VDC Green

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