there we need to have a governing body helping define and classify the different soils we use so the project manager, landscape architect or informed home owner can make the right soil decision.

Soil Series – Who is this Tom?

Soil Series – Who is this Tom? segment #2023

“Dirt is the stuff underneath your fingernails, whereas soil is an engineered composition of organic matter (sand, clay and organic matter) designed specifically for your project’s grow media and geographic location.”

– Tom McConkey

So who is this Tom McConkey?

Before we start talking about “soil”, we want to introduce our specialist.  Denbow is pleased to be offering this series of interviews with one of our valued partners,  Tom McConkey.  Tom is highly skilled in landscape and our local soil and has been in the field for over two decades.  Some of his specialty areas include Green Roofs, Urban Agriculture, Bio-Swales, Storm Water Management, Sustainable Turf Grass Practices.

Tom attended Carelton University and earned a degree to become an English teacher but as fate would have it, Tom decided to take a position at a local landscape company instead.

Early on there he got involved in the company’s compost practices. Tom became very interested in not only selling the compost, but understanding the composting process.

At the time, composting  was a new aspect of the landscape industry so Tom was given the freedom to get to really “dig in,” so to speak.  

During his early years, Tom was able cultivate his knowledge and skills while being mentored by Dr. Bill Herman of Pacific Soil Analysis.  Dr. Herman is a soil specialist who instructed Tom in both the composting process and more importantly, the beneficial use of organic matter as it pertains to the British Columbia Lower Mainland network.  

Denbow hopes this interview series with expert, Tom McConkey serves Landscape architects, city managers and landscapers as well as the average joe gardener in understanding the complexity of engineered soil and how it can assist their projects.  

A New Soil Series Called “Ask Tom”

“Ask Tom”

A new series by Denbow

At Denbow, we know how important soil is.  Some people might suggest that soil is just dirt but we’ve learned that soil is incredibly complex while the success of any environmental infrastructure depends heavily upon it. Soil is the most important variable of any growing medium, whether farm, garden or landscape. In our new series, “Ask Tom,  we are going to take a careful look at soil. Iron_rich_soil_in_hand

“Ask Tom” will be an interview series with Tom McConkey, a local soil and landscape specialist who has over 25 years of landscape and soil analysis under the mentorship of Dr. Bill Herman. Throughout the series, Tom will share his knowledge and experience about soil and help break down the different aspects of soil and the importance for both professionals and laymen.

We’ll ask Tom what he believes is the single most important aspect of soil.  With Tom’s help we’ll look at the quality of soil here in the lower mainland of BC. We live in a river delta, so should our soil be good everywhere? (Here’s a hint – it’s not and we’ll let you know why.)  

dirt-ball-in-handTom says, “Dirt is the stuff underneath your fingernails, whereas soil is an engineered composition of organic matter (sand, clay and organic matter) designed specifically for your project’s grow medium and geographic location.” As we dive into the different topics, please feel free to ask your questions in the comments or via email/web form. We would love to interact with you once we get things rolling. We want this series to be helpful to those in our geographic region.

Thanks for checking in!  We are looking forward to having conversations with Tom and understanding in more depth value of soil as it relates to sustainable development and the environment through urban development. We hope to cover a lot of ground…  






Structural Soil: An Innovative Medium Under Pavement

Structural Soil An Innovative Medium Under Pavement

Have you ever wondered how on earth trees grow in little square planters alongside roadways and sidewalks?

Unless you are one of the few in the know, you probably have. An education on structural soil help will help explain how growing trees below pavement is possible and how it provides urban areas opportunity to enhance the green space within city limits.

structural soil treesHere’s a great article from Cornell University that helps highlight the composition and uses of structural soil. There is a ton of valuable and informative information here, we hope you enjoy it as much as we did.
Read the full article here.


The major impediment to establishing trees in paved urban areas is the lack of an adequate volume of soil for tree root growth. Soils under pavements are highly compacted to meet load-bearing requirements and engineering standards. This often stops roots from growing, causing them to be contained within a very small useable volume of soil without adequate water, nutrients or oxygen. Subsequently, urban trees with most of their roots under pavement grow poorly and die prematurely. It is estimated that an urban tree in this type of setting lives for an average of only 7-10 years, where we could expect 50 or more years with better soil conditions. Those trees that do survive within such pavement designs often interfere with pavement integrity. Older established trees may cause pavement failure when roots grow directly below the pavement and expand with age. Displacement of pavement can create a tripping hazard. As a result, the potential for legal liability compounds expenses associated with pavement structural repairs. Moreover, pavement repairs which can significantly damage tree roots often result in tree decline and death.

The problems as outlined above do not necessarily lie with the tree installation but with the material below the pavement in which the tree is expected to grow. New techniques for meeting the often opposing needs of the tree and engineering standards are needed. One new tool for urban tree establishment is the redesign of the entire pavement profile to meet the load-bearing requirement for structurally sound pavement installation while encouraging deep root growth away from the pavement surface. The new pavement substrate, called ‘structural soil’, has been developed and tested so that it can be compacted to meet engineering requirements for paved surfaces, yet possess qualities that allow roots to grow freely, under and away from the pavement, thereby reducing sidewalk heaving from tree roots.

Convential Tree Pits are Designed for Failure Looking at a typical street tree pit detail, it is evident that it disrupts the layered pavement system. In a sidewalk pavement profile, a properly compacted subgrade of existing material often is largely impermeable to root growth and water infiltration and significantly reduces drainage if large percentages of sand are not present. Above the subgrade there is usually a structural granular base material. To maintain a stable pavement surface the base material is well compacted and possesses high bearing strength. This is why a gravel or sand material containing little silt or clay is usually specified and compacted to 95% Proctor density (AASHTO T-99). The base layer is granular material with no appreciable plant available moisture or nutrient holding capacity. Subsequently, the pavement surrounding the tree pit is designed to repel or move water away, not hold it, since water just below the pavement can cause pavement failure. Acknowledging that; the above generalizations do not account for all of the challenges below the pavement for trees, it is no mystery why trees are often doomed to failure before they are even planted.images

The subgrade and granular base course materials are usually compacted to levels associated with root impedance. Given the poor drainage below the base course, the tree often experiences a largely saturated planting soil. Designed tree pit drainage can relieve soil saturation, but does nothing to relieve the physical impedance of the material below the pavement which physically stops root growth.

A New System to Integrate Trees and Pavement

Structural soil’ is a designed medium which can meet or exceed pavement design and installation requirements while remaining root penetrable and supportive of tree growth. Cornell’s Urban Horticulture Institute, has been testing a series of materials over the past five years focused on characterizing their engineering as well as horticultural properties. The materials tested are gap-graded gravels which are made up of crushed stone, clay loam, and a hydrogel stabilizing agent. The materials can be compacted to meet all relevant pavement design requirements yet allow for sustainable root growth. The new system essentially forms a rigid, load-bearing stone lattice and partially fills the lattice voids with soil (Figure 1). Structural soil provides a continuous base course under pavements while providing a material for tree root growth. This shifts designing away from individual tree pits to an integrated, root penetrable, high strength pavement system.

This system consists of a four to six inch rigid pavement surface, with a pavement opening large enough to accommodate a forty year or older tree (Figure 2) .

The opening could also consist of concentric rings of interlocking pavers designed for removal as the buttress roots meet them. Below that, a conventional base course could be installed and compacted with the material meeting normal regional pavement specifications for the traffic they are expected to experience. The base course would act as a root exclusion zone from the pavement surface. Although field tests show that tree roots naturally tend to grow away from the pavement surface in structural soil. A geotextile could segregate the base course of the pavement from the structural soil. The gap-graded, structural soil material has been shown to allow root penetration when compacted. This material would be compacted to not less than 95% Proctor density (AASHTO T-99) and possess a California Bearing Ratio greater than 40 [Grabosky and Bassuk 1995,1996]. The structural soil thickness would depend on the designed depth to subgrade or to a preferred depth of 36 inches. This depth of excavation is negotiable, but a 24 inch minimum is encouraged for the rooting zone. The subgrade should be excavated to parallel the finished grade. Under-drainage conforming to approved engineering standards for a given region must be provided beneath the structural soil material.

code-walk---streetscapeThe structural soil material is designed as follows. The three components of the structural soil are mixed in the following proportions by weight, crushed stone: 100; clay loam: 20; hydrogel: 0.03. Total moisture at mixing should be 10% (AASHTO T-99 optimum moisture).

Crushed stone (granite or limestone) should be narrowly graded from 3/4 -1 1/2 inch, highly angular with no fines. The clay loam should conform to the USDA soil classification system (gravel <5%, sand 25-30%, silt 20-40%, clay 25-40%). Organic matter should range between 2% and 5%. The hydrogel, a potassium propenoate-propenamide copolymer is added in a small amount to act as a tackifier, preventing separation of the stone and soil during mixing and installation. Mixing can be done on a paved surface using front end loaders. Typically the stone is spread in a layer, the dry hydrogel is spread evenly on top and the screened moist loam is the top layer. The entire pile is turned and mixed until a uniform blend is produced. The structural soil is then installed and compacted in 6 inch lifts.

In a street tree installation of such a structural soil, the potential rooting zone could extend from building face to curb, running the entire length of the street. This would ensure an adequate volume of soil to meet the long term needs of the tree. Where this entire excavation is not feasible, a trench, running continuous and parallel to the curb, eight feet wide and three feet deep would be minimally adequate for continuous street tree planting.

There will be a need to ensure moisture recharge and free gas exchange throughout the root zone. The challenge may be met by the installation of a three dimensional geo-composite (a geo-grid wrapped in textile one inch thick by eight inches wide) which could be laid above the structural soil as spokes radiating from the trunk flair opening. This is currently in the testing stage. Other pervious surface treatments could also provide additional moisture recharge, as could traditional irrigation.

When compared to existing practice, additional drainage systems, and the redesigned structural soil layer represent additional costs to a project. The addition of the proposed structural soil necessitates deeper excavation of the site which also may be costly. In some regions this excavation is a matter of standard practice. However, this process might best be suited for new construction and infrastructure replacement or repair, since the cost of deep excavation is already incurred.

The Urban Horticulture Institute continues to work on refining the specification for producing a structural soil material to make the system cost effective. It is patent pending and will be sold with the trademark ‘CU-Soil’ to insure quality control. Testing over five years has demonstrated that stabilized, gap-graded structural soil materials can meet this need while allowing rapid root penetration. Several working installations have been completed in lthaca, NY, New York City, NY, Cincinnati, OH, Cambridge, MA and elsewhere. To date, the focus has been on the use of these mixes to greatly expand the potential rooting volume under pavement. It appears that an added advantage of using a structural soil is its ability to allow roots to grow away from the pavement surface, thus reducing the potential for sidewalk heaving as well as providing for healthier, long-lived trees.

Nina Bassuk, Director and Professor Urban Horticulture Institute, Cornell University, lthaca, NY
Jason Grabosky, Urban Horticulture Institute, Cornell University, lthaca, NY
Peter Trowbridge, FASLA, Professor Landscape Architecture, Cornell University, lthaca, NY
James Urban, FASLA, James Urban and Associates, Annapolis, MD