Soil Stability

Soil health and stability is crucial in intense Urban environments to be able to provide proper strength above for whichever paving system is utilized, while at that same time preventing compaction of the substrate where tree root growth is crucial for the long-term success of urban plantings.

Soils under pavement need to be compacted to around 95% density before pavements are laid in order to meet most load-bearing requirements for pedestrian and vehicle use. This compaction causes restricted growth of the tree roots beyond the tree well, thus causing the tree health and growth to suffer.

Recently, Vision Design Collaborative hosted a lunch & learn on the Deep Root Silva Cell product – an engineered structural cell that helps provide available soil, root, and utility space while providing structural support above for paving.

Energized by the lunch & learn we wanted do a little research on various alternatives for soil stability, which can be used in urban environments to help ensure the success of plantings & pavings. We are going to explore 3 approaches:

Photo: Copyright Permatill

1. 1. CU-Structural Soils

2. 2. Suspended pavement

3. 3. Sand-Based Structural Soil

CU-Structural Soils

CU-Structural Soil is a mixture of crushed gravel and soil with a small amount of hydrogel to prevent the soil and stone from separating during the mixing and installation process.

CU Structural Soil uses a carefully chosen aggregate with the proper stone to soil ratio which creates a medium for healthy root growth that can also be compacted to meet engineers’ load-bearing specifications. The proportion of soil to stone is approximately 80% stone to 20% soil by dry weight, with a small amount of hydrogel aiding in the uniform blending of the two materials. This proportion insures that each stone touches another stone, creating a rigid lattice or skeleton, while the soil almost fills the large pore spaces that are created by the stone.

The intention is to “suspend” the clay soil between the stones without over-filling the voids, which would compromise aeration and bearing capacity. This way, when compacted, any compactive load would be borne from stone to stone, and the soil in between the stones would remain uncompacted.

References: 1.The great soil debate – ASLA 2. Urban Horticulture Institute – Cornell University

Suspended Pavement

Suspended pavement involves using a modular building block for containing unlimited amounts of healthy soil beneath paving while supporting traffic loads and accommodating surrounding utilities. The engineered building block is filled with high-quality, uncompacted soil to grow trees and manage the rate, quality and volume of stormwater. The modular system can be easily sized to accommodate the needs of any site without compromising effectiveness or site design.

Photo: copyright DeepRoot Silva Cell

The use of structural cells solves the limitation of soil volume by placing a structure that supports the pavement above, allowing a much larger, low compaction soil volume for tree root growth. Without having to respond to the structural requirements of the pavement, the soil is free to meet the challenges of providing the tree with excellent water holding capacity drainage, fertility and long term soil biological functions. The soil, protected within the cells from compaction, can then support bulk densities that are ideal for tree growth.

Photo: copyright DeepRoot Silva Cell

A suspended pavement system integrates trees and soil with stormwater management, utilizing the proven capacity of soils to act as an underground bioretention system. Through soil filtration, bioremediation and evapotranspiration, this system can treat stormwater directly on-site

References: 1.The great soil debate – ASLA 2. DeepRoot Silva Cell brochure

Sand-Based Structural Soil

Sand-Based Structural Soil (SBSS) is a comprehensive system designed to create natural, sustainable growing environments beneath pavements. It provides an appropriate rooting medium for trees which can also support pavements. A typical profile consists of pavement, underlain by several inches of crushed stone, underlain by two to three feet of sand based structural soil, underlain by a drainage system. Aeration pipes are placed within the crushed stone layer to create an air/soil interface at the top of the structural soil. An irrigation system, typically drip irrigation within the aeration pipes, or harvested stormwater distributed though the aeration pipes, provides moisture and nutrients as needed.

Photo: The great soil debate – ASLA

The uniform gradation of the sand allows for a high degree of compaction yet bulk densities remain low and particles cannot pack into a hard mass. Excavations near street trees planted in SBSS have shown rapid root growth into the soil medium. Installations throughout the United States have demonstrated that SBSS supports pavements without settlements, yet is readily penetrated by plant roots.

Stormwater harvesting can be used with slot rains or catch basins. Collected water is distributed through the perforated aeration pipes. High rates of inflow cannot saturate SBSS soils and good aeration ensures a soil environment where aerobic microbes can thrive.

References: 1.The great soil debate – ASLA

Photo: The great soil debate – ASLA

Permeable Pavement Systems

Just recently, VDC hosted a lunch & learn at their office in downtown Asheville, showcasing Belgard paving systems. The main topic of the presentation was Sustainable Site Design & Stormwater Management with Permeable Pavement Systems and there was some great information on paver systems. Below is a general overview of what pervious pavement is, its benefit and several types of systems used today. There is also some great information related with permeable paver systems such as freeze thaw, maintenance & life cycle cost comparisons located here: http://atfiles.org/files/pdf/PermPavers.PDF

Pervious pavement is designed to allow percolation or infiltration of stormater through the surface into the soil below where the water is naturally filtered and pollutants are removed. This helps promote growth in trees and flower beds that are near the system. In addition to these benefits, it also cleans the water that drains into the natural waterways of our cities and neighborhoods and in some cases it can be used to help reduce the amount of stormwater retention and drains throughout a city by reducing the total volume of stormwater runoff and helping to remove the oils, fuels and other contaminants that are normally picked up by water on its way through the streets.

In contrast, normal pavement is an impervious surface that sheds rainfall and associated surface pollutants forcing the water to run off paved surfaces directly into nearby storm drains and then into streams and lakes.

These contaminants can kill not only vegetation throughout the city, but wildlife that drinks from the streams and rivers which is fed by this drainage fallout. Many wildlife species are in danger of becoming extinct due to their short life cycle.

Types of Pervious Pavement

Pervious/Permeable Pavers: This material can be used to create a porous surface with the aesthetic appeal of brick, stone, or other interlocking paving materials. They are most often used for driveways, entryways, walkways, or terraces to achieve a more traditional, formal appearance.

Permeable pavers are an environmentally friendly and attractive alternative to traditional paving methods. Unlike standard concrete, asphalt or even paver installations, permeable paver installations provide for drainage and filtration of water into underlying soils or water storage system. Permeable pavers allow water to percolate through joints or holes in the paver itself but also provide a solid surface that can withstand weight loads that are comparable to those of standard concrete or asphalt surfaces.

Permeable pavers come in many designs, and may be made from concrete, plastic or even combinations with recycled rubber. These pavers can be used for in many commercial and any residential paving application including patios, walkways, driveways, and parking lots.

Potential LEED credits for Permeable Pavers

Porous Asphalt: A great advantage to porous asphalt is that the same mixing and application equipment is used as for impervious asphalt. Only the formula for the paving material changes with porous bituminous pavement. For more details on the various layers of materials see, the Pennsylvania Stormwater Management Manual Porous pavement specification used by the City of Seattle Washington Park Department. The amount of asphalt binder required is about 6% by weight which is somewhat higher than required for standard impermeable asphalt mixes.

Bituminous permeable paving is appropriate for pedestrian-only areas and for very low-volume, low-speed areas such as overflow parking areas, residential driveways, alleys, and parking stalls. Permeable paving is an excellent technique for dense urban areas because it does not require any additional land. With proper design, cold climates are not a major limitation.

Porous Concrete: Again, the same equipment may be used as for standard concrete. Larger pea gravel and a lower water-to-cement ratio is used to achieve a pebbled, open surface that is roller compacted. This material was recently used in a parking area in Fair oaks, California as a way to reduce solar heat-gain solar from absorption. Project costs were reduced because no retention pond or connection to the municipal storm drain system was required.

Plastic or Concrete Grid Systems: High strength plastic or concrete grids (often made from recycled materials) filled with soil and planted with turf grass or a low-maintenance groundcover. Water passes through the turf block into a reservoir base of crushed aggregate, then infiltrates into the subgrade. Some are designed to be filled with gravel on top of an engineered aggregate material as well.

The grids provide a support structure for heavy vehicles, and prevent erosion. After heavy rains, the grids act as mini holding-ponds, and allow water to gradually absorb into the soil below.

Loose aggregate such as uniformly sized crushed stone can provide porous paving, although it is only suitable in light-traffic applications where it won’t quickly be displaced, ground down, or mixed with organic matter. Soft materials such as chipped bark or crushed seashells may be used as porous paving in pathways. Conventional dirt roads use mixed aggregate, including fines, and are largely impermeable.

Information found at the NAHB Research Center